Strategic Flood Risk Assessment (Level 1 SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
September 2016 (Revision 07) DRAFT
Royal Borough of Windsor and Maidenhead Planning and Property Services Town Hall St Ives Road Maidenhead SL6 1RF
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
EXECUTIVE SUMMARY
Introduction The Royal Borough of Windsor and Maidenhead (RBWM) is located in the east of Berkshire, between Slough, Wokingham and Bracknell. The River Thames forms the northern and eastern boundaries of the Royal Borough and is a major source of flood risk to properties within the Royal Borough, including core centres of future growth, such as Maidenhead, Windsor and Wraysbury). Tributaries of the River Thames including The Cut, the River Colne, the Bourne and Twyford Brook (a tributary of the River Loddon) also pose a flood risk to the Royal Borough. A considerable proportion of the Royal Borough is at risk of flooding from a number of sources including river flooding, localised runoff, sewer and groundwater flooding. Flooding can result not only in costly damage to property, but can also pose a risk to life, health and livelihood. It is therefore, essential that the Royal Borough of Windsor and Maidenhead in its role as the Local Planning Authority, Lead Local Flood Authority and Emergency Planning Authority is in a position where it is able to make informed decisions to ensure future development proposals are steered away from areas that are at the greatest risk from flooding, and ensuring that it does not exacerbate existing known flooding problems. The Royal Borough must also be in a position to make an informed decision on how best to balance flood risk with other planning constraints. The Royal Borough of Windsor and Maidenhead Strategic Flood Risk Assessment (SFRA) provides specific guidance and flood risk policy recommendations which should be adhered to by both developers and council officers in order to assist the Royal Borough in making informed decisions on future development to minimise and manage flood risk. The SFRA for the Royal Borough of Windsor and Maidenhead is divided into two documents; the Level 1 SFRA and the Level 2 SFRA. This report (and supporting mapping) represents the Level 1 SFRA (2016). The report provides the background information on flood risk and flood risk policy in the Royal Borough, along with requirements and guidance on assessing flood risk for site-specific development proposals. The report also provides guidance on land use planning and flood risk mitigation options for sites where flood risk has been identified as a potential constraint to future development. The accompanying Level 2 SFRA considers potential development allocation sites in more detail through the application of the National Planning Policy Framework (NPPF) Sequential Test and, where necessary, the need to apply the Exception Test. The Level 2 report sets out the methodology for applying the Sequential Test and the Exception Test in the Royal Borough, which has been applied in the testing of potential development allocation sites, and should also be used to inform the application of the tests to non-allocated development sites. The Level 1 and Level 2 SFRA should be read in conjunction when a potential site is being considered for development. Policy Context The NPPF requires local planning authorities to review the variation in flood risk across their district, and to steer vulnerable development (e.g. housing) towards areas of lowest risk. Where this cannot be achieved and development is to be permitted in areas that may be subject to some degree of flood risk, the NPPF requires the Council to demonstrate that there are sustainable mitigation solutions available that will ensure that the risk to property and life is minimised (throughout the lifetime of the development) should flooding occur. This information should be provided by a Strategic Flood Risk Assessment (SFRA); a study conducted by one or more local planning authorities to assess the risk of flooding to an area from all sources, now and in the future, taking into account the effects of climate change. At a more local scale, this SFRA forms an intrinsic part of the evidence base that informs the production of the Royal Borough of Windsor and Maidenhead Borough Local Plan. The Borough Local Plan is currently in draft and is anticipated to be adopted in 2017. The Borough Local Plan will set the overarching, long-term, spatial development strategy to support sustainable development in Borough up to 2032. The Borough Local Plan will also
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
include a number of allocated sites, on which the potential for development has been assessed through the various evidence documents that support the Royal Borough Local Plan, which include the SFRA. Flood Risk in the Royal Borough of Windsor and Maidenhead The existing flood risk to the Royal Borough has been assessed thorough review of the data sources held by the Environment Agency (which include hydraulic model outputs), Royal Borough of Windsor and Maidenhead and Thames Water. The Level 1 SFRA identifies where there are significant areas of flood risk within the Royal Borough from all sources, including fluvial, surface water, groundwater and sewer and drainage infrastructure. The Level 1 SFRA provides a number of maps which demonstrate the fluvial flood risk in the Royal Borough, including Flood Zone 3a, Flood Zone 3b developed and Flood Zone 3b functional floodplain extents. Maps are also provided showing the extent of flood risk from other sources, the effect of climate change and other supporting information. The principles behind the Royal Borough Local Plan Policy 45: Managing Flood Risk and Waterways are detailed, as well as guidance on the requirements that should be adhered to when producing site-specific Flood Risk Assessments (FRA). The report also provides information on reducing flood risk and making development safe, including Sustainable Drainage Systems (SuDS) and mitigation measures. The Level 1 SFRA was updated in September 2016. Both the Level 1 and Level 2 SFRA reports are living documents and will be periodically updated to reflect changes to flood risk in the Royal Borough.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Table of Contents
1 Introduction ...... 1 1.1 Context and Purpose ...... 1 1.2 Aim and Objectives ...... 1 1.3 Document Structure ...... 2 2 Legislative and Policy Framework ...... 4 2.1 National Legislative and Policy Context ...... 4 2.2 Local Policy Context ...... 4 3 Data Collection ...... 5 3.1 Overview...... 5 3.2 Flood Map for Planning (Rivers and Sea) ...... 5 3.3 Detailed Hydraulic Modelling ...... 5 3.4 Flood Defences ...... 6 3.5 Updated Flood Map for Surface Water ...... 6 3.6 Records of Historic Flooding ...... 7 3.7 Additional Information ...... 7 Royal Borough of Windsor and Maidenhead ...... 7 4 Understanding Flood Risk in RBWM ...... 9 4.2 Overview of Flood Risk in RBWM ...... 9 4.3 Climate Change ...... 15 5 Assessing Flood Risk in RBWM ...... 17 5.1 Fluvial Flooding - Delineation of the NPPF Flood Risk Zones ...... 17 5.2 Assessment of Flood Hazard ...... 19 5.3 Residual Risk of Flooding ...... 20 6 Flood Risk Policy in RBWM ...... 22 6.2 Managing Flood Risk for New Development and Applying the Sequential Approach ...... 22 6.3 Assessing Vulnerability and Flood Risk ...... 23 6.4 Site Evacuation Plan...... 24 6.5 Maintain Existing Overland Flowpaths ...... 24 6.6 Floodplain Storage Capacity...... 24 6.7 Ecological Habitat ...... 25 6.8 Development in Functional Floodplain ...... 25 6.9 Reduction in Onsite and Offsite Flood Risk ...... 25 6.10 Increased Floodplain Storage Capacity ...... 26 6.11 Sustainable Drainage Systems (SuDS) ...... 26 6.12 Flood Resilience and Resistance Measures ...... 27 6.13 Strategic Flood Risk Alleviation Schemes ...... 27 7 Site Specific Flood Risk Assessment ...... 29 7.1 Overview...... 29 7.2 Flood Risk Requirements ...... 29 7.3 Flood Risk Assessment Guidance ...... 33 7.4 Sources of Information to assist the production of Flood Risk Assessments ...... 35 7.5 Flood Risk Mitigation ...... 36 8 Assessment of Site Specific Allocations ...... 39 8.1 Introduction ...... 39 9 Conclusion ...... 42
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Appendices Appendix A National Policy Context Appendix B SFRA Fluvial Flood Maps (September 2016) Appendix C SFRA Local Flood Sources Maps (September 2016) Appendix D Climate Change Flood Maps (September 2016) Appendix E Overview of Flood Risk – Key Locations Appendix F Flood Defence Failure – The Simple Method, FD2320 Appendix G Strategic Flood Risk Assessment Summary of Updates
Figures Figure A Overview of Borough Figure B Geology Figure C Topography Figure D Index Map – Character Areas Figure E Areas Susceptible to Groundwater Flooding Figure F Depth of Fluvial Flooding Figure G Hazard Due to Flood Defence Failure – Simple Approach Figure H Velocity of Fluvial Flooding
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Glossary
AEP Annual Exceedance Probability e.g. 1% AEP is equivalent to a 1% probability of occurrence in any one year (or, on average, once in every 100 years).
Borough Local Plan A Development Plan Document which sets the long-term vision and (BLP) objectives for the area. It contains strategic policies required to deliver the vision including the broad approach to development as well as development management policies required for the delivery of windfall sites and site allocations. Catchment Flood Provides a catchment approach to managing flood risk and provides Management Plan key policies and actions on the catchment scale. (CFMP) CLG Communities and Local Government
De Facto Flood Defence A structure that provides a flood defence function, however has not been built and/or maintained for this purpose (e.g. boundary wall) Defra Department of Environment, Food and Rural Affairs
Development The carrying out of building, engineering, mining or other operations, in, on, over or under land, or the making of any material change in the use of a building or other land.
Development Plan A spatial planning document within the Council’s Local Development Document (DPD) 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. EA Environment Agency
Flood Risk Assessment An assessment of the likelihood and consequences of flooding in a (FRA) development area so that development needs and mitigation measures can be carefully considered.
Flood Zone Map Nationally consistent delineation of ‘high’ and ‘medium’ flood risk, published on a quarterly basis by the Environment Agency. FWMA Flood and Water Management Act (2010).
Formal Flood Defence A structure built and maintained specifically for flood defence purposes. Habitable Room The rooms within a dwelling that are used as living accommodation. Includes living rooms, bedrooms, dining rooms, studies. Kitchens larger than 13 square metres are also included. Bathrooms, toilets and kitchens smaller than 13 square metres are not included. Living rooms greater than 19 square metres and capable of sub-division count as two habitable rooms.
LLFA Lead Local Flood Authority
Local Development Will comprise of a portfolio of local development documents which Framework (LDF) will provide the framework for delivering the spatial strategy for the area.
Local Flood Risk A flood risk strategy document that is required to be produced by the Management Strategy Lead Local Flood Authority (LLFA) in accordance with the FWMA. The (LFRMS) strategy identifies how local flood risk is to be managed.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Major Development Developments of 10 dwellings or more; or equivalent non-residential or mixed development (as set out in Article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2010) National Planning Replaced Planning Policy Statement 25 in March 2012 and provides Policy Framework National Planning Policy. The NPPF is accompanied by a Planning (NPPF) Practice Guidance (PPG, April 2015) that replaces the NPPF Technical Guidance and provides additional guidance on implementing the policy in NPPF. Planning Policy A series of statements issued by the Government, setting out policy Statement (PPS) guidance on different aspects of planning. They were replaced by the National Planning Policy Framework in March 2012.
PPS25 Planning Policy Statement 25: Development and Flood Risk Department of Communities and Local Government, 2006. PPS25 was replaced by the NPPF in March 2012. Preliminary Flood Risk Assessment of local flood risk required under the Floods Directive. Assessment (PFRA) Undertaken by the Lead Local Flood Authority (LLFA) Previously Developed Land which is or was occupied by a building (excluding those used (Brownfield) Land 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. RBWM Royal Borough of Windsor and Maidenhead
Residual Risk Residual risks are those remaining after applying the sequential approach to the location of development and taking mitigating actions. Sustainability Appraisal Sustainability Appraisal is an appraisal of the environmental, (SA) economic and social effects of plans, strategies and proposals to test them against the four broad objectives set out in the Government’s sustainable development strategy. Strategic Strategic Environmental Assessment is a generic term used Environmental internationally to describe environmental assessment as applied to Assessment (SEA) policies, plans and programmes. The European ‘SEA Directive’ (2001/42/EC) does not in fact use the term strategic environmental assessment. It requires a formal ‘environmental assessment’ of certain plans and programmes, including those in the field of planning and land use. SHLAA Strategic Housing Land Availability Assessment
SuDS Sustainable Drainage Systems (SuDS) are used to manage surface water runoff in a way that mimicks natural drainage regimes, with the aim of reducing surface water flooding, improving water quality and enhancing the amenity and biodiversity value of the environment. SuDS achieve this by lowering flow rates, increasing water storage capacity and reducing the transport of pollution to the water environment. SuDS provide an alternative to the direct channelling of surface water through networks of pipes and sewers.
Supplementary Provides supplementary guidance to policies and proposals Planning contained within development plan documents. They do not form part Document (SPD) of the development plan, nor are they subject to independent examination.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Sustainable “Development that meets the needs of the present without Development compromising the ability of future generations to meet their own needs” (The World Commission on Environment and Development, 1987). Zone 3b Functional This zone comprises land where water has to flow or be stored in Floodplain1 times of flood. Defined as areas at risk of flooding in the 5% AEP (1 in 20 chance) design event
Zone 3a High Risk This zone comprises land assessed as having a 1% or greater annual probability of river flooding or a 0.5% or greater annual probability of flooding from the sea.
Zone 2 Medium Risk This zone comprises land assessed as having between a 1% and 0.1% annual probability of river flooding or between a 0.5% and 0.1% annual probability of sea flooding.
Zone 1 Low Risk This zone comprises land assessed as having a less than 0.1% annual probability of river or sea flooding.
Further information can also be found on the Planning Portal website - http://www.planningportal.gov.uk/ or the Environment Agency website - https://www.gov.uk/government/organisations/environment- agency.
1 Denoted as Zone 3b within the NPPF; refer to Section 5.2 for further explanation.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
1 Introduction
1.1 Context and Purpose 1.1.1 Many of the major towns and villages within the Royal Borough of Windsor and Maidenhead are situated adjacent to rivers which flow into, or from, adjoining authorities within the Thames Valley. The Environment Agency2 estimates that within the Royal Borough, approximately 11,000 properties are at ‘significant’ risk of flooding. This represents almost 15% of the total number of properties and nearly 60% of the total number of properties located within the Environment Agency Flood Zones within the Royal Borough3. These figures indicate the scale of the threat posed by flooding within the Royal Borough. 1.1.2 Flooding can result not only in costly damage to property, but can also pose a risk to life, health / wellbeing and livelihood. It is essential that future development is planned carefully, steering it away from areas that are at the greatest risk from flooding, and ensuring that it does not create or exacerbate existing flooding problems. 1.1.3 In the absence of careful management, future developments within the Royal Borough could influence the risk of flooding posed to residents proximal to development sites. Careless management decisions within adjoining Thames Valley authorities can also adversely impact the risk posed to residents of the Royal Borough of Windsor and Maidenhead. 1.1.4 To effectively manage the risks posed by flooding to both property and life, it is crucial that planning decisions are informed by, and take due consideration of, the flood risk posed to and from future development. 1.1.5 The National Planning Policy Framework (NPPF): Section 10 Meeting the challenge of climate change, flooding and coastal change4 has been developed to underpin decisions relating to future development within areas that are subject to flood risk. In simple terms, the NPPF requires local planning authorities to review the variation in flood risk across their district, and to steer vulnerable development (e.g. housing) towards areas of lowest risk. Where this cannot be achieved and development is to be permitted in areas that may be subject to some degree of flood risk, the NPPF requires the Council to demonstrate that there are sustainable mitigation solutions available that will ensure that the risk to property and life is minimised (throughout the lifetime of the development) should flooding occur. 1.1.6 This information should be provided by a Strategic Flood Risk Assessment (SFRA); a study conducted by one or more local planning authorities to assess the risk of flooding to an area from all sources, now and in the future, taking into account the effects of climate change. 1.1.7 The NPPF Planning Practice Guidance (PPG) for Flood Risk and Coastal Change5 recommends that an SFRA be prepared collaboratively with the Environment Agency, the local planning authorities’ own functions of emergency response and the local drainage authority.
1.2 Aim and Objectives
This SFRA aims to collate and present information to define flood risk within the Royal Borough of Windsor and Maidenhead and make recommendations for policies and guidance for the effective management of flood risk to property and life.
2 Estimate made using National Receptor Database and National Flood Risk Assessment Data, June 2013 3 Environment Agency (2016) Flood Map for Planning (Rivers and Sea), available at: maps.environment-agency.gov.uk/wiyby 4 Department for Communities and Local Government (2012) National Planning Policy Framework, available at: www.gov.uk 5 Department for Communities and Local Government (2015) Planning Practice Guidance: Flood Risk and Coastal Change, available at: planningguidance.communities.gov.uk/blog/guidance/flood-risk-and-coastal-change/
1 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
1.2.1 In order to meet this aim, this document will work to achieve the following objectives: A. To inform the review of the Royal Borough of Windsor and Maidenhead’s Borough Local Plan, including its spatial strategy, and to form a key part of the Council’s evidence base; B. To inform new or revised flooding policies as currently set out in the Royal Borough Local Plan and supplementary planning documents; C. To guide future detailed flood risk assessments, and the Royal Borough’s capacity to accommodate development; D. To input to the Council’s Sustainability Appraisal; E. To assist the development control process by providing a more informed response to development proposals affected by flooding; F. To inform future flood attenuation works; G. To support flood risk management and emergency planning; and, H. To contribute to the Council’s Sustainable Community Strategy.
