Strategic Flood Risk Assessment (Level 1 SFRA) A Technical Report Supporting the Royal Borough of Windsor & Maidenhead Borough Local Plan
Published in January 2014
Royal Borough of Windsor and Maidenhead Planning & 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
Updated Flood Mapping and January 2014 Flood Event in the Royal Borough
Subsequent to the production of this revised Level 1 Strategic Flood Risk Assessment (SFRA), it should be noted that in December 2013, the Environment Agency published updated Flood Maps. These and any subsequent revised Flood Maps will be taken into account in the next update of the SFRA.
This SFRA was also produced prior to the storm event of January 2014. As a result of heavy rainfall and existing ground conditions, a number of areas in the Royal Borough were affected by fluvial flooding from the River Thames and other watercourses, and other areas were affected by groundwater flooding or surface water flooding. The Council has sought to record information associated with this storm and any future update of this SFRA will take this into account.
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
ASSESSMENT REVISION
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
EXECUTIVE SUMMARY
Introduction
1. The Royal Borough of Windsor & Maidenhead extends from Hurley in the north west to Wraysbury in the south east. The Borough encompasses two major population centres, Windsor and Maidenhead. A large proportion of the Borough is designated Green Belt, interspersed by a number of smaller towns and villages including Cookham, Eton, Datchet, Old Windsor, Ascot, Bisham and Hurley.
2. The Council is currently preparing a Borough Local Plan in accordance with the Town and Country Planning (Local Planning) (England) Regulations 2012. The Royal Borough of Windsor and Maidenhead Local Plan (adopted June 2003) sets out the Council’s current proposals for the development and use of land in the Borough. This will be replaced by the Borough Local Plan. Development within the Borough is currently focussed very much upon the major centres of Windsor and Maidenhead. The emerging spatial strategy will retain this focus for future development within the Borough. It is noted that the Borough is currently delivering 94% of its housing development on previously developed land (based on RBWM data 2011 – 2012).
3. The River Thames and its tributaries are a dominant feature of the Royal Borough. A very large proportion of the local communities are situated adjacent to, or near, the river and/or its tributaries. Significant flooding from the River Thames has occurred no less than nine times within the last 100 years, most recently in 2003 in which a substantial number of homes and businesses within the Borough were affected.
4. The Windsor & Maidenhead SFRA was adopted by the Council for development control purposes and to inform the Local Development Framework in November 2007 (Revision A). The Council is committed to reviewing the SFRA on a regular basis in light of improved information relating to flood risk within the Borough, and/or a change in government policy. Jacobs was therefore commissioned in May 2008 to carry out a detailed review of the SFRA, triggered by the publication of revised flood zones for the River Thames (Environment Agency, March 2008), and the publication of the PPS25 Practice Guide (CLG, June 2008).
5. In February 2013 WSP was commissioned to carry out an update to the SFRA following updated Lower Thames Modelling and the publication of the National Planning Policy Framework (NPPF) and the Technical Guidance Note in March 2012.
6. This report (and the supporting mapping) represents the reviewed and updated SFRA for the Royal Borough of Windsor & Maidenhead. This document supersedes the November 2007 report and maps and the 2009 Revision report and maps. This report represents the Level 1 SFRA. The accompanying ‘Increased Scope’ SFRA considers potential development allocation sites in more detail. The ‘Increased Scope’ SFRA also includes a Sequential Test of potential allocation sites. The Level 1 and ‘Increased Scope’ SFRAs should be read in conjunction with each other.
Why carry out a Strategic Flood Risk Assessment (SFRA)?
7. Flooding can result not only in costly damage to property, but can also pose a risk to life and livelihood. It is essential that future development is planned carefully, steering it away from areas that are most at risk from flooding, and ensuring that it does not exacerbate existing known flooding problems.
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8. The National Planning Policy Framework (NPPF): Section 10 Meeting the challenge of climate change, flooding and coastal change has been developed to underpin decisions relating to future development (including urban regeneration) 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.