1.3 Document Structure 1.3.1 This document represents the Level 1 SFRA for the Royal Borough and provides the following evidence:
Information on known sources of flooding, including river, surface water (local drainage), sewers and groundwater, that may affect existing and/or future development within the Royal Borough;
Delineation of the areas that have a ‘low’, ‘medium’ and ‘high’ probability of flooding, as well as the Functional Floodplain, within the Royal Borough, in accordance with the National Planning Policy (NPPF);
Information on the Royal Borough’s policy on Flood Risk and Waterways;
Requirements for site-specific Flood Risk Assessments;
Recommendations for appropriate land uses in flood affected areas (in accordance with the NPPF Sequential Test) that will not unduly place people or property at risk of flooding; and,
Where flood risk has been identified as a potential constraint to future development, recommendations for possible flood mitigation solutions that may be integrated into the design (by the developer) to minimise the risk to property and life should a flood occur (in accordance with the NPPF Exception Test). 1.3.2 The accompanying Level 2 SFRA considers potential development allocation sites in more detail through the application of the sequential test and, where necessary, the need to apply the exception test. The Level 1 and Level 2 SFRA should therefore be read in conjunction with each other when a potential site is being considered. 1.3.3 This Level 1 SFRA report is broken down into the following sections:
Section 1 Introduction: This section introduces the context and purpose of the SFRA and the document aim and objectives.
Section 2 Policy Framework: This section details how the SFRA supports wider planning policy within the Royal Borough. Further information on the wider policy context related to the SFRA is provided in Appendix A.
Section 3 Data Collection: This section sets out the data sources that have been used to inform this SFRA, along with the reliability and method of collection for each source.
Section 4 Understanding Flood Risk in the Royal Borough of Windsor and Maidenhead: Section 4 provides baseline flood risk information for the Royal Borough, details on the delineation of the areas that have a ‘low’, ‘medium’ and ‘high’ probability of
2 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
flooding, as well as the Functional Floodplain and guidance on assessing flood risk in the Royal Borough.
Section 5 Flood Risk Policy in the Royal Borough of Windsor and Maidenhead: Section 5 sets out the Royal Borough’s policy on managing flood risk and provides guidance on why there is a need for the policy requirements.
Section 6 Site Specific Flood Risk Assessment: Guidance on the requirements for proposed developments in each Flood Zone is included in this section, along with recommendations for information that should be included in site-specific Flood Risk Assessments to meet these requirements.
Section 7 Assessment of Site Specific Allocations: This section provides an overview of the Sequential Test and Exception Test approach used in the Level 2 SFRA.
3 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
2 Legislative and Policy Framework
2.1 National Legislative and Policy Context 2.1.1 There are a number of national and regional policy directives in place which are relevant to the production of a SFRA, including the National Planning Policy Framework (NPPF) (2012), the Flood and Water Management Act (2010) and the EU Flood Directive and Flood Risk Regulations (2009). 2.1.2 Further information on the guidance and direction provided by each of these documents is detailed in Appendix A.
2.2 Local Policy Context 2.2.1 At a more local scale, this SFRA forms an intrinsic part of the evidence base that informs the production of the Royal Borough of Windsor and Maidenhead Borough Local Plan. 2.2.2 The Borough Local Plan is currently in Draft format. The latest version of the Draft Borough Local Plan was consulted on in summer 2015. Since the consultation closed the Royal Borough Local Plan has been revised to take account of the responses received and an updated plan was published in September 2016. A further consultation on this document will be undertaken before the final revision of the plan is submitted to the Secretary of State and Planning Inspector for examination. It is anticipated that the Royal Borough Local Plan will be adopted in 2017. 2.2.3 The plan will set an overarching, long-term, spatial development strategy for the Royal Borough, including site allocations and detailed development management and delivery policies to support infrastructure in the Royal Borough up to 2032. The Borough Local Plan sets out where best to accommodate homes, jobs and infrastructure in the most sustainable way. 2.2.4 The theme of sustainability runs through the plan and the policies within the document aim to provide a positive environment that encourages sustainable growth and development within the Royal Borough. 2.2.5 Local Plans are required to be based on adequate, up-to-date and relevant evidence about the economic, social and environmental characteristics and prospects of the area. A number of technical studies and other evidence base documents have been used to inform the Royal Borough Local Plan and its policies. In addition to this SFRA, these documents include a Strategic Housing Market Area Assessment, a Housing and Economic Land Availability Assessment and an Edge of Settlement Study (Volumes I and II). 2.2.6 A number of these studies have informed the review of potential development sites which may be considered for housing and employment allocations in the future as part of the Royal Borough Local Plan. There are currently 50 potential allocation sites in total. The flood risk to each of these potential allocation sites will be reviewed as part of the Level 2 SFRA. 2.2.7 The primary purpose of the SFRA is to provide recommendations for future sustainable flood risk management, both with respect to planning and development control decisions. It is then the responsibility of the Council to use these recommendations to establish robust policies that will ensure future sustainability with respect to flood risk. The Level 2 SFRA applies the recommendations detailed in the Level 1 document to review the suitability of the potential allocation sites with respect to flood risk. 2.2.8 The Borough Local Plan is also accompanied by a Sustainability Appraisal (SA). The SA process tests each of the policies included in the plan to determine whether each policy meets a number of objectives. One of the SA objectives that will assess the performance and delivery of the plan policies is to “reduce and manage the risk of flooding and the resulting detriment to people, property and the environment.” The evidence provided in this SFRA is therefore, intrinsic in ensuring that the policies within the Royal Borough Local Plan are suitably informed to meet this objective.
4 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
3 Data Collection
3.1 Overview 3.1.1 There is a considerable amount of existing knowledge related to flood risk within the Royal Borough. This data is primarily collected and recorded by the Environment Agency and the Royal Borough of Windsor and Maidenhead. The information held by these authorities has been used as part of this SFRA and in turn in the development of the Royal Borough Local Plan, to:
assess the flood risk to the Royal Borough;
to inform flood risk and water management policy; and’
to appraise the site allocation process. 3.1.2 The following section provides an overview of the core datasets that have been used in the SFRA process, including their source and their applicability.
3.2 Flood Map for Planning (Rivers and Sea) 3.2.1 The Flood Map for Planning (Rivers and Sea) has been prepared by the Environment Agency to show the natural floodplain, ignoring the presence of defences for the 1% AEP flood event occurring from rivers and the 0.1% AEP flood event occurring from rivers or the sea. The maps provide a reference point for flood risk for England and Wales. 3.2.2 The maps have been produced from a combination of a national generalised computer model, more detailed local modelling (where available), and some historic fluvial flood event outlines. 3.2.3 The Environment Agency’s knowledge of the floodplain is continuously being improved by a variety of studies, detailed models, data from river flow and level monitoring stations, and actual flooding information. The Environment Agency has a programme for improvements to the Flood Map made on a quarterly basis. 3.2.4 The Flood Map in the Royal Borough of Windsor and Maidenhead is provided in Figure A.
The Flood Map has been used in the SFRA to delineate Flood Zone 2 and Flood Zone 3, on which flood risk planning policy requirements for the Royal Borough are based. The Flood Zone delineations have also been used to assess flood risk to the Royal Borough and specifically to proposed development sites as part of this SFRA. This assessment will in turn inform the Sequential Testing of sites in the Level 2 SFRA report.
3.3 Detailed Hydraulic Modelling 3.3.1 A number of detailed flooding investigations have been carried out by the Environment Agency for watercourses throughout the Royal Borough, including
the River Thames (incorporating the Jubilee River);
the River Thames (from Sonning to Marsh Lock);
the River Colne;
the Lower Loddon (incorporating the Lower Loddon, Old Loddon and St. Patricks Stream);
the Twyford Brook;
the Billingbear Brook;
the Emm Brook (from Toutley Bridge to Loddon); and,
the Cut.
5 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
3.3.2 These studies generally incorporate the development of a detailed hydraulic model, providing a more robust understanding of the localised fluvial flooding regime in line with Section 105 (2) of the Water Resources Act 1991. 3.3.3 It should be noted that these detailed hydraulic models assume ‘typical’ conditions within the respective river systems. The predicted water levels may change if the operating regimes of the rivers involved are altered (e.g. engineering works which may be implemented in the future), or the condition of the river channel is allowed to deteriorate.
These detailed studies inform the Environment Agency’s Flood Map and in turn have been used to assess flood risk to proposed development sites and the wider borough as part of this SFRA.
3.4 Flood Defences 3.4.1 Flood defences are typically raised structures that alter natural flow patterns and prevent floodwater from entering property in times of flooding. They are categorised as either ‘formal’ or ‘de facto’ defences. A ‘formal’ flood defence is a structure designed for the purpose of managing flooding and is maintained by its respective owner. A ‘de facto’ flood defence is a structure that has often not been specifically built to retain floodwater, and is not maintained for this specific purpose. These can include railway / road embankments; large buildings / structures; or some impermeable boundary walls situated in flood risk areas. 3.4.2 Formal raised flood defences within the borough have been identified in consultation with the Environment Agency. The defences identified are located mainly on the River Thames, as highlighted in the Flood Zone maps in Appendix B. The main formal raised defences within the borough are as follows:
Cookham Bund;
North Maidenhead Bund;
Datchet Golf Course Bund (PNEU School Bund);
Battle Bourne;
Myrke Embankments;
Cookham Flood Wall 3.4.3 No de-facto flood defences have been specifically identified in the Royal Borough as part of the SFRA process.
Information on existing flood defences has been used to inform the assessment of flood risk to potential development sites as part of the SFRA.
3.5 Updated Flood Map for Surface Water 3.5.1 The Environment Agency has published mapping of surface water flood risk based on computational hydraulic modelling. In 2013 this map was updated and is referred to as the updated Flood Map for Surface Water (uFMfSW). The mapping provides flooding extents from surface water for three storm events; the 3.3% AEP, the 1% AEP and the 0.1% AEP. The mapping also provides information on the depth and velocity of flooding, as well as the hazard associated with the flood water. 3.5.2 The uFMfSW is considered to be a significant improvement on previous surface water flood maps and has been produced using improved modelling techniques & input data. It also benefits from incorporating locally produced mapping where available. 3.5.3 Although the uFMfSW is considered an improvement on previous mapping it still contains assumptions, the biggest of which relates to urban drainage capacity. Where available local drainage rates have been incorporated into the model. In other areas a single drainage rate
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based has been applied. Other benefits and limitations are included in the Environment Agency’s ‘What is the updated Flood Map for Surface Water?6’. 3.5.4 Flood maps for surface water are not intended to identify whether an individual property will flood but should be considered in the preparation of any site-specific FRA and the design of proposed development. It is important to note that these maps should not be used as the primary factor guiding the site allocation process. The intention of the map is to act as a starting point to highlight areas where the potential for surface water flooding needs particular assessment and scrutiny. These maps should not be used in isolation in terms of assessing surface water flooding issues. Additional studies such as historical records should also be used as supporting evidence.
The updated Flood Map for Surface Water has been used as part of the evidence base for assessing flood risk to the wider Borough and potential development sites as part of the SFRA. This assessment will in turn help to inform the Sequential Testing of sites in the Level 2 SFRA report.
3.6 Records of Historic Flooding 3.6.1 Historic records of flooding within the Royal Borough are held by the Environment Agency, RBWM and Thames Water. The quality of the data held varies, depending on whether the following information is known for the record:
extent of the flood event;
source of the flooding; and,
date of the flood event. 3.6.2 Historical information has also been captured through the Preliminary Flood Risk Assessment (PFRA) process7. This information identifies locations of flood events that have occurred in the Royal Borough from local sources of flooding. Those areas known to have been susceptible to localised flooding in recent years (but have not been subsequently mitigated through the provision of an improvement scheme) have been highlighted in the SFRA Local Flood Sources maps (refer to Appendix C). A number of these properties sit outside of the designated flood zones, making them particularly noteworthy in this SFRA. They act as a reminder that flood risk is not restricted to fluvial flooding alone.
Incidences of historic flooding within the Royal Borough have been used to assess the flood risk to potential development sites as part of the SFRA. Historic flood records have only been used to inform this process however, if they have at least 2 of the 3 quality criteria listed above.
3.7 Additional Information 3.7.1 Consultation has formed a key part of the data collation phase for the Windsor and Maidenhead SFRA. The following key stakeholders have been consulted:
Royal Borough of Windsor and Maidenhead 3.7.2 The Borough planning team has been consulted to identify areas under pressure from development and/or regeneration. The Borough drainage team has been consulted to identify areas potentially at risk from river flooding and/or urban drainage. The Borough Emergency
6 ‘What is the updated Flood Map for Surface Water? (November 2013): https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/297432/LIT_8988_0bf634.pdf 7 RBWM Preliminary Flood Risk Assessment (May 2011)
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Planning team has been consulted to identify issues that may be of relevance to the planning of an emergency response to a flooding event within the Royal Borough.
Environment Agency 3.7.3 The Environment Agency is a statutory consultee under the Duty to Co-operate and has therefore been consulted to source specific flood risk information to inform the preparation of this SFRA. The Environment Agency have been consulted to discuss potential flood risk mitigation measures and planning recommendations. The Environment Agency must be satisfied with the findings and recommendations for sustainable flood risk management into the future.
Thames Water 3.7.4 Thames Water is a statutory consultee as part of the emerging Borough Local Plan and has been consulted on the previous SFRAs. Thames Water is responsible for the management of urban drainage (surface water) and sewerage within the Royal Borough. As a statutory undertaker for water supply and sewerage in the Royal Borough, Thames Water has also been involved in other parts of the evidence base related to the Royal Borough Local Plan. In particular this relates to the Infrastructure Needs Assessment and development of the Charging Schedule for the Community Infrastructure Levy, which will ensure that infrastructure is provided when required by new development. 3.7.5 It is recognised that issues associated with failures of the underground drainage/sewer systems are often relatively localised, and should not preclude development. Specific problems have been highlighted by the SFRA process (refer to the flood maps) through consultation with the Royal Borough and the Environment Agency.
Community Stakeholders 3.7.6 A small number of community representatives were also consulted during the preparation of earlier iterations of the SFRA process. This consultation sought their contributions in terms of local knowledge with respect to flood risk within the Royal Borough and with respect to emerging policy and development control recommendations.
Information obtained through consultation with stakeholders has been used to inform the assessment of flood risk to potential development sites as part of the SFRA where relevant.
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4 Understanding Flood Risk in the Royal Borough of Windsor and Maidenhead 4.1.1 The study area for this SFRA comprises the entirety of the Royal Borough of Windsor and Maidenhead. The Borough is located in the east of Berkshire, between Slough, Wokingham and Bracknell (refer to Figure A) and covers an area of 19,844 hectares. Over 83% of the Royal Borough is designated Green Belt. 4.1.2 The Borough encompasses the larger, historic settlements of Windsor and Maidenhead, as well as the smaller towns of Cookham, Ascot, Sunninghill, Sunningdale, Eton, Old Windsor, Horton, Datchet and Wraysbury, which are covered by 15 parish councils (including Eton Town Council).The Borough’s population is 144,560 (2011 census, an increase of 8.25% since 2001) and the dwelling count is 60,901 (an increase of 7.9% since 2001). 4.1.3 The Borough is regarded as a prosperous area with a thriving local economy and a high employment record. The main settlements of Windsor and Maidenhead are the primary focus for employment related development. The Borough has excellent transport links, being located near to Heathrow airport, the motorway network (including M4, M40 and M25) and the national rail network. Geology 4.1.4 Geological information for the Royal Borough has been retrieved from the British Geological Survey (BGS). The solid bedrock geology of the Royal Borough is characterised by chalk to the North (underlying Maidenhead and the surrounding areas) and London Clay to the South (underlying Windsor and surrounding areas). These distinct geological units are separated by a suite of beds, including the Oldhaven, Blackheath, Woolwich, Reading and Thanet beds, which broadly follow the M4 corridor through the Borough. 4.1.5 The underlying geology is particularly relevant in considering the potential risk of groundwater flooding to a site and when designing sustainable drainage systems for a proposed development. 4.1.6 An overview of the Royal Borough’s geology is provided in Figure B. Topography 4.1.7 Topographic information has been provided by the Environment Agency in the form of LiDAR. LiDAR is available as a detailed Digital Elevation Model (DEM) that, in simple terms, offers a three dimensional representation of the local topography. The developed nature of the river valleys within urban areas of the Royal Borough may have an adverse effect on the otherwise high vertical accuracy of LiDAR (within 250mm in some cases). 4.1.8 The topography of the Royal Borough is generally undulating, with a lack of distinctly steep slopes and/or river valleys. A considerable portion of the Royal Borough slopes gently towards the North and the River Thames. A significant number of the properties at risk within the Royal Borough are situated within the Thames Valley, towards the east. The potential risk of surface water flooding exists across the whole of the Royal Borough however, as the undulating topography creates the opportunity for localised ponding to occur. 4.1.9 An overview of the Royal Borough’s topography is provided in Figure C.