9. The Strategic Flood Risk Assessment (SFRA) is the first step in this process, and it provides the building blocks upon which the Council’s planning and development control decisions will be made.
What is a Strategic Flood Risk Assessment (SFRA)?
10. The Royal Borough of Windsor & Maidenhead Strategic Flood Risk Assessment (SFRA) has been carried out to meet the following key objectives:
¾ To collate all known sources of flooding, including river, surface water (local drainage), sewers and groundwater, that may affect existing and/or future development within the Borough;
¾ To delineate areas that have a ‘low’, ‘medium’ and ‘high’ probability of flooding, as well as the Functional Floodplain, within the Borough, in accordance with the National Planning Policy (NPPF), and to map these:
- Functional Floodplain incorporates areas of the region susceptible to flooding within which “water has to flow or be stored in times of flood”1;
- Areas of ‘high’ probability of flooding are assessed as having a 1 in 100 or greater chance of river flooding (>1%) in any year, and are referred to as Zone 3 High Probability;
- Areas of ‘medium’ probability of flooding are assessed as having between a 1 in 100 and 1 in 1000 chance of river flooding (1% to 0.1%) in any year, and are referred to as Zone 2 Medium Probability;
- Areas of ‘low’ probability of flooding are assessed as having a less than 1 in 1000 chance of flooding (<0.1%) in any year, and are referred to as Zone 1 Low Probability.
¾ Within flood affected areas, to recommend appropriate land uses (in accordance with the NPPF Sequential Test) that will not unduly place people or property at risk of flooding; and
¾ Where flood risk has been identified as a potential constraint to future development, recommend 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 As defined by NPPF
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STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
The Sequential Test
11. The primary objective of the NPPF is to steer vulnerable development towards areas of lowest probability of flooding, i.e. Flood Risk Zone 1 Low Probability. 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 seek to allocate sites for future development within areas of lowest flood risk. 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 is referred to as the Sequential Test. The NPPF Sequential Test is identified within Paragraph 101 of the NPPF and paragraph 5 of the Technical Guide for the NPPF (March 2012)) and Section 6.4.1 of this document.
12. Following the application of the Sequential Test, the NPPF stipulates permissible development types (refer Table 2, the NPPF Technical Guidance and duplicated in Appendix G) within each flood zone. This considers both the probability of flooding within a 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.
The Exception Test
13. Many towns within England are situated adjacent to rivers, and are at risk of flooding. The future sustainability of these communities relies heavily upon their ability to grow and prosper. The NPPF recognises that, in some local authorities, restricting residential development from areas designated as Zone 3a High Probability may heavily compromise the viability of existing communities.
14. For this reason, if necessary, the NPPF provides an Exception Test. Following the application of the Sequential Test, and where there are wider sustainability objectives that support the development proceeding, it will be necessary for the Council to demonstrate that the Exception Test can be satisfied.
15. For the Exception Test to be passed:
¾ it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a Strategic Flood Risk Assessment where one has been prepared; and
¾ a site-specific Flood Risk Assessment (FRA) must demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall. 2
Both elements of the test will have to be passed for development to be allocated or permitted.
Outcomes of the Windsor & Maidenhead SFRA
16. The Borough of Windsor and Maidenhead has been delineated into zones of low, medium and high probability of flooding, based upon existing available information provided by the Environment Agency. Detailed flood risk mapping has been made available for the River Thames, the River Colne and the Cut. The Environment Agency Flood Maps (obtained April 2013) have been adopted as the basis for the SFRA for other watercourses.
17. A number of towns and villages are situated within the 1% (1 in 100) flood extent (i.e. within Zone 3 High Probability), including the principal centres of Windsor and Maidenhead which form the primary focus for future development within the Borough. Considerable investment has been made in flood defence improvements within the area, increasing the standard of protection provided to Maidenhead, Cookham, Eton and
2 The NPPF (2012) paragraph 102
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STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Windsor. There is always a residual risk, however, that these defences may be overtopped in more extreme flood events or fail in some way, and therefore it is crucially important that future development takes careful consideration of the possible risk to life should a flood occur.