4.2 Overview of Flood Risk in the Royal Borough of Windsor and Maidenhead 4.2.1 There are a number of different types of flooding that affect the Royal Borough of Windsor and Maidenhead. An overview of how each of these affects the Royal Borough is provided in this section. It is important to recognise that the cause of flooding is not always certain and cannot always be attributed to just one source of flooding. In addition it should be noted that the most severe flooding is often caused when different sources combine. Main River and Ordinary Watercourse Flooding 4.2.2 The northern and eastern boundaries of the Royal Borough of Windsor and Maidenhead are delineated by the River Thames. Many of the key population centres within the Royal Borough are situated along the length of the river, including Maidenhead, Windsor, Old Windsor, and Cookham. These locations are typically the areas of the Royal Borough with the highest flood
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risk as the historically natural floodplain has been ‘reclaimed’ over centuries for the building of these towns adjacent to crucially important transport corridors. As a result a number of these locations are at risk of flooding. Significant flooding from the River Thames has affected the Royal Borough on a number of occasions within the past 125 years; the Royal Borough experienced major floods in 1894, 1947 and 2014, with floods of lesser severity in 1954, 1959, 1974, 1981, 1990, 2000, 2003, 2007 and 2012. 4.2.3 In addition to the River Thames, a risk of flooding has also been identified in association with other large main rivers in the Borough, including Colne Brook, the River Colne, The Cut, Strand Water, and White Brook. A number of smaller local watercourses also pose a potential risk of flooding, including Bourne Ditch, the Battle Bourne, Wraysbury Drain, and Horton Drain. 4.2.4 These rivers and drains affect fewer properties within the Royal Borough than the River Thames; however they are more susceptible to flash flooding as a result of localised intense rainfall. With our changing climate patterns it is expected that flash flooding will become increasingly common. As incidents of flash flooding are difficult to predict (as opposed to fluvial flood events) it is vitally important that planning decisions recognise the potential risk that these watercourses pose to property, and that development is planned accordingly so that future sustainability can be assured.
The Royal Borough has experienced significant flooding from the River Thames on a number of occasions in the past. Population centres located along the length of the River Thames in the historically natural floodplain are at the greatest risk of flooding. A risk of flooding has also been identified in association with other large, main rivers in the Royal Borough. These rivers are more susceptible to flash flooding, however, due to localised, intense rainfall, which is expected to become increasingly common with changing climate patterns.
4.2.5 The Environment Agency Flood Map for Planning (Rivers and Sea) illustrates the areas of the Royal Borough that are at risk of flooding from fluvial sources. These maps indicate that 26% of the total Borough area is at risk of flooding during a flood event with an annual probability of 0.1%. Within this area, there are approximately 13,289 residential properties and 1,890 commercial properties at risk8. 4.2.6 Of these 15,179 properties, 7677 residential and 1,151 commercial are also at risk of flooding during a flood event with an annual probability of 1%. The area of the Royal Borough at risk during such an event is approximately 20%. 4.2.7 Substantial investment has been made in recent years to alleviate the risk of flooding from fluvial sources, including (but not limited to) the Maidenhead, Windsor and Eton Flood Alleviation Scheme and the Cookham Flood Alleviation Scheme remedial works, improving the standard of protection provided to selected properties within the Royal Borough. 4.2.8 It is essential to recognise that flood defences do not fully remove the risk of flooding to properties within the Royal Borough. In many areas, the standard of protection provided by the defences is less than 1% AEP. More importantly however, it should be recognised that there is a risk to properties situated behind the defences as a result of groundwater flooding and/or surface water flooding, exacerbated by high river levels. 4.2.9 The Environment Agency Flood Maps show significant areas located adjacent to the River Thames, The Cut and the Twyford Brook to be located within Flood Zones 2 and 3. Flood Zone 2 comprises land assessed as having between a 1% and 0.1% annual probability of river flooding whilst Flood Zone 3 is land with a greater than 1% annual probability of river flooding. 4.2.10 An overview of fluvial flood risk is provided in Appendix B (Fluvial Flood Maps) and Appendix E (key locations).
8 Environment Agency (2013) National Receptor Database (NRD)
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In recent years, substantial investment has been made to alleviate the risk of flooding within the Royal Borough, by implementation of schemes such as Windsor and Eton Alleviation Scheme. It is important, however, to recognise that flood defences do not fully remove the risk of flooding and a residual risk will always remain. Properties behind flood defences within the Borough are at risk of flooding from other sources, such as groundwater or surface water flooding.
Surface Water Flooding 4.2.11 Localised flooding as a result of local catchment runoff and/or sewer system failure following heavy rainfall is also a known risk to properties in the Royal Borough, including those in defended areas. The most recent significant flood event associated with surface water flooding within the Royal Borough occurred during summer 2007, when widespread flooding affected large areas of England. Whilst there was little river flooding within the Royal Borough during this event, there is clear evidence of surface water flooding affecting homes and businesses locally. It is understood that at least four schools were closed as a result of flooding in the towns of Windsor and Maidenhead, approximately 30 homes in Maidenhead were directly affected by flood waters, and the commercial centre in Maidenhead (Nicholson’s Walk) was closed due to storm water flooding. 4.2.12 Consultation has been carried out with the Environment Agency and the Council to identify known and/or perceived problem areas. These drainage problems may be attributed to inundation from floodwaters from open drains and watercourses or increased overland flow due to development and/or exceptionally wet weather. In some instances these problems may be due to poor maintenance, associated with (for example) culvert blockages. 4.2.13 Maps providing an overview of surface water flood risk across the Royal Borough are provided in Appendix C (Local Sources Flood Maps) and an estimate of the number of properties at risk of surface water flooding in the Royal Borough based on these maps and the Environment Agency’s National Receptor Database (NRD) is provided in Table 1. Table 1: Estimated numbers of properties flooded from the updated Flood Maps for Surface Water (uFMfSW)
Number of Properties at Risk of Flooding
Residential Non-residential
uFMfSW 1 in 30 < 0.3m deep 49 111
uFMfSW 1 in 30 0.3m and < 0.6m deep 43 109
uFMfSW 1 in 30 0.6m deep 25 52
TOTAL number of properties at risk from the 1 in 30 117 272 year flood event
uFMfSW 1 in 100 < 0.3m deep 137 211
uFMfSW 1 in 100 0.3 m and < 0.6m deep 63 152
uFMfSW 1 in 100 0.6m deep 77 92
TOTAL for number of properties at risk from the 1 in 277 455 100 year flood event
uFMfSW 1 in 1000 < 0.3m deep 939 554
uFMfSW 1 in 1000 0.3m and < 0.6m deep 313 313
uFMfSW 1 in 1000 0.6m deep 307 258
TOTAL number of properties at risk from the 1 in 1000 1,559 1,125 year flood event
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Localised surface water flooding due to local catchment runoff and/or sewer failure following heavy rainfall is a risk to properties within the Royal Borough. The most recent significant surface water flood event occurred during the summer of 2007. Drainage problems in the Borough leading to surface water flooding can be attributed to a combination of floodwater inundation, and increased overland flow due to developments and poor maintenance of the drainage system.
Groundwater Flooding 4.2.14 There is a known risk of groundwater emergence along the River Thames due to the presence of ‘Thames Gravels’. This is a term commonly used to describe the highly permeable superficial deposits beneath the historical floodplain of the River Thames. High water levels in the river result from high underlying groundwater levels in these superficial deposits. As a consequence, properties situated away from the direct influence of the river are also at risk of flooding due to high groundwater levels. 4.2.15 Equally, where flood defences have been constructed to mitigate the risk of fluvial flooding, a residual risk of groundwater emergence may remain. After a period of intense or prolonged rainfall, groundwater moves through the Thames Gravels, causing localised flooding behind the river defences. Fluvial defences could also impede the natural flow of groundwater into the river, thus resulting in a backing up of groundwater behind the defences, potentially exacerbating the risk of groundwater flooding. 4.2.16 The risk of groundwater flooding is highly variable and heavily dependent upon local conditions at any particular time. In an endeavour to provide an indication of areas throughout the country that may be at risk of groundwater flooding, the Environment Agency has produced strategic scale maps known as Areas Susceptible to Groundwater Flooding (AStGWF) to be used within the LLFAs Preliminary Flood Risk Assessment. The maps are based on a grid format and show the proportion of each 1km grid square where geological and hydrogeological conditions show that groundwater might emerge. They do not show the likelihood of groundwater flooding occurring. In common with the majority of datasets showing areas which may experience groundwater emergence, this dataset covers a large area of land, and only isolated locations within the overall susceptible area are likely to suffer the consequences of groundwater flooding. The data should not be interpreted as identifying areas where groundwater is likely to flow or pond, thus causing flooding, but may be of use to LLFAs in identifying where, for example, further studies may be useful. The outcomes of this modelling and mapping exercise within the Royal Borough are presented in Figure E.
It is important to remember that flood defences do not eliminate all flood risk and a residual risk of groundwater flooding remains. The highly permeable soils underlying the historic River Thames floodplain are an effective medium through which groundwater can flow. The risk of groundwater flooding is highly variable, however, and depends on local conditions at any particular time.
Sewer Flooding 4.2.17 The overloading of the sewer system due to inflows exceeding the underground system capacity (i.e. resulting in surcharging) is also a known problem in some parts of the Royal Borough. Thames Water is the responsible authority for sewer flooding in the Royal Borough and as part of this role collects and records incidents of sewer flooding on the DG5 register. 4.2.18 Given the heavily urbanised character of key town centres within the Royal Borough, it is inevitable that localised flooding problems arising from under capacity drainage and/or sewer systems will occur, particularly given the mounting pressure placed upon ageing systems as a result of climate change. Furthermore, sewer systems are generally designed (in accordance with current Government guidance) to cater for the 3.33% AEP storm, and highway drainage systems are generally designed for only 10% AEP storms. Storms over and above these design events will exceed the drainage system capacity, causing overland flow, often in an uncontrolled manner, which can then result in localised flooding.
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4.2.19 Flooding can also occur due to the ingress of groundwater into the sewer network leading to overloading of the sewer system and subsequent flooding. Increasing pressure on an ageing sewer system from urbanisation, climate change and groundwater ingress means overloading of the sewer system and the resultant localised flooding is a risk in the Royal Borough.
Highway Flooding 4.2.20 Within the Royal Borough of Windsor and Maidenhead there are a number of urbanised towns including Windsor, Maidenhead, Cookham and Eton. This urbanisation gives rise to localised flooding issues from surface water drainage and/or sewer systems. Urban drainage systems are generally designed to cater for a 3.33% AEP storm (in accordance with Government guidance) and highway drainage systems are designed for only a 10% AEP storm. Future storms over and above these design events are likely to exceed the capacity of the drainage system, resulting in overland flows and localised flooding. Reservoir Flooding 4.2.21 Reservoir Flood Maps have been produced by the Environment Agency for large reservoirs over 25,000 cubic metres of water. Flood maps are not displayed for smaller reservoirs and details of flood depth and flow are not publically available. In the Royal Borough there is only one reservoir identified within the mapping; the Queen Mother Reservoir, located to the south east of Windsor. The mapping also indicates that the maximum flood extent from the Wraysbury, King George VI and Staines Reservoirs, located to the south east of the Royal Borough boundary, would also impact parts of the Royal Borough. 4.2.22 These reservoirs are situated above ground, and a sudden failure of the embankments retaining the stored water could have a catastrophic effect on properties situated in the path of the discharging water. The reservoirs are, however, monitored and very stringently managed by Thames Water and the Environment Agency (Spitafield Reservoir). 4.2.23 Thames Water9 is responsible for water supply infrastructure located within (and adjacent to) the Royal Borough of Windsor and Maidenhead which could cause flooding should any of the infrastructure fail. This infrastructure includes large water supply reservoirs and several deep large diameter pipes. Thames Water was previously consulted10 to discuss the risk of flooding associated with the failure of the above mentioned water supply infrastructure, in particular the breaching of the water supply reservoirs. It is understood that the risk of flooding associated with the failure of such reservoirs has been determined under a study sponsored by Defra to be in the order of 2 x 10-5% AEP. Further information on the reservoirs is not publically available. 4.2.24 The possibility of a failure of the water mains network is greater; however the consequence is much lower. Such a failure was experienced within the Royal Borough at Datchet and St Leonard’s in which an underground pipe collapse resulted in the flooding of a number of properties. It is notoriously difficult to measure in real terms the potential risk of a structural failure of this nature occurring. 4.2.25 It is noted that the Water Act 2003 amended the Reservoirs Act 1975, requiring the preparation of dedicated Flood Plans for reservoirs that hold over 25,000 cubic metres. A Flood Plan is a set of documents that describe the arrangements to be put into operation in response to a risk of a sudden large release of water from a reservoir that could pose a threat to property and life downstream. A Flood Plan will include an assessment of the impacts of dam failure, a review of the measures that can be taken by the reservoir operator to prevent the catastrophic failure, and an assessment of the emergency response mechanism required to minimise risk to life and property should a failure occur. Thames Water has confirmed that a Reservoir Surveillance Management Process is used on all of its reservoirs that is externally accredited under the ISO9000 system. This includes the appointment of a Supervising Panel Engineer and regular inspections of all reservoirs to the requirements of the Reservoirs Act by suitably qualified
9 Three Valleys Water and South East Water also provide clean water to the Royal Borough. 10 Thames Water was consulted during the drafting of the April 2009 Level 1 SFRA
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engineers. In addition to the statutory requirement, Thames Water undertakes inspections of the reservoirs using trained individuals at a frequency agreed by the Supervising Panel Engineer. 4.2.26 The Environment Agency has also prepared mapping to show the inundation downstream following a breach or failure of a reservoir. This mapping has been prepared to support Local Resilience Forums11 and reservoir operators to prepare reservoir Flood Plans. 4.2.27 It is important to emphasise that issues of this nature should not preclude development, and therefore do not affect the outcomes and/or recommendations of the SFRA process.
There is only one reservoir identified in the Royal Borough, the Queen Mother Reservoir, located to the south east of Windsor. The risk of reservoir flooding in the Royal Borough is considered to be low and should not affect the outcomes and/or recommendations of the SFRA process.
Historical Flooding 4.2.28 The Borough has a considerable flood history, with significant flooding from the River Thames recorded no less than ten times within the past 100 years. 4.2.29 In January 2003, flows in the River Thames triggered the operation of the Jubilee River, constructed between 1999 and 2002, which comprises part of the Maidenhead, Windsor and Eton Flood Alleviation Scheme (MWEFAS). Approximately 1,000 properties were estimated to have been protected within these areas, however, 128 houses downstream of the Jubilee River were affected, many of which were located in the Ham Island and Wraysbury areas of the Royal Borough. 4.2.30 Widespread flooding occurred throughout England during the summer of 2007, with the Environment Agency issuing a flood alert for the Maidenhead area. Despite being predicted within the lower reaches of the River Thames, no significant river flooding was reported within the Royal Borough, evidence that the MWEFAS was working to the designed standard. It should be noted, however, that a large proportion of the flooding occurring throughout the country during the 2007 event was a result of surface water flooding (i.e. intense rainfall that exceeded the capacity of the existing drainage system). The Borough was no exception to this, with flash flooding affecting town centres throughout the Royal Borough. 4.2.31 November and December 2012 saw two River Thames flooding events, caused partly by saturated ground producing greater runoff into rivers. The Environment Agency produced a report into the events, indicating that six properties experienced internal flooding within the Royal Borough in the November event; no properties in the borough were flooded internally during the December event. 4.2.32 Heavy rains, driven by a succession of deep Atlantic low pressure systems caused widespread flooding during the winter of 2013/14, between December and February. The winter was the wettest recorded in the UK since records began in 1766, with areas of the South-East receiving almost 2.5 times the winter rainfall average. Many towns in the Royal Borough were badly affected, including Cookham, Eton and Windsor. Approximately 170 properties were confirmed as experiencing internal flooding from a variety of sources including main rivers (145), fluvial gravels from main rivers (25) and groundwater (1). Although a significant and prolonged event, the Environment Agency records indicate that the flooding experienced during this 2013/14 event was between a 5% AEP (1 in 20 year) and a 10% AEP (1 in 10 year) flood event.
11 Multi-agency partnerships made of emergency services, local authorities, the EA and others to plan and prepare for localised incidents and catastrophic emergencies. Cat. 1 Responders.
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The Borough has a considerable flood history, with significant flooding from the River Thames recorded ten times in the last 10 years. Notable flood events in the Borough have occurred in the summer of 2007, the winter of 2012 and the winter of 2013/14. Historic floods have been the result of a combination of fluvial, groundwater and surface water flooding.
4.2.33 Historically, a number of properties within the Borough have been affected as a result of groundwater flooding and/or failure of the sewer system. It is worth noting that, in some areas, the cause of flooding has been addressed through dedicated investment in improvement works. These improvement works include capital works at Windsor (Bourne Ditch) and Old Windsor (Burfield Road Ditch), undertaken during the early 1990s to improve the standard of protection provided to adjacent homes to 1% AEP. Summary 4.2.34 In summary, there are a number of potential sources of flood risk affecting properties within the Royal Borough of Windsor and Maidenhead. In addition to the relatively large number of properties falling within Zone 3a High Probability (i.e. at risk of river flooding), many more are potentially at risk from localised runoff, groundwater flooding and/or sewer overload. Flooding can affect lives and livelihoods, and it is absolutely essential that future development (particularly residential development) is not placed within areas of the Royal Borough within which the safety of residents cannot be assured in times of flood.
4.3 Climate Change 4.3.1 A considerable amount of research is being carried out worldwide in an endeavour to quantify the impacts that climate change is likely to have on flooding in future years. Climate change is perceived to represent an increasing risk to low lying areas of England, and it is anticipated that the frequency and severity of flooding will change measurably within our lifetime. 4.3.2 The Environment Agency Flood Risk Assessments: Climate Change Allowances12 (March 2016) states that within the Thames River Basin District peak river flow is expected to increase between 10% and 25% within the next 25 years, rising to between 15% and 35% within the next 65 years. Within the next 100 years peak river flow is expected to increase between 25% and 70% within the Thames River Basin District. 4.3.3 The Environment Agency Guidance also provides the anticipated changes in extreme rainfall intensity for across the country. Over the next 100 years, the increase in extreme rainfall intensity is expected to be between 20% and 40%. 4.3.4 Whilst present day flood extents should be used to establish flood zones at a development site (refer to Appendix B), the NPPF requires that developers consider the possible change in flood risk over the lifetime of the development as a result of climate change. The likely increase in flow and rainfall intensity over the lifetime of the development should be assessed proportionally to the guidance provided by the Environment Agency and the requirements detailed in Section 5 and 6. The Environment Agency guidance specifies the exact climate change increase that should be applied for each type of development, depending on the Flood Zone in which it is located and flood source to which it is exposed. 4.3.5 The Environment Agency detailed modelling of the River Thames hitherto considered alternative allowances for the potential impact of climate change over the next 100 years. The Climate Change mapping for the River Thames was withdrawn by the Environment Agency in 2015 due to errors in the model. At the time of writing the Environment Agency model had not been updated to account for the revised climate change allowances set out in the Environment Agency Flood Risk Assessments: Climate Change Allowances and a date for when an updated model would be available could not be provided.
12 Environment Agency (March 2016) Flood Risk Assessments: Climate Change Allowances, available at www.gov.co.uk
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4.3.6 In order to account for the absence of this information the SFRA utilises the current 0.1% AEP flood outline as a proxy for the 1% AEP plus climate change event. Experience has shown that in other areas (i.e. in which detailed modelling is available), in simple terms, the anticipated extent of the 1% AEP flood affected area for the period from 2025 to 2115 can be approximated by the current 0.1% AEP flood outline, i.e. Zone 2 Medium Probability. Within the Borough, this indicates a relatively small increase in the number of properties at risk of flooding. Therefore, in the absence of Flood Zone 3a climate change extents from the Environment Agency’s latest model, the climate change flood maps in this SFRA rely upon the Flood Zone 2 flood extent (refer Appendix D). When the Environment Agency’s Flood Zone 3a climate change flood extent becomes available the maps will be updated. 4.3.7 It should also be noted that those properties (and areas) that are currently at risk of flooding may be susceptible to more frequent, more severe flooding in future years. It is essential therefore that the development control process (influencing the design of future development within the Royal Borough) carefully mitigates against the potential impact that climate change may have upon the risk of flooding to the property.
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5 Assessing Flood Risk in the Royal Borough of Windsor and Maidenhead
5.1 Fluvial Flooding - Delineation of the NPPF Flood Risk Zones 5.1.1 It is emphasised that the risk of an event (in this instance a flood event) is a function of both the probability that the flood will occur, and the consequence to the community as a direct result of the flood. The NPPF requires the assessment of the likelihood (or probability) of flooding, categorising the Royal Borough into zones of low, medium and high probability. It then provides recommendations to assist the Council to manage the consequence of flooding in a sustainable manner. 5.1.2 To this end, a key outcome of the SFRA process is the establishment of flood maps that will inform the application of the Sequential Test in accordance with Paragraph 100 of the NPPF and Paragraph 6 of the Flood Risk and Coastal Change PPG. To inform the planning process, it is necessary to delineate the area into zones that depict the likelihood (or probability) that flooding will occur. 5.1.3 The Borough has been delineated into the flood zones summarised below:
Zone 3b Functional Floodplain: 5.1.4 Zone 3b Functional Floodplain is defined as those areas in which “water has to flow or be stored in times of flood”. 5.1.5 The definition of functional floodplain remains somewhat open to subjective interpretation. The NPPF13 states that “Local planning authorities should identify in their Strategic Flood Risk Assessments areas of functional floodplain and its boundaries accordingly, in agreement with the Environment Agency. The identification of functional floodplain should take account of local circumstances and not be defined solely on rigid probability parameters. But land which would flood with an annual probability 5% or greater in any year or is designed to flood in an extreme (0.1%) flood, should provide a starting point for consideration and discussions to identify the functional floodplain”.
For the purposes of the SFRA, Zone 3b Functional Floodplain has been defined in the following manner:
land subject to flooding in the 5% AEP flood event;
land which provides a function of flood conveyance (i.e. free flow) or flood storage, either through natural processes, or by design (e.g. washlands and flood storage areas);
land where the flow of flood water is not prevented by flood defences or by permanent buildings or other solid barriers during times of flood;
5.1.6 Detailed modelled flood extents for the 5% AEP design event were adopted for the River Thames, River Colne and River Loddon for the basis of Zone 3b Functional Floodplain delineation. Areas without detailed modelling have been identified on the SFRA Flood Zone maps. 5.1.7 In areas where the 5% AEP flood event has not been identified in the SFRA, a precautionary approach should be applied; assuming that all of the 1% AEP flood extent is the functional floodplain until the extent of the 5% AEP flood extent is adequately demonstrated by a Site specific flood risk assessment.
13 Table 1: Flood Zones NPPF PPG – Flood Risk and Coastal Change
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Zone 3b Developed 5.1.8 Discussions with the Environment Agency have confirmed that, due to the obstructions to overland flow paths posed by existing development within flood affected areas, existing buildings (that are considered impermeable to floodwater) should not be considered as falling within the functional floodplain. For this reason, these areas have been delineated as Zone 3b Developed for planning purposes, and a suite of recommended planning responses have been established accordingly. It is important to highlight that the land surrounding existing buildings form important flow paths and flood storage areas and these must be protected.
For the purposes of the SFRA, Zone 3b Developed has been defined as land subject to flooding in the 5% AEP flood event on which permanent buildings or other solid barriers are located.
5.1.9 It is also important to recognise that all areas within Zone 3b Developed are subject to relatively frequent flooding – on average, there is a 5% probability every year that these areas will flood. There are clear safety, sustainability and insurance implications associated with future development within these areas, and informed planning decisions must be taken with particular care.
Zone 3a High Probability
5.1.10 The modelling outputs developed by the Environment Agency, where available (i.e. River Thames, Henley to Hurley, River Colne, Lower Loddon, Chalvey Ditches and the Cut) have been adopted for the delineation of Zone 3a High Probability. The detailed flood outlines are also reflected in the current Environment Agency Flood Zone Maps. 5.1.11 The Environment Agency also produces mapping to show the areas within Flood Zone 3a and 3b that benefit from current defences. 5.1.12 In other areas where detailed modelling has not been completed, detailed topography has been used to carry out a ‘sensibility check’ of the flood zone maps. This check has sought to ensure that the predicted floodplain extents are sensible in light of surrounding ground levels. No alterations have been made to the maps in this instance.
Zone 2 Medium Probability
Zone 2 Medium Probability is defined as those areas of the Royal Borough that are situated between the 0.1% AEP and the 1% AEP flood extents.
5.1.13 In this instance, Zone 2 Medium Probability is defined by the Environment Agency Flood Zone Map. 5.1.14 It is noted that, given the relatively rapid rise in topography at the periphery of the floodplain, the increase in the predicted flood extents between Zone 3a High Probability and Zone 2 Medium Probability is marginal. Consequently, throughout much of the area, the difference in the Zone 3a and Zone 2 flood extents is limited.
Zone 1 Low Probability
Zone 1 Low Probability is defined as those areas of the Royal Borough that are situated outside of the 0.1% AEP flood extent. For SFRA purposes, this incorporates all land that is outside of the shaded Zone 2, Zone 3a and Zone 3b flood risk areas (as defined above).
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5.1.15 For SFRA purposes, this incorporates all land that is outside of the shaded Zone 2, Zone 3a and Zone 3b flood risk areas (as defined above). 5.1.16 It is highlighted that a small number of areas within the Royal Borough that fall into Zone 1 Low Probability are ‘dry islands’. These areas may be surrounded by flood water for a considerable period of time. Whilst there is no direct risk to life and / or property as a result of water ingress, residents are unlikely to have ready access to medicines, food, water and utilities (i.e. electricity, telephone and sewerage). It is essential that any future development within these areas considers carefully the emergency response in times of flood.
5.2 Assessment of Flood Hazard
Definition of Flood Hazard 5.2.1 The assessment of flooding has thus far considered the maximum extent to which flooding will occur during a particular flood event (i.e. the probability of an area experiencing flooding). Of equal importance is the speed and depth with which flooding occurs as water levels rise. The inundation of floodwaters into low lying areas can pose a considerable risk to life. 5.2.2 Substantial research has been carried out internationally into the risk posed to pedestrians during flash flooding. This research has concluded that the likelihood of a person being knocked over by floodwaters is related to the depth of flow, and the speed with which the water is flowing. 5.2.3 To ensure that the risk posed by floodwaters is assessed consistently, Defra (in collaboration with the Environment Agency) produced a Flood Hazard equation. Variations of this equation are within two of their guidance documents entitled FD2321 Flood Risks to People and FD2320 Flood Risk Assessment Guidance for New Development TR1 and TR2. These two guidance documents were supplemented by an additional Note14 which reconciles the information provided in FD2321 and FD2320. 5.2.4 The Flood Hazard equation provides criteria for determining the degree of danger that is posed to life, assessed as a product of flood depth and flow velocity with an additional ‘debris factor’ factored (i.e. depth x (velocity + 0.5) + debris factor). The guidance states that if this product is below 0.75, then caution should be exercised due to “shallow flowing water or deep standing water”. In contrast, if the product exceeds 2.0 then the hazard posed to life is extreme with “deep fast flowing water”, representing a danger to all.
If safe access and egress cannot be achieved in accordance with NPPF guidelines and a low hazard rating cannot be achieved, then new development should not be permitted unless a Flood Evacuation Plan is submitted to and approved by the Local Planning Authority that shows the danger to site users can be appropriately managed.
5.2.5 For further guidance on assessing safe access and egress for more vulnerable developments in Flood Zone 3a and highly vulnerable development in Flood Zone 2, refer to Section 6.
Flood Hazard due to River Thames Flooding 5.2.6 Modelling of the River Thames throughout a relatively large proportion of Windsor and Maidenhead (i.e. upstream of Windsor) was undertaken using 1-Dimensional hydraulic modelling, based on representative cross sections taken at defined locations across the river. This form of modelling provides a single ‘average’ flow velocity across the river and its floodplain, and therefore is unreliable for assessing flood hazard within overbank areas. Notwithstanding this, however, an assessment of the variation in depth of flooding within the Thames floodplain for the 1% AEP design event has been carried out. This is presented in
14 Environment Agency and HR Wallingford (2008) Supplementary Note on Flood Hazard Ratings and Thresholds for Development Planning and Control Purpose
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Figure F, indicating areas within which the risk to life may be greater than others (i.e. as a result of much deeper flood waters). 5.2.7 The Defra guidance FD2321 indicates that flood depths exceeding 1.25m may represent a danger to some. This assumes standing water, therefore this risk will increase where the water is flowing and where a debris factor is applied. FD2320 considers ‘people vulnerability’ and reclassified still water with depths between 0.3 – 0.5m as danger to some. Whilst from a NPPF perspective these areas are all delineated as Zone 3a High Probability, the Council should consider the likely depth of flooding as depicted in Figure F, steering development away from areas of deep flowing water. 5.2.8 Hazard mapping for this reach of the River Thames was withdrawn by the Environment Agency in 2015 due to errors in the model. At the time of issue of this SFRA, the revised model was not available to inform this report.
Despite DEFRA FD2320 guidance classifying areas with still water depths between 0.3-0.5m as dangerous, these areas are delineated as Zone 3a High Probability from an NPPF perspective. The Council should, however, consider the likely depth of flooding as depicted in Figure F, in order to steer development away from areas of deep flowing water.
5.3 Residual Risk of Flooding 5.3.1 Whilst it is necessary to minimise the risk of flooding over the lifetime of the development in all instances, it is also important to recognise that flood risk can never be fully mitigated, and there will always be a residual risk of flooding. 5.3.2 This residual risk is associated with a number of potential risk factors including (but not limited to):
a flooding event that exceeds that for which the flood risk management measures have been designed;
the structural deterioration of flood defence structures (including de facto structures acting as a flood defence) over time; and/or
general uncertainties inherent in the prediction of flooding. 5.3.3 A small number of raised defences have been identified within the Borough, indicated on the fluvial flood maps in Appendix B, providing localised protection against fluvial flooding. 5.3.4 There is always a residual risk that these defences may fail, as a result of either overtopping and/or breach failure. The latter could result in rapid inundation into areas behind the defence, posing a potential risk to residents, pedestrians and property that may be in the path of the floodwaters. 5.3.5 The raised defences highlighted in the flood maps typically all exceed 1m in height. Given a worst case scenario of a catastrophic structural failure of one of these raised defences occurring during high water levels within the river, then a wave of flood water could rapidly inundate the area immediately behind the location of the breach. This may pose a risk to life to those immediately behind the defence at the time of failure. 5.3.6 It is highlighted that a breach failure of the River Thames defences could, over a period of time, result in the inundation of a relatively large area. Following the initial ‘burst’ of water through the defences, however, the flood wave will be relatively shallow and unlikely to pose a risk to life. Only the zone of rapid, deep and fast flowing water immediately behind the breach is considered in the current context, potentially knocking pedestrians off their feet. 5.3.7 To provide an indication of areas that may be at risk as a result of catastrophic defence failure within the Royal Borough, a series of ‘danger envelopes’ have been established using the
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principles of FD2320 (i.e. adopting the Simple Approach15). These are presented in Figure G. The ‘simple approach’ provides a straightforward relationship between the height of the raised defence, and the distance that the flood wave might extend should the flood wall collapse suddenly. This methodology assumes a flat floodplain behind the defences, and is therefore a conservative assessment of the area that may be at risk. The adopted methodology is described in Appendix F. 5.3.8 The structural integrity of the existing flood defences is integral to the sustainability of both existing and future development in Cookham, Maidenhead, Windsor and Eton. Without the raised defences, the consequence and probability of flooding in these areas will increase. It is essential that the detailed site based Flood Risk Assessment for all potential future development situated within the highlighted risk envelopes (refer Figure G) considers both the probability and consequence of defence failure in the vicinity of the site. 5.3.9 The SFRA process has carried out a review of flood risk within the Royal Borough in accordance with the NPPF, identifying a number of areas that fall within Zone 3a High Probability. The modelling of flood flows and flood levels has limitations in the methodologies used for prediction, and the models developed are reliant upon observed flow data for calibration. Certain flow and level data sets have a relatively short lifespan relating to their collection, although other datasets along the Thames have been collected for hundreds of years with varying accuracy. For this reason, there are inherent uncertainties in the prediction of flood levels used in the assessment and management of flood risk. 5.3.10 The adopted flood zones underpinning the SFRA are based upon the flood mapping within the area adjoining the River Thames. Whilst these provide a reliable depiction of flood risk for specific modelled conditions, all detailed modelling requires the making of core assumptions and the use of empirical estimations relating to (for example) rainfall distribution and catchment response. 5.3.11 Taking a conservative approach for planning purposes, it is understood that the Environment Agency (Thames Region) generally adopt a +/-150mm allowance for uncertainty within areas that have been modelled in some detail. The degree of uncertainty in areas reliant upon the Environment Agency’s national generalised computer model will be somewhat higher (300mm). The implications for development control and developers are that development proposals, especially floor levels and safe access routes, should take account of these uncertainties. 5.3.12 It is worth noting that the likelihood of a rapid river level rise within the River Thames, resulting in quick inundation of urban areas within the Royal Borough of Windsor and Maidenhead is considered to be low. This is primarily due to the large River Thames system and its substantial upper contributing catchment area. The underlying chalk geology also plays a significant part in reducing this risk. It should therefore be possible to provide timely flood warnings to those who have signed up to receive them. The Environment Agency aims to provide a warning of at least two hours to those who are registered to their flood warning service.