18. The Environment Agency shows Areas Benefiting from Defences (ABD) on their flood mapping. The ABD mapping shows areas that, because they are defended, are not expected to flood in 1% AEP fluvial events or 0.5% AEP coastal / tidal events (but which would flood if they were not defended). In planning and policy terms these areas are still treated as Flood Zone 3a (refer below). In accordance with the requirement of paragraph 103 of the NPPF, the developer must use a Flood Risk Assessment to prove the suitability of a site’s location in an ABD, proposing appropriate finished floor levels and flood resilient & resistant mitigation if required. Paragraph 103 states that local planning authorities should only consider development appropriate in areas of risk where it can be demonstrated that:
¾ “development is appropriately flood resilient and resistant, including safe access and escape routes where required, and that any residual risk can be safely managed, including by emergency planning; and it gives priority to the use of sustainable drainage systems”.
Zone 3b (Functional Floodplain)
19. Areas subject to flooding in events up to (and including) the 5% AEP3 (1 in 20) design event have been delineated. These areas have subsequently been sub-delineated on the basis of current land use such that:
¾ Areas of existing open space have been defined as Zone 3b Functional Floodplain; ¾ Areas that are ‘previously developed’ have been defined as Zone 3b Developed.
20. Within the context of the SFRA ‘previously developed’ areas, delineated as Zone 3b Developed for planning purposes, relate to sites within which there are existing buildings that are considered to be impermeable to floodwaters. It is important to recognise that the land surrounding these buildings are critical flow paths and/or flood storage areas, and must be retained.
21. It is also important to recognise that all areas within Zone 3b Functional Floodplain and Zone 3b Developed are areas that are subject to relatively frequent flooding, and may be subject to fast flowing and/or deep water. Very careful consideration must be given to future sustainability and safety issues within this area, and development may only be considered following application of the Sequential Test.
22. No development is permissible in Zone 3b Functional Floodplain apart from water compatible uses and essential infrastructure, if required, and only then if the Sequential Test is passed. Essential infrastructure use also needs to pass the Exception Test. Specific planning responses have been developed for both Zone 3b Functional Floodplain and Zone 3b Developed, and these are set out in Section 6.4.
Zone 3a High Probability
23. Areas with a 1% probability of flooding in any one year (within the 1 in 100 year flood extent) have been delineated as Zone 3a High Probability. Development within these areas may only be considered following application of the Sequential Test and if required the Exception Test. Water Compatible and Less Vulnerable development is compatible with this zone if it passes the Sequential Test. More Vulnerable development should be avoided wherever possible.
3 Annual Excedance Probability (AEP)
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STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
24. The SFRA has outlined specific development control recommendations that should be placed upon development within Zone 3a High Probability to minimise the damage to property, the risk to life in case of flooding, and the need for sustainable drainage systems (SuDS). It is essential that the developer carries out a detailed Flood Risk Assessment to consider the site-based constraints that flooding may place upon the proposed development.
25. Re-development can provide an opportunity to reduce the causes and impacts of flooding for example by incorporating SuDS and resilient building design, creating flood storage, re-creating functional floodplain and setting back defences.
Zone 2 Medium Probability
26. Areas subject to flooding in events exceeding the 1% (1 in 100) event, and up to (and including) the 0.1% (1 in 1000) event have been delineated as Zone 2 Medium Probability. Development within these areas may only be considered following application of the Sequential Test. ’Highly Vulnerable Development’, for example emergency services, should be avoided in these areas and is only permissible if it has passed the Exception Test.
27. There are generally no other restrictions placed upon land use in these areas, however it is important to ensure that the developer takes account of possible climate change impacts to avoid a possible increase in the risk of flooding in future years (achieved through completion of a simple Flood Risk Assessment).
Zone 1 Low Probability
28. All areas outside of Zones 2 and 3 have been delineated as Zone 1 Low Probability. There are no restrictions placed on land use within Zone 1 Low Probability (i.e. all remaining areas of the Borough) by the NPPF. It is essential however that consideration is given to the potential risk of flooding from other sources (outlined in ‘Localised Flooding Issues’ below), ensuring that future development is not inadvertently placed at risk. It is also essential to ensure that future development does not exacerbate the current risk posed to existing homes and businesses.