15 FD2320 Flood Risk Assessment Guidance for New Development
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6 Flood Risk Policy in the Royal Borough of Windsor and Maidenhead 6.1.1 The Royal Borough of Windsor and Maidenhead is committed to ensuring that new development is not at risk of flooding and to minimising the existing risk of flooding in the Royal Borough. Flood risk and watercourses within the Royal Borough must therefore, be managed appropriately. 6.1.2 This Section details the flood risk and watercourse management policy requirements (boxed text) for the Royal Borough as detailed in Borough Local Plan Policy 45: Managing Flood Risk and Waterways16. Guidance on why there is a need for each of the policy requirements and how the requirements should be adhered to is set out beneath each of the policy extracts.
6.2 Managing Flood Risk for New Development and Applying the Sequential Approach
The Borough will guide development to areas of lowest flood risk from all sources of flooding by adapting to climate change through the careful management of flood risk and the application of a sequential test approach. The Borough will only support development proposals where an appropriate flood risk assessment has been carried out and it has been demonstrated that development is located and designed to ensure that flood risk from all sources of flooding is acceptable in planning terms. Development proposals must show how they have regard to the availability of suitable alternative sites in areas of lower flood risk (the Sequential Test).
6.2.1 All sources of flood risk associated with a proposed development must be thoroughly and accurately assessed in order to identify the risk to both the development itself and neighbouring areas. It is important that all possible sources of flood risk are assessed including surface water, groundwater, sewer and drainage infrastructure and man-made features, as well as the risk of fluvial flooding to ensure flood risk is not increased in the Royal Borough for any source. The impact of climate change must be considered as part of this assessment to ensure that flood risk is not increased in the Royal Borough in the future. 6.2.2 The potential impacts of climate change will affect not only the risk of flooding posed to property as a result of river flooding, but it will also potentially increase the frequency and intensity of localised storms over the Royal Borough, which may in turn exacerbate localised drainage problems. It is important therefore that any site specific Flood Risk Assessment takes due consideration of climate change in accordance with the Environment Agency Flood Risk Assessments: Climate Change Allowances guidance to ensure that the development does not increase, and remains protected against, flood risk, over its lifetime. 6.2.3 Applications for proposed developments that are not wholly located within Flood Zone 1 must demonstrate that the proposed development has been sequentially tested against other proposed development sites within the Royal Borough. The Sequential Test is designed to ensure that areas at little or no risk of flooding from any source are developed in preference to areas at higher risk. The aim is to keep development out of Flood Zone 2 and Flood Zone 3 and other areas affected by other sources of flooding where possible. Other forms of flooding should be treated consistently with fluvial flooding when mapping probability and assessing vulnerability for the sequential approach. Where it is not feasible to locate development within Flood Zone 1, the Flood Risk Assessment must demonstrate that the Proposed Development will not be subject to unacceptable flood risk and that users are able to safely use the development for its planned lifetime to ensure the level of development within the Royal Borough at risk of flooding is not increased as a result of new development.
16 Royal Borough of Windsor and Maidenhead (2016) Draft Borough Local Plan
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6.2.4 For certain types of development in Flood Zone 2 and Flood Zone 3, the Exception Test will need to be passed in order to demonstrate that flood risk to people and property within these higher risk areas will be managed satisfactorily and that again, to ensure the level of development within the Royal Borough at risk of flooding is not increased as a result of new development. 6.2.5 Further guidance on sequentially testing individual development sites is provided in Section 8. Additional information about the Sequential Test and Exception Test is provided in the NPPF Flood Risk and Coastal Change PPG17.
6.3 Assessing Vulnerability and Flood Risk
Development proposals must show how they have regard to: The vulnerability of the proposed use and the number of people, property or infrastructure at risk of flooding; The present and future flood risk; and The scale of potential consequences.
6.3.1 The vulnerability of a development is dictated by its use. The NPPF Flood Risk and Coastal Change PPG groups potential uses into five vulnerability categories: Essential Infrastructure; Highly Vulnerable; More Vulnerable; Less Vulnerable; and Water-Compatible Development. The vulnerability of a development dictates its suitability for use in each Flood Zone and determines whether the Exception Test must be passed. 6.3.2 All future development in the Royal Borough must demonstrate that the use is suitable for the Flood Zone in which it is located in accordance with Table 2 and 3 of the NPPF Flood Risk and Coastal Change PPG. Where suitable sites at lower risk of flooding are not available, evidence must be provided to demonstrate that flood risk to people and property will be managed to ensure it is safe for its lifetime, without increasing flood risk elsewhere and where possible reduce flood risk in the Royal Borough. 6.3.3 Development lifetime is dictated by the proposed use. Residential development, for example, should be considered with a minimum lifetime of 100 years, whereas non-residential development lifetime is dependent on the type and location of the development. Within the Royal Borough all non-residential development should be considered with a minimum life of 60 years unless robust evidence is available to demonstrate otherwise. 6.3.4 Proposed development must not cause new or exacerbate existing flooding problems, either on the proposal site or elsewhere. A key consideration of the NPPF is to ensure that new development does not increase existing flood risk or introduce risk. The existing flood risk for all sources must be clearly stated and evidenced in the FRA, along with a demonstration that the risk will not be increased or new flood risk introduced as a result of the proposed development. 6.3.5 Where proposed development will have an impact on flood risk, either for the development itself or wider area, the scale of the consequence must be detailed as part of the planning application. If the proposed development will impact on the flood extent within the vicinity of the site, the impact must be assessed using hydraulic modelling and the results shown on a plan included with the FRA. Where the potential consequences relate to the residual flood risk associated with the proposed development, the FRA must contain details of how this risk will be managed and communicated to site users, e.g. Flood Evacuation Plan. This information must be included with the FRA to enable the Royal Borough to assess whether the scale of the impact is acceptable and to determine whether the development has been adequately designed to mitigate for the potential consequences.
17 Department for Communities and Local Government (2015) Flood Risk and Coastal Change Planning Practice Guidance
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6.4 Site Evacuation Plan
For all sites at risk of flooding during a 1% AEP or greater event, development proposals must include a site evacuation plan demonstrating how the site will be evacuated in the event of potential flooding.
6.4.1 For all sites at risk of flooding, the means of evacuation must be detailed in a site evacuation plan in order to demonstrate that site users will not be at risk during a flood event. The plan will need to demonstrate how site users should be able to evacuate the site without emergency assistance during the 1% AEP plus climate change event and before a more extreme event by either foot or vehicle. The vehicular access route for the site should be suitable for use by the emergency services and be designed to be functional for the lifetime of the development. 6.4.2 The Borough will approve site evacuation plans as part of its Emergency Planning role. The plans must be in accordance with the Royal Borough’s Generic Emergency Plan18 and Flood Plan19, which detail the emergency planning systems and procedures in the Royal Borough. One of the key functions of the Royal Borough’s Flood Plan is to provide a framework for responding to flooding; site specific evacuation plans must be consistent with this framework to ensure a safe and consistent response to flooding across the Royal Borough.
6.5 Maintain Existing Overland Flowpaths
Development must not itself, or cumulatively with other development materially impede the flow of flood water.
6.5.1 The impediment of flood water has the potential to change the flood risk both to the proposed development site and the wider area by changing the area(s) where flood water flows and collects. . Therefore, wherever possible, development must not be located in existing overland flowpaths. 6.5.2 For constrained sites where development must be located in an existing overland flow path, the development must be designed so as to continue to allow water to flow, such as through the use of voids beneath the building or raising the building on stilts.
6.6 Floodplain Storage Capacity
Development must not itself, or cumulatively with other development materially reduce the capacity of the floodplain to store water.
6.6.1 The floodplain plays an intrinsic role in reducing flood risk to developed parts of the Royal Borough. As previously stated, a considerable number of the Royal Borough’s towns are located alongside the River Thames in areas that were historically part of the River’s floodplain. If the capacity of the floodplain to store water is further reduced, the water that can no longer be stored in the existing floodplain will flood areas that were not previously at risk of flooding. 6.6.2 All development proposals that involve altering the floodplain must demonstrate how the overall volume of the floodplain is not altered on a level-for-level basis. Alterations to the floodplain must be tested using hydraulic modelling undertaken by a suitably qualified engineer. For further information, refer to Section 7.
18 Royal Borough of Windsor and Maidenhead (2013) Generic Emergency Plan, available at: https://www3.rbwm.gov.uk/downloads/file/81/generic_emergency_plan 19 Royal Borough of Windsor and Maidenhead (2011) Flood Plan, available at: https://www3.rbwm.gov.uk/downloads/file/78/generic_flood_plan
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6.7 Ecological Habitat
Development must not itself, or cumulatively with other development materially reduce the waterway’s viability as an ecological network or habitat for notable species of flora or fauna.
6.7.1 Waterways are at the core of the natural ecosystem, supporting numerous flora and fauna species. These environments are fundamental to supporting life, both human and aquatic, generating and sustaining economic growth and regulating our climate. 6.7.2 New development has the potential to damage the water environment during both the construction and operation stage. During the construction stage damage to the water environment must be mitigated for through the use of a Construction Environmental Management Plan (CEMP), which outlines how the construction of the proposed development will avoid, minimise or mitigate effects on the environment and surrounding area. Potential harm to the water environment during the operational stage should be mitigated for through the use of water treatment SuDS features, which work to remove pollutants from surface water runoff before they can cause damage to the wider environment.
6.8 Development in Functional Floodplain
Only water compatible uses and essential infrastructure development will be supported by the Royal Borough in the area defined as functional floodplain.
6.8.1 Land designated as Functional Floodplain is defined in the NPPF Flood Risk and Coastal Change PPG as ‘land where water has to flow or be stored in times of flood’. Development in these areas should be kept to a minimum order to continue to allow water to flow and/or be stored. Development within this area must either consist of development that is compatible with the water environment or development that has to be located in this area and has passed the Exception Test. Such development must be designed and constructed to remain operational and safe for users in times of flood; result in no net loss of floodplain storage; not impede water flows; and not increase flood risk elsewhere.
6.9 Reduction in Onsite and Offsite Flood Risk
Development proposals are expected to reduce flood risk both within and beyond sites wherever practical.
6.9.1 The NPPF considers not only the risk of flooding posed to new development, but also seeks to positively reduce the risk of flooding posed to existing properties. Within the Royal Borough development proposals should deliver a positive reduction in flood risk, whether that be by reducing the frequency or severity of flooding (for example, through the introduction of SuDS), or by reducing the impact that flooding may have on the community (for example, through a reduction in the number of people within the site that may be at risk). 6.9.2 This should not be seen as an onerous requirement, and indeed if integrated into the design at the conceptual stage, should place no added demands upon the development and/or planning application process. 6.9.3 Possible risk reduction measures for consideration may include the following:
The integration of SuDS to reduce the runoff volume and rate from both the site and the wider catchment. These features can be designed to attenuate and manage a larger volume of water than just the surface water runoff generated from the development, reducing flood risk in the wider area;
A change in land use to reduce the vulnerability of the proposed development;
A reduction in the building footprint area and intensity of use. This is to prevent intensification through the addition of storeys (or other conversion) within the same footprint;
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Incorporating flood resilience / resistance into building design, for example, the raising of internal floor levels and flood proofing (within existing buildings) to reduce potential flood damage;
The rearrangement of buildings within the site to remove obstructions to overland flow paths, to ensure that water does not pond and cause localised flooding. This risk reduction method should only be considered where the rearrangement of buildings will not increase the risk of flooding elsewhere; and,
Apply the sequential approach at a site level to minimise risk by directing the most vulnerable development to areas of lowest flood risk, matching vulnerability of land use to flood risk. 6.9.4 A clear statement will be required within each detailed FRA that concisely summarises how a reduction in flood risk has been achieved within the proposed (re)development and where possible how benefits have been achieved in reducing flood risk overall. This may be specified as (for example) a reduction in flow from the site, a reduction in water levels within (or adjacent to) the site, or a reduction in the consequences of flooding.
6.10 Increased Floodplain Storage Capacity
Development proposals are expected to increase the storage capacity of the floodplain where possible.
6.10.1 One way of reducing flood risk at a borough-wide scale is to increase the capacity of the floodplain within the Royal Borough to enable a greater volume of water to be stored safely during a flood event. This in-turn will reduce the volume of flood water affecting developed areas of the Royal Borough. 6.10.2 The re-development of sites located within the floodplain should be designed so as to reduce the overall built development footprint on the site by a minimum of 5%, so as to increase the area of the site that could be allowed to flood (refer to Section 7 for further information). 6.10.3 New developments located within the floodplain must provide floodplain compensation. The area that will compensate for the lost floodplain must provide a minimum of 5% more storage than the area that will be lost as a result of the proposed development on a level-for-level basis.
6.11 Sustainable Drainage Systems (SuDS)
Development proposals are expected to incorporate Sustainable Drainage Systems in order to restrict or reduce surface water run-off.
6.11.1 Sustainable Drainage Systems (SuDS) manage the volume and rate of discharge of surface water from development sites in a way that mimics natural processes. Restricting the volume and rate of discharge of surface water from a development site to those associated with the greenfield conditions at the site is one of the most effective ways of reducing and managing flood risk. Wherever possible within brownfield20 areas, the developer should seek to reduce the volume and rate of runoff from the site to greenfield runoff conditions (i.e. the runoff generated from the site assuming an open grassed area). 6.11.2 In catchment terms, the cumulative effect of applying SuDS to a number of sites can have a significant affect in reducing the volume of water entering a watercourse. The integration of SuDS into a site design can also provide broader benefits, including an improvement in the water quality of runoff discharged from the site, the capture and re-use of site runoff for irrigation
20 Land which is or was occupied by a building (excluding those used for agriculture or 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.
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and/or non-potable uses, and the provision of green space areas offering recreation and/or aesthetic benefits and opportunities to enhance biodiversity. 6.11.3 Following the issue of House of Commons: Written Statement HCWS16121, major development (defined as developments of 10 dwellings or more; or equivalent non-residential or mixed development (as set out in Article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2010) must ensure that SuDS are put in place for the management of runoff, unless demonstrated to be inappropriate. 6.11.4 In order to guide the design of SuDS, Defra has produced the Non-statutory technical standards for sustainable drainage22, which sets out the requirements that surface water drainage strategies for proposed new and re-developments must meet. These Standards should be used in conjunction with the NPPF and Flood Risk and Coastal Change PPG and the CIRIA SuDS Manual (C753). 6.11.5 The appropriate application of a SuDS scheme to a specific development is heavily dependent upon the topography and geology of the site (and its surrounds). Careful consideration of the site characteristics must be given to ensure the future sustainability of the adopted drainage system.
6.12 Flood Resilience and Resistance Measures
Development proposals are expected to be constructed with adequate flood resilience and resistance measures suitable for the lifetime of the development.
6.12.1 Development proposals located in areas of flood risk must be designed so as to be resilient to flooding for the lifetime of the development to ensure site users are not at risk from flooding. Such measures may include raising internal floor levels above the design flood level, with an allowance for climate change, raising electrical sockets and appliances above the flood level or flood proofing of external building facades. For further information on flood resilient measures refer to Improving the flood performance of new buildings: flood resilient construction23 and the Flood Risk and Coastal Change PPG. 6.12.2 All floor levels, access routes, drainage systems and flood mitigation measures should be designed with an allowance for climate change. This provides a robust and sustainable approach to the potential impacts that climate change may have upon the Royal Borough over the next 100 years, ensuring that future development is considered in light of the possible increases in flood risk over time.
6.13 Strategic Flood Risk Alleviation Schemes
The Borough will support in principle, appropriate comprehensive flood risk management measures within or affecting the Royal Borough which are agreed by the Environment Agency and or the Royal Borough as Local Lead Flood Authority
The Borough will support and safeguard from further development land associated with strategic flood relief measures, including the proposed flood relief channel from Datchet to Wraysbury. The Borough will support development that facilitates the improvement and integration of waterways in Maidenhead, including the completion of the Maidenhead Waterway Project.
21 Department for Communities and Local Government Written Statement made by: The Secretary of State for Communities and Local Government (Mr Eric Pickles) on 18 Dec 2014. 22 Non-Statutory Technical Standards for Sustainable Drainage Systems (2015) DEFRA 23 Improving the flood performance of new buildings: flood resilient construction (2007) Department for Communities and Local Government
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6.13.1 Strategic flood risk alleviation schemes have a fundamental role to play in reducing existing flood risk in across the Royal Borough. A number of areas in the Royal Borough are at fluvial risk of flooding as a result of historic development in the floodplain. Flood risk on such a scale cannot typically be mitigated via property and/or site specific measures and instead requires strategic schemes that address the risk at a catchment scale. 6.13.2 Strategic flood risk alleviation schemes can also be necessary to reduce flood risk associated with localised surface water flooding, which has resulted from a reduction in the permeable area in developed parts of the Royal Borough. 6.13.3 The Borough recognises the importance of delivering such strategic schemes to provide benefit at a catchment level and will work to ensure that the land and support is available to bring such schemes forward in the future.