Localised Flooding Issues
29. Properties and infrastructure within the Royal Borough of Windsor & Maidenhead are also at risk of flooding from other sources. These include groundwater flooding, the surcharging of the underground sewer system, the blockage of culverts and gullies (which results in overland flow), and surface water flooding. Evidence of localised flooding of this nature has been captured through consultation with the Council and the Environment Agency and is reflected in the flood maps contained within this SFRA. 30. The NPPF does not address issues of this nature within its delineation of flood zones and what development is acceptable within them. It does, however, require that a sequential approach is used in areas known to be at risk from all forms of flooding. After having applied a sequential approach, if necessary, incidents of this nature can often be addressed through the design process. However, the recent flooding throughout England (including the Royal Borough of Windsor and Maidenhead) in 2007 highlights that this is certainly not always the case and uncontrolled flooding as a result of particularly heavy rains can create significant damage and disruption. 31. The NPPF Technical Guidance (March 2012) advocates the application of a sequential approach when allocating land, taking into consideration all sources of flooding. 32. It is essential not to overlook the potential risk of localised flooding (i.e. flooding from groundwater or surface water) during the design process. Whilst the incidents that have been identified may not typically result in widespread damage or disruption, a proactive approach to risk reduction through design can mitigate the potential for damage, both to the development itself and elsewhere. Specific development control recommendations have been provided accordingly (refer Section 6.4).
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STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
33. As a minimum, the implementation of sustainable drainage systems (SuDS) must be ensured and careful consideration to overland flow routes (and avoidance of their obstruction), as part of the site design, should be encouraged.
A Proactive Approach – Reduction in Flood Risk
34. It is crucial to recognise that the NPPF considers not only the risk of flooding posed to new development. It also seeks, where possible, to reduce the risk of flooding posed to existing properties within the Borough. It is recommended that this principle should be adopted as the underlying ‘goal’ for developers and the Council’s development control teams within Windsor & Maidenhead. 35. Developers will be expected to demonstrate that their proposal will deliver a reduction in flood risk to the Borough, 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). The redevelopment and refurbishment of existing buildings in the floodplain to achieve resilience and/or resistance, and to make space for water, are a very important part of the planning solution needed to reduce flood risk within the Borough. 36. Where flood risk reduction is possible this should be reflected through the inclusion of a positive statement within the detailed FRA that clearly and concisely summarises how this reduction in flood risk will be delivered.
The Way Forward
37. A considerable proportion of the Royal Borough of Windsor & Maidenhead is at risk of flooding. The risk of flooding posed to properties within the Borough arises from a number of sources including river flooding, localised runoff, sewer and groundwater flooding.
38. Substantial investment in flood defence has been delivered within the Royal Borough, providing a degree of protection to existing property in the form of raised embankments, flood control structures and diversion channels (e.g. the Jubilee River). A residual risk of flooding remains, however, associated both with an event that may exceed the design capacity of the defences, and/or a structural failure. It is imperative that this risk is interrogated, and safely mitigated through design, as part of the detailed site based Flood Risk Assessment (FRA).
39. 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 all urban centres within the Borough.
40. If after having undertaken the Sequential Test it has been identified that there are no reasonably available sites in areas not at risk of flooding, specific recommendations have been provided to assist the Council and the developer apply the Exception Test where required (refer Section 6.4). These should be considered when writing new policies as part of the Borough Local Plan, as well as in the determination of planning applications.
41. Council policy is essential to ensure that the development control recommendations in this SFRA can be imposed consistently at the planning application stage. This is essential to achieve future sustainability within the Borough with respect to flood risk management. Current Local Plan (adopted) Policy F1 (Flood Risk) is being reviewed as part of the emerging new Borough Local Plan. The 2013 SFRA will help shape new flood risk policy.
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STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
42. Emergency planning is crucial for the minimisation to the risk to life posed by flooding within the Borough. The Council’s Emergency Planning Unit should use the SFRA as a guide for risk identification, supporting the generic Flood Plan that will guide the emergency response in case of flooding.
A Living Document
43. The Royal Borough of Windsor & Maidenhead SFRA has been developed, and subsequently reviewed, in accordance with the NPPF and associated Technical Guidance. As part of this review process, the SFRA has been developed building heavily upon the best available knowledge with respect to flood risk within the Royal Borough at the time of writing (November 2013). The Environment Agency regularly review and updates their Flood Zone Maps (on a quarterly basis) and a rolling programme of detailed flood risk mapping within the South East region is underway. This will progressively improve the current knowledge of flood risk within the Borough, and may alter predicted flood extents. This may therefore influence future development control decisions within these areas.
44. In summary, it is imperative that the SFRA is adopted as a ‘living’ document and is reviewed periodically in light of emerging policy directives, significant hydraulic modelling updates and an improving understanding of flood risk within the Borough.
Disclaimer
It is important to recognise that the information provided within the SFRA comprises the best available data at the time of writing (November 2013). The mapping of flood risk is not an exact science, and there may be some uncertainties in the information presented. The SFRA is a strategic document that is intended to support the spatial planning process. It recommends where more detailed site-based Flood Risk Assessments should be undertaken when future development is being considered (following application of the Sequential Test), and it is expected that the FRA will improve the level of accuracy in the flood extents from a localised perspective.
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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 Study Area ...... 1 2 SFRA Approach ...... 2 3 Policy Framework ...... 5 3.1 Introduction ...... 5 3.2 National Policy ...... 5 3.2.1 Overview ...... 5 3.2.2 Flood and Water Management Act 2010 ...... 5 3.2.3 National Planning Policy Framework (NPPF) ...... 5 3.2.4 Technical Guidance to the National Planning Policy Framework 6 3.3 Regional Planning Guidance ...... 6 3.3.1 South East Plan ...... 6 3.4 Local Development Plan ...... 7 3.4.1 Royal Borough of Windsor & Maidenhead Local Plan ...... 7 3.4.2 Emerging Borough Local Plan and its Context ...... 7 3.4.3 Proposals Map...... 8 3.4.4 Neighbourhood Plans ...... 8 4 Data Collection ...... 9 4.1 Overview...... 9 4.2 Consultation ...... 9 4.3 Environment Agency Flood Zone Maps ...... 10 4.4 Historical Flooding ...... 10 4.5 Detailed Hydraulic Modelling ...... 11 4.6 Flood Defences ...... 12 4.7 Geology & Topography ...... 12 5 Flood Risk in Windsor & Maidenhead ...... 14 5.1 Overview...... 14 5.2 Fluvial Flooding - Delineation of the NPPF Flood Risk Zones ...... 15 5.2.1 Delineation of Zone 3b Functional Floodplain ...... 15 5.2.2 Existing Development Affected by the 5% (1 in 20) Design Event - Zone 3b Developed ...... 16 5.2.3 Delineation of Zone 3a High Probability ...... 16 5.2.4 Delineation of Zone 2 Medium Probability ...... 16 5.2.5 Delineation of Zone 1 Low Probability ...... 17 5.3 Assessment of Risk to Life (Flood Hazard) ...... 17 5.3.1 Definition of Flood Hazard ...... 17 5.3.2 Flood Hazard due to River Thames Flooding ...... 18 5.3.3 Flood Hazard due to Flood Defence Failure ...... 18 5.4 Local Drainage Issues ...... 19 5.5 Groundwater Issues...... 20 5.6 Water Supply Infrastructure Failure ...... 21 5.7 Climate Change ...... 22 5.8 Residual Risk of Flooding ...... 23 6 Sustainable Management of Flood Risk ...... 25 6.1 Overview...... 25 6.2 Responsibility for Flood Risk Management ...... 25 6.2.1 Responsibilities...... 25 6.3 Strategic Flood Risk Management - The Environment Agency ...... 26 6.3.1 Overview ...... 26