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7 Site Specific Flood Risk Assessment
7.1 Overview 7.1.1 A site-based Flood Risk Assessment (FRA) is an integral part of the planning application for new and redevelopments.
A FRA is required for all proposed developments located:
Partially or wholly within Flood Zone 2 or 3; Within Flood Zone 1 and covering an area greater than 1 hectare; and, Within Flood Zone 1 and at risk from other non-main river flooding.
7.1.2 The detail provided in the FRA should be commensurate with the risk of flooding to the proposed development. For example, where the risk of flooding to the site is negligible there is little benefit to be gained in assessing the potential risk to life and/or property as a result of flooding. Rather, emphasis should be placed on ensuring that runoff from the site is controlled safely on-site and does not exacerbate flooding lower in the catchment. 7.1.3 All proposed sites that are not allocated as part of the Royal Borough Local Plan will need to demonstrate that the Sequential Test has been applied (in accordance with the NPPF and the requirements detailed in Section 7 of this document).
7.2 Flood Risk Requirements 7.2.1 The requirements for proposed development in each flood zone in the Royal Borough are outlined below. All proposed developments must meet the requirements detailed in the first reference page – “All Proposed Development”. In addition to these requirements, proposed developments in Flood Zone 2 must also meet the requirements set out on the second reference page – “Proposed Development in Flood Zone 2”. Proposed development in in Flood Zone 3 must meet the requirements set out in “All Proposed Development” and those on the third reference page – “Proposed Development in Flood Zone 3”. 7.2.2 Failure to meet the relevant requirements will result in refusal of the planning application.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
1 - All Proposed Development All proposed development must ensure that: x The surface water drainage system is designed to achieve a positive reduction in flood risk within the Royal Borough. x Sustainable drainage techniques are employed to ensure surface water runoff from the proposed development does not exceed greenfield runoff rates and volumes. o For brownfield sites where it is not feasible to restrict runoff to the greenfield rate and volume, the proposed development must deliver an improvement on the existing conditions that is as close as feasibly possible to the rate and volume associated with the greenfield site. x All sources of flood risk to and from the proposed development are assessed and mitigated for. Development may contribute to an increase in flood risk elsewhere if not carefully mitigated. x Safe access and egress can be provided from the proposed development to an area outside of the 1% AEP plus climate change flood extent. The route should be on publicly accessible/permissible land to allow safe access and egress along the entirety of the route by all. x A minimum 8m buffer zone is provided to ‘top of bank’ within sites immediately adjoining a river corridor. This relates to both open waterways and culverted waterway corridors in accordance with the Environment Agency’s Living on the Edge guidance. x The proposed development does not result in an increase in maximum flood levels within adjoining properties. This may be achieved by ensuring that the existing building footprint is not increased, that overland flow routes are not truncated by buildings and/or infrastructure, or hydraulically linked compensatory storage is provided within the site (or upstream). x Where existing buildings are being retained, seek that their refurbishment increases their resilience / resistance to flooding. x Opportunities to recreate river corridors and wetland habitats in urban areas are explored, ensuring that space for water, habitat, wildlife and recreation is designed into the proposed development. Opportunities to work with partners to assess the viability of land swapping in those areas where there is a risk of flooding should be explored where possible. x Reinstate the natural open waterway within existing culverted reaches wherever possible.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
2 – Proposed Development in Flood Zone 2 Proposed development in Flood Zone 2 must also ensure that: x The Sequential Test has been applied for the proposed development (and where necessary the Exception Test). x Built development must be located within the site in accordance with the sequential approach; the most vulnerable uses should be located in the areas of lowest flood risk. x All building(s) are resilient to flooding in order to enable the safe use of all buildings for the lifetime of the development. A key element of this is raising the finished floor levels within the building(s) to a minimum of 300mm above the 1% AEP plus climate change (20%) level for the site. Where information on climate change is not available, finished floor levels must be set 600mm above the existing 1% AEP level. x It is designed to account for an acceptable level of residual risk of flooding (after existing and proposed flood management and mitigation measures are taken into account). Measures may include flood defences, flood resistant and resilient design, provision for escape/evacuation, effective flood warning and emergency planning. x The potential impacts of climate change on flood risk associated with the proposed development are taken into account in accordance with the Environment Agency Flood Risk Assessments: Climate Change Guidance1 to minimise the risk to future site users.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
3 – Proposed Development in Flood Zone 3 Proposed Development in Flood Zone 3 must also ensure: x The Sequential Test has been applied for the proposed development (and where necessary the Exception Test). x Built development must be located within the site in accordance with the sequential approach; the most vulnerable uses should be located in the areas of lowest flood risk. The surface water drainage strategy is designed so as to remain operational during all flood events up to and including the 1% AEP plus climate change event (20-40%). For developments partially within Flood Zone 3, SuDS must be used to manage surface water runoff on the site; however these features must also be located in areas outside of Flood Zone 3. Where this is not feasible, or where the site is wholly located in Flood Zone 3, the proposed development must utilise blue / green roofs and green walls to manage surface water runoff, where feasible. If additional storage is required, below ground storage must be used; this must be designed to remain operational during a flood event. x Existing overland flow paths are maintained through the site. For constrained sites where development is required in areas through which an existing overland flow path is indicated, the development must be designed so as to continue to allow water to flow, such as through the use of voids beneath the building or raising the building on stilts. x The potential impacts of climate change on flood risk associated with the proposed development are taken into account in accordance with the Environment Agency Flood Risk Assessments: Climate Change Guidance. Proposed developments in Flood Zone 3b must always use the Upper End scenario when designing for the impact of climate change to minimise the risk to future site users. x All building(s) are resilient to flooding in order to enable the safe use of all buildings for the lifetime of the development. A key element of this is raising the finished floor levels within the building(s) to a minimum of 300mm above the 1% AEP plus climate change level for the site. Where information on climate change is not available, finished floor levels must be set 600mm above the existing 1% AEP level. x It is designed to account for an acceptable level of residual risk of flooding (after existing and proposed flood management and mitigation measures are taken into account). Measures may include flood defences, flood resistant and resilient design, provision for escape/evacuation, effective flood warning and emergency planning. x Areas of floodplain compensation provide a minimum increase in storage volume of 5% of the existing volume that will be lost as a result of the proposed development on a level-for-level basis. Increasing the area available for storing flood water will help reduce flood risk to both the proposed development and neighbouring areas. x The building footprint of redevelopments is reduced by a minimum of 5% to increase floodplain capacity. This could be achieved by designing external areas of the site to flood, including car parks, public open space or roads that are not part of the safe access route for the site. This could also be achieved through a void design or through raising the development up on stilts to enable water to flow beneath it. x The presence of both formal and de-facto flood defences within the proximity of the site are considered. Any change in the flooding regime as a result of a flood defence must be thoroughly understood using hydraulic modelling undertaken by a suitably qualified engineer. The integrity of the defence must be determined and the potential impact of a defence failure must be considered as well as the residual risk of flooding that always remains with a flood defence. This residual risk may be due to an extreme event that overtops the design ‘height’ of the defence, changing climatic conditions which increase the frequency and severity of extreme flooding, a structural failure of the constructed flood defence system, or flooding behind the defences due to local runoff or groundwater. It is key that the level and integrity of defence provided can be assured for the lifetime of the development.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
7.3 Flood Risk Assessment Guidance 7.3.1 The following section provides guidance on the information that should be provided as part of a Flood Risk Assessment for sites in each of the Flood Zones to demonstrate that the above requirements have been met. 7.3.2 The level of information provided in a FRA should be commensurate with the risk of flooding to the proposed development and the following list of requirements is far from exhaustive. 7.3.3 The following requirements should be prepared as part of all FRAs:
An assessment of the risk to and from the proposed development from all sources of flooding and details of how the potential risk of flooding has been accounted for in the proposed development design in order to minimise the risk to development users;
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 a suitably qualified engineer. It is emphasised that the detailed assessment of potential impacts elsewhere should not be limited (in a geographical sense) to the Royal Borough. Future development within the Royal Borough may adversely affect sites within adjoining boroughs, and it is essential that this is mitigated;
Details of the SuDS features that will be applied at the site with appropriate justification for the selection;
A description of how water will be collected and dealt with across the site by the proposed SuDS techniques including what volumes are to be stored where;
Details of how the SuDS features to be used at the site will be designed to account for the climate change, topographical, groundwater and geological conditions at the site;
A description of the water quality treatment measures applied to surface water as it drains the site;
Details of the proposals for the adoption and maintenance of the entire drainage system(s). For major developments24 a written agreement in principal from the adopting/maintaining body must be provided;
Identification of the destination of discharge (in accordance with Building Regulation Part H hierarchy) with appropriate justification for the selection and identification of proposed points of connection(s);
Evidence of agreement in principal from key third parties whose systems the proposed drainage system will need to connect / impact (Environment Agency, Lead Local Flood Authority, Thames Water or adjacent landowner);
Where necessary a description of how the proposed system(s) will deal with surface water overland flow flooding originating from off-site areas;
24 Developments of 10 dwellings or more; or equivalent non-residential or mixed development (as set out in Article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2010)
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Where necessary, an explanation of why it is not reasonably practicable to achieve one or more of the non-statutory technical standards for sustainable drainage systems (March 2015);
Where proposing discharge to ground; evidence to support proposed infiltration rates used in initial calculations. This should include nearby borehole logs, geological mapping and if available any geotechnical investigations, infiltration tests and records of groundwater monitoring. Consideration shall also be given to the implications of any local Source Protection Zones;
Where necessary, identification of any mechanical or electrical features necessary for the effective operation of the drainage strategy;
Where the proposed development is identified as being in area where there is a possible risk of groundwater flooding, groundwater monitoring results must be submitted with the FRA. The FRA must also consider the impact that groundwater may have upon the effectiveness of any adopted SuDS system; and,
A clear and concise statement summarising how the proposed (re)development has contributed to a positive reduction in flood risk within the borough. 7.3.4 In addition to the above requirements, proposed development sites located in Flood Zone 2 should also meet the following points:
The assessment must detail the vulnerability of the development to flooding over its lifetime, including the potential impacts of climate change for all sources of flooding. The potential impacts of climate change on flood risk associated with the proposed development must be modelled in accordance with the appropriate allowances set out in the Environment Agency Flood Risk Assessments: Climate Change Guidance. The Environment Agency may have carried out detailed flood risk mapping (with respect to fluvial flooding) within localised areas that could be used to underpin this assessment. Where available, this will be provided at a cost to the developer. All modelling must be undertaken by suitably qualified engineers. The extent of modelling required will be proportionate to the nature and scale of the development.
Details of existing site levels, proposed site levels and proposed ground floor levels. All levels should be stated relevant to Ordnance Datum. 7.3.5 As well as the requirements detailed in Section 7.3.3 and 7.3.4, sites in Flood Zone 3 should meet the following requirements:
Details of floodplain compensation requirements for the proposed development, including calculations demonstrating that level-for-level floodplain compensation can be provided, plus a minimum of 5% additional volume to help mitigate flood risk offsite;
A plan indicating the location of the area in which compensation is to be located and a cross-section demonstrating that level-for-level compensation can be provided;
An assessment of the flooding regime accounting for the presence of any formal and de-facto flood defences. It is recommended this assessment is undertaken using hydraulic modelling undertaken by a suitably qualified engineer. Details of the integrity of any defences must be provided to demonstrate that the defence will be structurally sound throughout the lifetime of the proposed development. Where this is not shown to be the case, the proposed development must be designed to account for the flood risk to the site without defences. Details of a
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
potential defence failure must be provided, along with details of how users of the proposed development will remain safe.
7.4 Sources of Information to assist the production of Flood Risk Assessments Strategic Flood Risk Assessment 7.4.1 The SFRA provides some helpful tools to assist in the assessment of flood related issues, irrespective of the location and/or scale of the proposed development within the Royal Borough. The information outlined below should be used as a guide and does not negate the need for a detailed site-specific assessment.
The risk of flooding to the site from rivers is summarised in the SFRA flood maps in Appendix B and Appendix D.
The risk of flooding to the site from surface water flooding is summarised in the SFRA local sources flood maps in Appendix C.
The locality of formal flood defences, and the areas that benefit from the presence of these defences, are presented in Appendix B. Areas within which the risk of defence failure may pose a potential risk to life and/or property is presented in Figure G, triggering the need for a more detailed investigation as part of the site based FRA.
Figure C offers a broad indication of the topography of the Royal Borough. Developers should consider the route that overland flow can be expected to take during an intense storm event that exceeds the capacity of the existing drainage system (i.e. overland flow paths). The blockage of these routes by buildings may result in localised flooding and, consequently, this should be avoided wherever possible.
The SFRA local sources flood maps in Appendix C, and the discussions in Appendix D, provide a summary of locations that have been susceptible to localised flooding historically. This is not a comprehensive record of flooding, and relies upon community reports of flooding made to the Council. It is a good indication of areas that may be susceptible to localised flooding however, and reiterates the importance of considering flood risk related issues in areas that are outside of the designated NPPF flood zones.
Within all areas of the Royal Borough, groundwater levels and soil permeability should be assessed on site at an early stage, and this should be used to inform the design of buildings and sustainable drainage systems (SuDS). An overview of the geology of the Royal Borough is provided in Figure B. Environment Agency 7.4.2 To assist local planning authorities, the Environment Agency has produced standing advice to inform on their requirements regarding the consultation process for planning applications on flood risk matters. Full details of their Flood Risk Standing Advice can be found on the website www.gov.uk/government/organisations/environment-agency. 7.4.3 The Environment Agency is an excellent source of information to inform the development of the detailed FRA. The external relations team should be contacted as early as possible to source information relating to (for example) historical flooding, and hydraulic modelling. It is emphasised that the information provided within the SFRA is the best available at the time of writing. More up to date information may be available, and contact should always be made with the Environment Agency at an
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
early stage to ensure that the detailed site based FRA is using the most current datasets, avoiding unnecessary re-work. 7.4.4 It is strongly recommended that a draft of the detailed FRA is provided to the Environment Agency for review and comment before being submitted with the planning application, thereby reducing potentially costly delays to the planning process. 7.4.5 The Environment Agency introduced a cost recovery process for pre-application inquiries in January 2014. Some elements of pre-application advice such as a review of any FRA are likely to incur a charge. Please contact the Environment Agency directly for more information.