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
6.3.2 Catchment Flood Management Plan (CFMP) ...... 26 6.3.3 The Lower Thames Flood Alleviation Scheme ...... 28 6.3.4 The Jubilee River...... 29 6.3.5 Cookham Flood Alleviation Scheme ...... 29 6.3.6 Other Schemes...... 29 6.3.7 Green Infrastructure ...... 29 6.4 Application of the NPPF within Windsor & Maidenhead ...... 29 6.4.1 Planning Solutions to Flood Risk Management ...... 29 6.4.2 A Proactive Approach – Positive Reduction of Flood Risk through Development ...... 31 6.4.3 Localised Flood Risk within the Planning Process...... 32 6.4.4 Spatial Planning & Development Control Recommendations ... 33 6.5 SFRA Interpretation ...... 34 6.5.1 The NPPF Constraints upon Potential Site Allocations ...... 35 6.6 Detailed Flood Risk Assessment (FRA) – The Developer ...... 36 6.6.1 Scope of the Detailed Flood Risk Assessment ...... 36 6.6.2 Raised Floor Levels & Basements (Freeboard) ...... 39 6.6.3 Sustainable Drainage Systems (SuDS) ...... 39 6.7 Local Community Actions to Reduce Flood Damage ...... 42 6.7.1 Flood Proofing ...... 43 6.8 Emergency Planning...... 43 6.9 Insurance ...... 45 7 Conclusion & Recommendations ...... 47
Appendices
Appendix A SFRA Flood Maps (November 2013) (Appendix B Not used) Appendix C Safe Access & Egress – Environment Agency Design Recommendations Appendix D Rainfall & Runoff from Development – Interim National Procedure Appendix E Overview of Flood Risk – Key Locations Appendix F Potential Impacts of Climate Change Appendix G Land Use Vulnerability – Technical Guidance to the NPPF Table 2 (March 2012) Appendix H Flood Defence Failure – The Simple Method, FD2320 Appendix I Strategic Flood Risk Assessment – Summary of Updates
Figures
Figure A Overview of Borough Figure B Historical Flooding (from all sources) Figure C Geology Figure D Topography Figure E Index Map – Character Areas Figure F River Thames Flood Depth 1% (1 in 100) Design Event Figure G Areas Potentially at Risk of Defence Failure Figure H Areas Susceptible to Groundwater Flooding Figure I Flood Map for Surface Water
STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Glossary
Annual Excedance Probability e.g. 1% AEP is equivalent to 1% probability AEP of occurring in any one year (or, on average, once in every 100 years)
A Development Plan Document which sets the long-term vision and objectives for the area. It contains strategic policies required to deliver the Borough Local Plan vision including the broad approach to development as well as (BLP) development management policies required for the delivery of development and site allocations. Catchment Flood Provides a catchment approach to managing flood risk and provides key Management Plan policies and actions on the catchment scale. (CFMP)
CLG Communities and Local Government
A structure that provides a flood defence function, however has not been De Facto Flood Defence built and/or maintained for this purpose (e.g. boundary wall)
Defra Department of Environment, Food and Rural Affairs
The carrying out of building, engineering, mining or other operations, in, on, Development over or under land, or the making of any material change in the use of a building or other land.
A spatial planning document within the Council’s Local Development Development Plan Framework which set out policies for development and the use of land. Document (DPD) Together with the Regional Spatial Strategy they form the development plan for the area. They are subject to independent examination.
EA Environment Agency
An assessment of the likelihood and consequences of flooding in a Flood Risk Assessment development area so that development needs and mitigation measures can (FRA) be carefully considered.
Nationally consistent delineation of ‘high’ and ‘medium’ flood risk, published Flood Zone Map 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.
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 Habitable Room 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 will Framework (LDF) provide the framework for delivering the spatial strategy for the area.
Local Flood Risk This is a flood risk strategy that is required to be produced by the Lead Management Strategy Local Flood Authority (LLFA) in accordance with the FWMA. This strategy (LFRMS) will identify 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
Replaced Planning Policy Statement 25 in March 2012 and provides National Planning Policy National Planning Policy. The NPPF is accompanied by a Technical Framework (NPPF) Guidance note (March 2012) that provides additional guidance on implementing the policy in NPPF. A series of statements issues by the Government, setting out policy Planning Policy guidance on different aspects of planning. They were replaced by the Statement (PPS) National Planning Policy Framework in March 2012. Planning Policy Statement 25: Development and Flood Risk PPS25 Department of Communities & 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) Land which is or was occupied by a building (excluding those used for Previously Developed agriculture and forestry). It also includes land within the curtilage of the (Brownfield) Land building, for example a house and its garden would be considered to be previously developed land.
RBWM Royal Borough of Windsor & Maidenhead
A measure of the outstanding flood risks and uncertainties that have not Residual Risk been explicitly quantified and/or accounted for as part of the review process
Sustainability Appraisal (SA) is an appraisal of the environmental, economic and social effects of plans, strategies and proposals to test them against SA the four broad objectives set out in the Government’s sustainable development strategy.
Strategic Environmental Assessment (SEA) is a generic term used internationally to describe environmental assessment as applied to policies, plans and programmes. The European ‘SEA Directive’ (2001/42/EC) does SEA 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 Urban Drainage System
Provides supplementary guidance to policies and proposals contained Supplementary Planning within development plan documents. They do not form part of the Document (SPD) development plan, nor are they subject to independent examination.
“Development that meets the needs of the present without compromising Sustainable Development the ability of future generations to meet their own needs” (The World Commission on Environment and Development, 1987).
This zone comprises land where water has to flow or be stored in times of Zone 3b Functional 4 flood. Defined as areas at risk of flooding in the 5% AEP ( 1 in 20 chance) Floodplain design event
This zone comprises land assessed as having a 1 in 100 or greater annual Zone 3a High Probability probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year
This zone comprises land assessed as having between a 1 in 100 and 1 in Zone 2 Medium 1000 annual probability of river flooding (1% – 0.1%) or between a 1 in 200 Probability and 1 in 1000 annual probability of sea flooding (0.5% – 0.1%) in any year
This zone comprises land assessed as having a less than 1 in 1000 annual Zone 1 Low Probability probability of river or sea flooding in any year (<0.1%)
Further information can also be found on the Planning Portal website - http://www.planningportal.gov.uk/ or the Environment Agency website - http://www.environment-agency.gov.uk/
4 Denoted as Zone 3b within 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
45. Many of the major towns and villages within the Royal Borough of Windsor & Maidenhead are situated adjacent to rivers. The Environment Agency5 estimates that, within Windsor and Maidenhead, approximately 11,000 properties are at ‘significant’ risk of flooding, representing almost 15% of the total number of properties within the Borough and nearly 60% of the total number of properties located within flood zones on the Environment Agency’s flood maps. Flooding represents a risk to both property and life. It is essential therefore that planning decisions are informed by, and take due consideration of the risk posed to (and by) future development by flooding.
46. National Planning Policy Framework (NPPF) (March 2012) requires that local planning authorities prepare a SFRA in consultation with the Environment Agency. The primary purpose of the SFRA is to determine the variations in flood risk across the Borough. Robust information on flood risk is essential to inform and support the Council’s revised flooding policies in its emerging Borough Local Plan (BLP).
47. The Royal Borough of Windsor & Maidenhead SFRA was originally adopted for development control purposes and to inform the LDF in November 2007 (Revision A). The Council is committed to reviewing the SFRA on a regular basis, and as a result Jacobs was commissioned in May 2008 to carry out a detailed review of the SFRA.
48. Subsequently, in 2013 WSP were commissioned to review the SFRA and undertake an ‘increased scope’ SFRA for individual sites. This report (and the supporting mapping) represents the reviewed and updated SFRA for the Royal Borough of Windsor & Maidenhead.