7.5 Flood Risk Mitigation Raised Floor Levels & Basements (Freeboard) 7.5.1 The raising of floor levels above the 1% AEP plus climate change fluvial flood level will ensure that the damage to property is minimised. In order to mitigate for the residual risks the floor level should be raised above the flood level. This additional height is referred to as the ‘freeboard’. 7.5.2 Given the anticipated increase in flood levels due to climate change, the finished floor level of proposed development must have a minimum freeboard of 300mm for the 1% AEP plus climate change flood level. Where climate change information is not available, finished floor levels must have a minimum freeboard of 600mm above the 0.1% AEP flood level. 7.5.3 The use of basements within flood affected areas should be discouraged. Where basement uses are permitted, it is necessary to ensure that the basement access points are situated 300mm above the 1% AEP plus climate change flood level, or 600mm above the 0.1% AEP flood level if no climate change data is available. The basement must be of a waterproof construction to avoid seepage during flooding conditions, without impacting on groundwater flows and quality. Habitable (sleeping) uses of basements within flood affected areas will not be permitted. Other Forms of Mitigation 7.5.4 Other forms of mitigation that could be considered to mitigate proposed or extant developments in flood risk areas include on or offsite floodplain compensation; flood voids; and floodable or floating buildings. Any proposed mitigation should be agreed with the Environment Agency and / or RBWM planners prior to its implementation. Sustainable Drainage Systems (SuDS) 7.5.5 SuDS are used to manage surface water drainage in a way that mimics the natural environment. The management of rainfall (surface water) is considered an essential element of reducing future flood risk to both the site and its surroundings. Indeed reducing the volume and rate of discharge from urban sites to greenfield conditions is one of the most effective ways of reducing and managing flood risk within the Royal Borough. The integration of SuDS into a site design can also provide broader benefits, including an improvement in the water quality of runoff discharged from the site, the capture and re-use of site runoff for irrigation and/or non-potable uses, and the provision of green space areas offering recreation and/or aesthetic benefits and opportunities to enhance biodiversity. 7.5.6 SuDS may improve the sustainable management of water for a site by25:
25 Interim Code of Practice for Sustainable Drainage Systems National SuDS Working Group, 2004
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
reducing peak flows to watercourses or sewers and potentially reducing the risk of flooding downstream;
reducing volumes and the frequency of water flowing directly to watercourses or sewers from developed sites;
improving water quality over conventional surface water sewers by removing pollutants from diffuse pollutant sources;
reducing potable water demand through rainwater harvesting;
improving amenity through the provision of public open space and wildlife habitat;
replicating natural drainage patterns, including the recharge of groundwater so that base flows are maintained;
contribute to the enhanced amenity and aesthetic value of developed area; and
provide habitats for wildlife in urban areas and opportunities for biodiversity enhancement. 7.5.7 In catchment terms, the cumulative effect of applying SuDS to a number of sites can have a significant affect in reducing the volume of water entering a watercourse. 7.5.8 The appropriate application of a SuDS scheme to a specific development is heavily dependent upon the geology of the site (and its surrounds) as well as the local groundwater regime. For example, infiltration techniques are generally most suitable in areas of permeable soils and geology. It is important to highlight that a shallow water table will compromise the operation of an infiltration system, and it is essential that groundwater levels (in addition to soil permeability) are assessed on site as an integral part of the design process. The geology of the Royal Borough is summarised in Figure B. 7.5.9 The topography of the site is also an essential consideration for the selection of an appropriate SuDS system. For example, areas of steeply sloping ground are generally unsuitable for techniques that rely on the storage and/or infiltration of runoff on the surface. An overview of the topography of the Royal Borough is included in Figure C. 7.5.10 Other constraints that may restrict or preclude the use of a particular SuDS technique (such as land contamination), or that may impose additional requirements on the performance of a particular system are identified within Chapter 8 of the CIRIA document C753 ‘The SuDS Manual’ (which should be the first point of reference for SuDS). 7.5.11 Where possible consideration should also be given to how SuDS can provide opportunities to enhance biodiversity at a site and facilitate movement of wildlife through a development through creation of green corridors. 7.5.12 For more guidance on SuDS, the following documents and websites are recommended as a starting point:
C753 The SuDS Manual (Woods Ballard B; Kellagher R et al, 2015 – available from the CIRIA bookshop www.ciria.org)
Non-Statutory Technical Standards for Sustainable Drainage (Defra, 2015 – Free download from: www.gov.uk)
Rainfall Runoff Management for Developments -Report – SC030219 (Kellagher R, 2013 – Free download from: www.gov.uk/government/organisations/environment-agency)
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Interim code of practice for sustainable drainage (National SuDS Working Group, 2004) - Free download from CIRIA: www.ciria.org, or the Environment Agency: www.gov.uk/government/organisations/environment-agency)
C644 Building greener. Guidance on the use of green roofs, green walls and complementary features on buildings (Early P; Gedge D; Newton J; Wilson S, 2007- available from the CIRIA bookshop: www.ciria.org)
C635 Designing for excedance in urban drainage – good practice (C Digman, D Balmforth, R Kellagher, D Butler, available from the CIRIA bookshop: www.ciria.org)
C625 Model agreements for sustainable drainage systems (Shaffer et al, 2004 - available from the CIRIA bookshop: www.ciria.org)
Report 156 Infiltration drainage – manual of good practice (Bettess R, 1996 - available from the CIRIA bookshop: www.ciria.org)
C582 Source control using constructed pervious surface: hydraulic, structural and water quality performance issues (Pratt et al, 2002 - available from the CIRIA bookshop: www.ciria.org)
W12 Sustainable water management in schools (Duggin & Reed, 2006 – Free download from: www.ciria.org)
Harvesting rainwater for domestic uses: an information guide (Environment Agency, 2003 - Free download from: www.gov.uk/government/organisations/environment-agency)
National Planning Policy Framework (NPPF) and PPG Flood Risk and Coastal Change (Department for Communities and Local Government, 2015 – Free download available from: http://planningguidance.planningportal.gov.uk)
The Building Regulations: Approved document H – Drainage and waste disposal (Office of the Deputy Prime Minister, 2010 - Free download from: www.communities.com)
www.ciria.org.uk/SuDS/
Design Guide 365 Soakaway design (BRE)
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
8 Assessment of Site Specific Allocations
8.1 Introduction 8.1.1 The Royal Borough of Windsor and Maidenhead has identified a series of potential development site allocations for which the flood risk needs to be assessed as part of the SFRA. The list of potential development sites has resulted from the 2016 Edge of Settlement Study, the Functional Economic Market Area (FEMA) Study (2016), the Strategic Housing Market Assessment (SHMA) (2016), the Strategic Housing Land Availability Assessment (SHLAA) (2014) and the Employment Land Review (ELR) (2009), as well as the 2015 and 2014 consultations on the draft Borough Local Plan. 8.1.2 The Level 2 SFRA provides an initial review of flood risk for the proposed development allocation sites, to enable the Sequential Test to be applied. Where necessary, the Exception Test is then applied to any of the sites where the vulnerability of the proposed development negates the need for further assessment (in accordance with Table 2 and 3 of the NPPF Flood Risk and Coastal Change PPG). 8.1.3 The following section provides further information on the Sequential Test and the Exception Test. The Sequential Test 8.1.4 The NPPF states that Local Planning Authorities should, when allocating land, apply a sequential approach, taking into consideration all sources of flooding. 8.1.5 The ideal solution to effective and sustainable flood risk management is a planning led one, i.e. steer development away from areas that are susceptible to flooding. The NPPF advocates a sequential approach that will guide the planning decision making process (i.e. the allocation of sites). In simple terms, this requires planners to allocate sites for future development within areas of lowest flood risk in the initial instance. Only if it can be demonstrated that there are no suitable sites within these areas should alternative sites (i.e. within areas that may potentially be at risk of flooding) be contemplated. This sequential approach is referred to as The Sequential Test, and is summarised in Paragraph 5 of the PPG Flood Risk and Coastal Change (April 2015).
It is absolutely imperative to highlight that the SFRA does not attempt, and indeed cannot, fully address the requirements of the NPPF Sequential Test. As highlighted in Paragraph 5 of the PPG Flood Risk and Coastal Change, it is necessary for the Council to demonstrate that sites for future development have been sought within the lowest flood risk zone (i.e. Zone 1 Low Probability). Only if it can be shown that suitable sites are not available within this zone can alternative sites be considered within the areas that are at greater risk of possible flooding (i.e. Zone 2, and finally Zone 3). The Level 2 SFRA outlines the Sequential Test methodology undertaken for the Royal Borough, as well as the results of the test for the potentially allocated sites.
8.1.6 It is important to remember that the NPPF stipulates permissible development types. This considers both the degree of flood risk posed to the site, and the likely vulnerability of the proposed development to damage (and indeed the risk to the lives of the site tenants) should a flood occur. Following the Sequential Test, the Council should restrict development to the permissible land uses summarised in the PPG – Flood Risk and Coastal Change (Table 2). This may involve seeking opportunities to ‘swap’ more vulnerable allocations at risk of flooding with areas of lesser vulnerability. 8.1.7 The principles of the sequential approach are applicable throughout the planning and development cycle, and refer equally to the forward planning process (delivered by the Council as part of the BLP) as they do to the assessment of windfall sites. A site
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
specific FRA will be required to demonstrate the careful and measured consideration of whether there is an alternative site available within an area of lesser flood risk, in accordance with the NPPF Sequential Test26. A sequential approach should also be applied within the site boundary locating the higher vulnerability uses in the parts of the site at the lowest probability of flooding. 8.1.8 The Royal Borough of Windsor and Maidenhead SFRA has been developed to inform the Sequential Test. The Borough has prepared a Sequential Testing report on the basis of the information contained in the updated 2016 Level 1 SFRA, allocating potential sites for future development accordingly. Developers proposing sites in Zone 3b, Zone 3a or Zone 2 will be required to demonstrate within a detailed Flood Risk Assessment that the Sequential Test has been applied, and (where appropriate) that the risk of flooding has been adequately addressed in accordance with the NPPF. Developers should also show that for a particular site land uses have been located in the areas of least flood risk according to their vulnerability and Flood Zone ‘compatibility’.
The Council’s Planning Policy Unit is able to provide further advice in relation to the expected scope of sequential tests for individual planning proposals. Please refer to the planning pages of the Council’s website for contact details.
The Exception Test 8.1.9 A relatively large proportion of the Royal Borough lies within Flood Zone 3, including the major urban and retail centres of Maidenhead and Windsor. Prohibiting future development within these areas may have a detrimental impact upon the economic and social welfare of the existing community. It is essential that a sequential approach is taken to underpin all planning decisions as stipulated above. However, pressing planning arguments (that outweigh flood risk) may remain, putting into place a requirement to investigate further the possibility of regeneration and/or future development within areas at risk of flooding. 8.1.10 Should this be the case, the Council and potential future developers are required to work through the Exception Test (NPPF Paragraph 102) where applicable. It is important to remember that the Sequential Test should always be carried out prior to the Exception Test. For the Exception Test to be passed:
“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; and
a site-specific flood risk assessment must demonstrate that the development will be safe for its lifetime taking account the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall” 8.1.11 The first point set out in the Exception Test is a planning consideration. A planning solution to acceptably reducing flood risk must be sought at each specific location, seeking to relocate the proposed allocation to an area of lower flood risk (i.e. Zone 1 Low Probability or Zone 2 Medium Probability) wherever feasible. 8.1.12 The management of flood risk throughout the Royal Borough must be assured should development be permitted to proceed, addressing the second critical element of the Exception Test. The SFRA has provided specific recommendations that ultimately should be adopted as design features, with evidence provided of how they will be fulfilled prior to permission being granted for all future development. It is the
26 It is strongly recommended that developers agree the application of the Sequential Test with the Council before embarking upon a detailed site based FRA, thereby ensuring that the site can be taken forward on planning grounds prior to considering potential design solutions.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
responsibility of the prospective developer to build upon these recommendations as part of a detailed Flood Risk Assessment to ensure that the specific requirements of the NPPF can be met. 8.1.13 An overview of flood risk throughout the council has been provided in Appendix B, Appendix C and Appendix D.
Future planning decisions should consider the spatial variation in flood risk across the council, as defined by the delineated flood zone that applies at the specified site location, and apply the recommendations provided below accordingly.
It is reiterated that the NPPF applies to allocated sites identified within the emerging BLP and to future windfall sites. Developers undertaking site based detailed Flood Risk Assessment should take due consideration of climate change (refer to Section 4 and Appendix D) to ensure that the development does not increase, and remains protected against, flood risk, over its lifetime.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
9 Conclusion 9.1.1 This Level 1 SFRA provides an overview of the flood risk to the Royal Borough of Windsor and Maidenhead and sets out the policy ( and basis) for ensuring this risk is accounted for in all new and re-development. The report provides guidance for developers and Borough officers on how the Royal Borough’s policy should be applied, including specific requirements that must be adhered to for site-specific development proposals. 9.1.2 The consistent application of these requirements across the Royal Borough at the planning application stage is essential to achieving future sustainability within the Royal Borough with respect to flood risk management. 9.1.3 A planning solution to flood risk management should be sought wherever possible, steering vulnerable development away from areas affected by flooding in accordance with the NPPF Sequential Test. Specific planning recommendations have been provided for new and re-developments within different areas of flood risk within the Royal Borough. 9.1.4 The SFRA has been developed building heavily upon existing knowledge with respect to flood risk within the Royal Borough. The Environment Agency review their flood maps on a quarterly basis to ensure they provide the most accurate, up-to-date, representation of flood risk, taking account of revised flood modelling and any incidences of flooding that may occur. The Royal Borough of Windsor and Maidenhead SFRA will be periodically reviewed to account for changes in this information. 9.1.5 The SFRA has been developed based upon the best available information at the time of publication (September 2016). It is essential to recognise that all recommendations and assumptions have been made on the basis of this data, and as improved information comes to light, these may be subject to change. 9.1.6 The information provided in the adjoining SFRA maps is intended purely to inform strategic planning decisions. It will always be necessary to rigorously review this information on a local scale as part of a detailed Flood Risk Assessment at the planning application stage.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Appendix A National and Regional Policy Context
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
National and Regional Policy Context
National Policy Context National planning policy is set out through the National Planning Policy Framework (NPPF) and the Planning Practice Guidance (PPG) - Flood Risk and Coastal Change. The NPPF covers a full range of planning issues drawing on the central issue of sustainable development. Central themes include the re-use of previously developed land and the wish to steer inappropriate (or vulnerable) development away from areas at risk of flooding. It is a requirement that the BLP is consistent with Government planning policy. EU Flood Directive & Flood Risk Regulations (2009) These regulations require the preparation of Preliminary Flood Risk Assessments, flood risk maps, hazard maps and flood risk management plans for flood risk from all sources of flooding. Areas of nationally significant risk must also be identified, and flood risk maps, hazard maps and management plans must be produced for those areas. Flood and Water Management Act 2010 The Flood and Water Management Act 2010 provides duties on the Environment Agency, Local Authorities, Developers and other bodies to manage flood risks. The Act establishes a SuDS Approving Body (SAB) at county or unitary local authority level. The Act sets out the requirements for local flood risk for England and defines the Lead Local Flood Authority for an area as the Unitary Council or County Council. "A lead local flood authority for an area in England must develop, maintain, apply and monitor a strategy for local flood risk management in its area (a local flood risk management strategy)." The Lead Local Flood Authority will be responsible for ensuring the strategy is put in place however local partners can agree how to develop it. Local flood risk, as quoted above, takes into consideration all forms of flooding including surface runoff, groundwater and ordinary watercourses (including lakes and ponds). The Act received Royal Assent on the 8th April 2010. The Act was put through commencement in parts and not as a whole document and therefore some parts of the Act are still to be implemented or will now not be implemented. National Planning Policy Framework (NPPF) The National Planning Policy Framework (NPPF) was released in March 2012 and superseded Planning Policy Statement 25 (PPS25). It underpins the process with which local planning authorities are to account for flood risk as an integral part of the planning process. The NPPF aims to rationalise previous planning policy and guidance. The over- arching principles set out by the NPPF for the management of flood risk at local planning authority level are broadly encapsulated in Section 10 paragraph 100 of the document: “Inappropriate development in areas at risk of flooding should be avoided by directing development away from areas at highest risk, but where development is necessary, making it safe without increasing flood risk elsewhere. Local Plans should be supported by Strategic Flood Risk Assessment and develop policies to manage flood risk from all sources, taking account of advice from the Environment Agency and other relevant flood risk management bodies, such as lead local flood authorities and internal drainage boards. Local Plans should apply a sequential, risk-based approach to the location of development to avoid where possible flood risk to people and property and manage any residual risk, taking account of the impacts of climate change, by: – applying the Sequential Test;
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
– if necessary, applying the Exception Test; – safeguarding land from development that is required for current and future flood management; – using opportunities offered by new development to reduce the causes and impacts of flooding; and – where climate change is expected to increase flood risk so that some existing development may not be sustainable in the long-term, seeking opportunities to facilitate the relocation of development, including housing, to more sustainable locations.” These broad planning objectives effectively set the scope for the specific outcomes of the SFRA process. The SFRA in turn then informs planning and development control decisions to ensure that the objectives set out above can be achieved. The guidance in the NPPF also indicates that sustainability appraisals should be informed by the SFRA for their area. Under the Town and Country Planning (Local Planning) (England) Regulations 2012, a Sustainability Appraisal (SA) is required for a local plan. The purpose of SA is to promote sustainable development through better integration of sustainability considerations in the preparation and adoption of plans. It is important to stress that flood risk guidance is not applied in isolation as part of the planning process. The formulation of Council policy and the allocation of land for future development must also meet the requirements of other planning policy advice set out in the NPPF. As such, a careful balance must be sought, 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. Planning Practice Guidance (PPG) to the National Planning Policy Framework In March 2014 the PPG - Flood Risk and Coastal Change was published to replace the NPPF Technical Guidance and support the NPPF. This document provides and supports the requirements of the planning policy. This provides further guidance on the implementation of the NPPF specifically for flood risk providing guidance on vulnerability classifications and climate change. The guide provides an indication of the ways in which the principles of the NPPF might be applied in practice. The Flood Risk and Coastal Change PPG was updated in April 2015.
Regional Planning Guidance South East Plan The Regional Spatial Strategy for the South East, known as the South East Plan, was revoked on 25 March 2013. Policy NRM6: Thames Basin Heaths Special Protection Area (SPA) was not revoked as part of this process, and continues to form part of the statutory development plan for the Royal Borough.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
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Appendix B SFRA Fluvial Flood Maps (September 2016)
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
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Appendix C SFRA Local Sources Flood Map (September 2016)
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Appendix D SFRA Climate Change Flood Maps (September 2016)
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Appendix E Overview of Flood Risk Key Locations
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Overview of Flood Risk Key Locations
Hurley
Assessment of Flood Risk Refer Character Area WM1 (Figure D)
Parts of Character Area WM1 (i.e. those areas adjacent to the river) are within ZONE 3B and are affected by flooding from the River Thames. A number of localised ‘high points’ within the floodplain are delineated as ZONE 2 MEDIUM PROBABILITY.
The greater part of Hurley village, a Conservation Area, falls within ZONE 3B the only exception being the historical priory that is elevated above the 1% AEP peak design flood level. The village is not only directly affected by flooding, but is also entirely surrounded by low lying ground, isolating the area during flooding conditions. This represents a major concern with respect to public safety.
The River Thames drains a considerable catchment area and flooding is typically a result of long duration, regional rainfall events. The Environment Agency issue warnings of anticipated river flooding within this reach, and due to the relatively long catchment response times, timely forewarning of a River Thames flood should be possible. The Environment Agency endeavours to provide a two hour warning prior to a flood event for main rivers. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
The remainder of the Character Area to the south of Hurley is predominantly ZONE 1 LOW PROBABILITY.
Bisham and Cookham Dean
Assessment of Flood Risk Refer Character Area WM2 (Figure D)
The open space areas of Character Area WM2 immediately adjoining the River Thames are largely contained within ZONE 3B, subject to relatively frequent fast flowing and/or deep flooding. In contrast, the relatively elevated developed areas of WM2, including the villages of Bisham and Cookham Dean, are situated in ZONE 1 LOW PROBABILITY.
Although part of Bisham is located in Flood Zone 1 it is important to highlight that the village is surrounded by low lying ground, isolating the area during flooding conditions. The Chiltern Way hills also rise to the south east which constricts fluvial flows to the Quarry Wood Road area north east of Bisham. Surface water runoff from the hills can exacerbate the flooding in this area. This represents a major concern with respect to public safety. Notwithstanding this, the River Thames drains a considerable catchment area and flooding is typically a result of long duration, regional rainfall events.
The Environment Agency issues warnings of anticipated river flooding within this reach, and due to the relatively long catchment response times, timely forewarning of a River Thames flood should be possible. The Environment Agency endeavours to provide a two hour warning prior to a flood event for main rivers. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
Cookham
Assessment of Flood Risk Refer Character Area WM3 (Figure D)
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
The part of Character Area WM3 situated between the River Thames (to the east) and Strand Water (to the west) is affected by fluvial (river) flooding. A large proportion of this area is open space and subject to relatively frequent (fast flowing and/or deep) flooding, falling within ZONE 3B.
The village of Cookham is on slightly higher ground, situated within ZONE 2 MEDIUM PROBABILITY. Whilst not affected by the 1% AEP flood extent at the current time, it is noted that Cookham is expected to be at risk in future years as a result of increased flows associated with climate change.
Like Bisham, although Cookham is located partly within Flood Zone 2, it is important to highlight that the village is entirely surrounded by low lying ground, isolating the area during flooding conditions. This represents a major concern with respect to public safety. The Environment Agency issue warnings of anticipated River Thames flooding at this location, and due to the relatively long catchment response times, timely forewarning of a River Thames flood should be possible. The Environment Agency endeavours to provide a two hour warning prior to a flood event for main rivers. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
Cookham itself does benefit from the presence of localised flood defences, constructed in 2002 as part of the Maidenhead, Windsor and Eton Flood Alleviation Scheme. Further south a number of properties in the Strande Lane area were affected by flooding in the 2003 flood event however since then the Cookham Flood Alleviation Scheme has been carried out by the Environment Agency which includes a raised bund across Cookham Marsh and the lowering of the sill levels at Cookham (Odney) weir. This scheme was constructed to alleviate flooding in the Strands Lane area of Cookham. .
The remainder of Character Area WM3, including Cookham Rise, is situated within ZONE 1 LOW PROBABILITY.
Maidenhead (North)
Assessment of Flood Risk Refer Character Area WM4 (Figure D)
The part of Character Area WM4 situated between the River Thames (to the east) and Maidenhead Ditch (to the west) is affected by fluvial (river) flooding.
Maidenhead is defended against flooding from the River Thames by the North Maidenhead Bund constructed in 2002 as part of the Windsor and Eton Flood Alleviation Scheme. High flows from the River Thames are also diverted into the Jubilee River during flood conditions, reducing flood levels through Maidenhead town centre.
The detailed flood modelling of this area indicates a proportion of WM4 is subject to relatively frequent flooding. In physical terms, the flood defence system does provide considerable protection against flooding. Experience has shown however that a relatively high susceptibility to flooding remains in these areas. This is due to elevated groundwater levels associated with flooding within the River Thames, and localised runoff resulting in ponding in low lying areas. For this reason, it is considered prudent to highlight these areas as ZONE 3B.
A large proportion of Character Area WM4 is situated within ZONE 3A HIGH PROBABILITY. The current flood risk mapping is currently under review, but the number of properties affected by flooding in the 1% AEP River Thames event is not expected to change substantially. This is discussed further in Section 4.4.
It is noted that the Environment Agency issues warnings of anticipated River Thames flooding within Maidenhead, and due to the relatively long catchment response times, timely forewarning of a River Thames flood should be possible. The Environment Agency endeavours to provide a two hour warning prior to a flood event for main rivers. This enables
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
The remainder of the character area is situated within ZONE 2 MEDIUM PROBABILITY and ZONE 1 LOW PROBABILITY.
Maidenhead (South) and Bray
Assessment of Flood Risk Refer Character Area WM5 (Figure D)
Character Area WM5 is situated adjacent to the River Thames, encompassing the southern areas of Maidenhead town centre, and the village of Bray.
A large proportion of Character Area WM5 is situated within ZONE 3A HIGH PROBABILITY, at risk of flooding in the 1% AEP River Thames event. The current flood risk mapping assumes operation of the Jubilee River however the number of properties affected by flooding in the 1% AEP River Thames design event is not expected to reduce substantially.
Once again it is noted that the Environment Agency issues warnings of anticipated River Thames flooding within this region, and due to the relatively long catchment response times, timely forewarning of a River Thames flood should be possible. The Environment Agency endeavours to provide a two hour warning prior to a flood event for main rivers. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
The remainder of the character area is situated within ZONE 1 LOW PROBABILITY.
Holyport
Assessment of Flood Risk Refer Character Area WM6 (Figure D)
Character Area WM6 is affected by flooding both from the River Thames, and The Cut.
The detailed flood modelling of the River Thames in this area indicates a proportion of WM5 is subject to relatively frequent flooding. Experience has also shown a relatively high susceptibility to flooding due to elevated groundwater levels. This is associated with flooding within the River Thames, and localised runoff resulting in ponding in low lying areas. For this reason, it is considered prudent to highlight these areas as ZONE 3B FUNCTIONAL FLOODPLAIN.
A relatively limited number of properties within Holyport (and surrounding areas) are situated within ZONE 3A HIGH PROBABILITY. Detailed modelling of The Cut highlights a very small number of properties within Stud Green that are within ZONE 3B FUNCTIONAL FLOODPLAIN. There are also several properties in Holyport and Coz Green within ZONE 2 MEDIUM PROBABILITY.
The remainder of the character area is situated within ZONE 1 LOW PROBABILITY. Discussions with the Council however have identified historical localised flooding problems that have occurred in both Stud Green and Fifield. This reiterates the importance of a Drainage Impact Assessment (refer below) as development proceeds within this apparent low flood risk areas.
Windsor (West) and Eton Wick
Assessment of Flood Risk Refer Character Area WM7 (Figure D)
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Character Area WM7 spans the River Thames, encompassing the low lying areas of Windsor adjoining the river corridor, and the village of Eton Wick.
The detailed flood modelling of this area indicates a relatively large proportion of WM7 immediately adjoining the River Thames is subject to relatively frequent flooding. It is considered prudent therefore to retain these areas as ZONE 3B FUNCTIONAL FLOODPLAIN. The slightly higher (developed) areas immediately to the south of the Royal Windsor Race Course (south of Maidenhead Road) have been designated ZONE 3A HIGH PROBABILITY for planning purposes.
The remaining areas of Character Area WM7, including a proportion of the village of Eton Wick, are generally within ZONE 3A HIGH PROBABILITY. It is noted that, although much of Eton Wick does appear to be situated above the 1% AEP flood extent, the surrounding area is affected by flooding from the River Thames. It should also be noted that Eton Wick is expected to be at further risk in future years as a result of increased flows associated with climate change. This poses a potential threat to public safety and should be carefully considered in planning terms. To take due consideration of this fact, it is recommended that the village is considered as situated wholly within the high risk zone for planning purposes.
Once again, it is noted that the Environment Agency issues warnings of anticipated River Thames flooding, and due to the relatively long catchment response times, timely forewarning of a River Thames flood should be possible. The Environment Agency endeavours to provide a two hour warning prior to a flood event for main rivers. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
Windsor (East) and Eton
Assessment of Flood Risk Refer Character Area WM8 (Figure D)
Character Area WM8 spans the River Thames, encompassing both Windsor and Eton.
The detailed flood modelling of this area indicates a relatively large proportion of WM8 immediately adjoining the River Thames is subject to relatively frequent flooding. It is considered prudent therefore to retain these areas as ZONE 3B under the definition provided by NPPF.
The remaining areas of Character Area WM8, including most of Eton, are generally within ZONE 3A HIGH PROBABILITY. It is noted that the Environment Agency issues warnings of anticipated River Thames flooding, and due to the relatively long catchment response times, timely forewarning of a River Thames flood should be possible. The Environment Agency endeavours to provide a two hour warning prior to a flood event for main rivers. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
Datchet
Assessment of Flood Risk Refer Character Area WM9 (Figure D)
A large proportion of Character Area WM9 adjoining the River Thames is contained within ZONE 3B, affected by flooding from the River Thames. Much of this area is low lying open space that should be protected against future development.
The remaining areas of Character Area WM9, including the town of Datchet, are generally within ZONE 3A HIGH PROBABILITY. It is noted that the Environment Agency issues warnings of anticipated River Thames flooding, and due to the relatively long catchment response times, timely forewarning of a River Thames flood should be possible. The Environment Agency endeavours to provide a two hour warning prior to a flood event for main
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
rivers. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
Old Windsor
Assessment of Flood Risk Refer Character Area WM10 (Figure D)
A large proportion of Character Area WM10 adjoining the River Thames is contained within ZONE 3B, affected by flooding from the Thames, including a number of properties on Ham Island. These homes are subject to relatively frequent flooding at depth.
The remaining areas Old Windsor are generally within ZONE 3A HIGH PROBABILITY.
It is noted that the Environment Agency issues warnings of anticipated River Thames flooding, and due to the relatively long catchment response times, timely forewarning of a River Thames flood should be possible. The Environment Agency endeavours to provide a two hour warning prior to a flood event for main rivers. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
Wraysbury and Horton
Assessment of Flood Risk Refer Character Area WM11 (Figure D)
A large proportion of Character Area WM11 is contained within ZONE 3B, affected by flooding from the River Thames and the River Colne including parts of the village of Wraysbury.
The majority of the village of Horton is situated within ZONE 2 MEDIUM PROBABILITY but parts are located within ZONE 3A HIGH PROBABILITY.
The Environment Agency issues warnings of anticipated flooding from both the River Thames and the River Colne, and due to the relatively long catchment response times, substantial forewarning of pending flood events can generally be provided. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.
Waltham St Lawrence
Assessment of Flood Risk Refer Character Area WM12 (Figure D)
Character Area WM12 is affected by flooding from the upper reaches of the River Loddon system.
A relatively limited number of properties within Waltham St Lawrence and Paley Street are situated within ZONE 3A HIGH PROBABILITY.
The remainder of the character area is situated within ZONE 1 LOW PROBABILITY. Discussions with the Council however have identified historical localised flooding problems that have occurred in White Waltham. This reiterates the importance of a Drainage Impact Assessment (refer below) as development proceeds within this apparent low flood risk areas.
Ascot and Sunninghill
Assessment of Flood Risk Refer Character Area WM13 (Figure D)
Character Area WM13 is affected by flooding from the uppermost reaches of The Bourne.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
A relatively limited number of properties within Ascot are situated within ZONE 3A HIGH PROBABILITY. There has not been any detailed flood modelling of the uppermost reaches of The Bourne, and it is expected therefore that there is some uncertainty surrounding the depiction of the high probability flood zone within this area (currently based on the Environment Agency Flood Map). A detailed Flood Risk Assessment at the development application stage is therefore imperative to ensure that the ‘real’ risk of flooding to the proposed development is understood.
The remainder of the character area is situated within ZONE 1 LOW PROBABILITY. Discussions with the Council however have identified a number of localised flooding issues within Ascot, and the area is considered particularly susceptible to flooding as a result of local surface water runoff. It is essential therefore that a Drainage Impact Assessment (refer below) is completed, demonstrating that the proposed development will not exacerbate existing localised flooding problems.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Appendix F Flood Defence Failure The Simple Method, FD2320
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Simple Approach
The simplest assessment of the risk behind flood defences uses information on the danger to people from flooding in defended areas. This is illustrated in Figures 1 and 2, which show how flood depths for a particular breach scenario can be interpreted as danger to people.
Max Flood Depth (m)
Reduces
Breach Breach Flood defence
Flood Hazard
Danger for all x km Danger for most y km z km Danger for some
Figure 1 - Plan view of danger to people for a breach scenario
Flood defence
Max Flood Depth (m)
Flood Hazard
Danger for all Danger Danger Reduces for most for some
x km y km
Figure 2 - Cross-section view of danger to people for a breach scenario
The results of this type of modelling have been used to create generic lookup tables relating the level of danger to people to the distance from the defence following a breach scenario, where the hazard is related to the water level above the floodplain.
It is advisable not to construct new buildings near to defences because of the risk to people and potential for damage to buildings, particularly on the coast. The distance depends on the likely head above crest level or breach level.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Table 1 is a simple lookup table that can be used as a guide to the danger to people27 at various distances behind flood defences for breaching (assuming that this will occur during the lifetime of the development).
Table 1 - Danger to people from breaching relative to distance from defence
Distance from Head above floodplain (m) Key: breach (m) 0.5 1 2 3 4 5 6 Danger for some 100 1.26 2.34 4.62 7.13 10.11 13.47 18.07 Danger for most 250 0.89 1.49 2.73 4.00 5.37 6.86 8.94 Danger for all 500 0.62 1.10 1.97 2.81 3.65 4.50 5.68 1000 0.36 0.76 1.42 2.02 2.61 3.18 3.97 1500 0.00 0.46 1.09 1.61 2.10 2.58 3.22 2000 0.00 0.00 0.75 1.27 1.72 2.15 2.72 2500 0.00 0.00 0.09 0.87 1.35 1.77 2.31 3000 0.00 0.00 0.00 0.09 0.88 1.35 1.90 3500 0.00 0.00 0.00 0.00 0.02 0.73 1.43 4000 0.00 0.00 0.00 0.00 0.00 0.00 0.63 4500 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5000 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Implications of Using the Simple Approach
General
In this simple approach, the danger to people decreases as the distance from the defence increases. A more detailed analysis would identify ‘pinch points’ on floodplains where flow velocity and, therefore, hazard can be high, for example at openings beneath embankments. A more detailed analysis would also identify areas where the hazard would be lower, for example due to localised high ground.
Table 1
This table has been generated for a breach of 100 metres wide, breaching onto a flat floodplain. There may be greater spatial variation in the hazard on complex floodplains and for different sized breaches. This uncertainty is expected to be relatively large.
Hazard to people increases as the head of water against the defence increases.
For small defences (say 2m high or less) the zone of high hazard only extends for the first few hundred metres if the defence is breached.
For large defences (say 5m high or more) the zone of high hazard can extend for 2km behind the defence, if the defence is breached.
In general, this suggests that development should be avoided within the first few hundred metres of the defence because there is a risk to all people exposed to floodwater. The distance depends on the head of water above the floodplain. In addition, the velocities in this zone will be relatively high and therefore there is a clear risk of damage to property.
27 For details regarding the danger classifications of ‘danger to all’, ‘danger to most’ and ‘danger to some’ reference should be made to HR Wallingford (2003) Flood Risks to People Phase 1, Environment Agency\Defra R&D Technical Report FD2317/TR, July 2003.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Behind large defences it would be advisable not to build within the first 500m to 1km due to the potential hazard of breaches with large heads of water. However, it is important to consider the probability of such a breach occurring.
Note
An additional source of information relating to flood risk and flood defences is available from the National Flood Risk Assessment (NaFRA) mapping published by the Environment Agency in 2013. NaFRA mapping can only be used to 1 in 10,000 scale but shows flood risk taking into account Standard of Protection, type and condition of defences at the time of modelling. The method used to assess the breach scenario has been extracted from NPPF guidance (FD2320), however the NaFRA shows the chance of flooding from rivers and the sea, taking into account flood defences and the condition they are in, presented in flood risk likelihood categories. It is updated using the Modelling and Decision Support Framework (MDSF2) software. NaFRA mapping is mainly intended for the insurance industry. The Environment Agency’s Flood Map for Planning remains the primary tool for assessing flood risk in relation to development proposals as flood defences may be overtopped or fail as noted in section 4.6.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Appendix G Strategic Flood Risk Assessment Summary of Updates