49. This report represents the updated Level 1 SFRA. The accompanying ‘Increased Scope’ SFRA considers potential development allocation sites in more detail. The ‘Increased Scope’ SFRA also includes a Sequential Test of potential allocation sites. The Level 1 and ‘Increased Scope’ SFRAs should be read in conjunction with each other.
1.2 Study Area
50. The study area includes the whole of the Royal Borough of Windsor and Maidenhead. The Royal Borough is located in the east of Berkshire, between Slough, Wokingham and Bracknell (refer Figure A). The area of the Borough is 19,844 hectares.
51. The Borough includes the large settlements of historic Windsor and Maidenhead. There are also a number of smaller settlements within the Royal Borough including Cookham, Ascot, Sunninghill, Sunningdale, Eton, Old Windsor, Horton, Datchet and Wraysbury which are covered by 15 parish councils (including Eton Town Council).
52. The Royal Borough is regarded as a prosperous area with a thriving local economy and low unemployment record. The Royal Borough has excellent communications being located near to Heathrow, the motorway network (including M4, M40 and M25) and the national rail network.
53. The main settlements of Windsor and Maidenhead are the primary focus for employment related development. The Council is currently achieving 94% of housing development on previously developed urban sites6. 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). Over 83% of the Royal Borough is designated Green Belt.
5 June 2013 6 Housing Completions 2011/12
November 2013 1 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 2 SFRA Approach
54. There are eight main intended applications of the Royal Borough of Windsor & Maidenhead SFRA:
¾ To inform the review of the Local Development Plan, including its spatial strategy, and to form a key part of the Council’s evidence base; ¾ To inform new or revised flooding policies as currently set out in the RBWM Local Plan incorporating alterations accepted in 2003, and supplementary planning documents; ¾ To guide future detailed flood risk assessments, and the Borough’s capacity to accommodate development; ¾ To input into the Council’s Sustainability Appraisal; ¾ To assist the development control process by providing a more informed response to development proposals affected by flooding; ¾ To provide the basis for any future flood attenuation works; ¾ To support flood risk management and emergency planning; and ¾ To contribute to the Council’s Sustainable Community Strategy.
55. The primary objective of the SFRA is to inform the revision of flooding policies, including the allocation of land for future development, within the emerging Borough Local Plan (BLP). The SFRA has a broader purpose, however, and in providing a robust depiction of flood risk across the Borough it can:
¾ Assist the development control process by providing a more informed response to development proposals affected by flooding, influencing the design of future development within the Borough; ¾ Help to identify and implement strategic solutions to flood risk, providing the basis for possible future flood attenuation works; ¾ Support and inform the Council’s emergency planning response to flooding.
56. The Government provides no specific methodology for the SFRA process. Therefore, to meet these broader objectives, the SFRA has been developed in a pragmatic manner in close consultation with both the Council and the Environment Agency.
57. A considerable amount of knowledge exists with respect to flood risk within the Borough, including information relating both to historical flooding, and the predicted extent of flooding under extreme weather conditions (i.e. as an outcome of detailed flood risk modelling carried out by the Environment Agency). The Windsor & Maidenhead SFRA has built heavily upon this existing knowledge, underpinning the delineation of the Borough into zones of ‘high’, ‘medium’ and ‘low’ probability of flooding, as well as the functional floodplain, in accordance with the NPPF. These zones have then been used to provide a robust and transparent evidence base for the development of flooding related policy, and to inform the site allocation process.
November 2013 2 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
58. A summary of the adopted SFRA process is provided in Figure (i) below, outlining the specific tasks undertaken, and the corresponding structure of the SFRA report.
Collation of existing information relating to flooding
Delineation of ‘high’, ‘medium’ and ‘low’ Assessment of the possible risk probability zones in accordance with the NPPF to life (Flood Hazard) should a flood occur
Application of the Sequential Test & Recommend Appropriate land uses within the flood affected areas in accordance with NPPF
Assessment of the Assessment of the residual risk of potential impacts of flooding to the Borough flooding to the Borough of climate change
Application of the Exception Test Make development recommendations to mitigate the risk of flooding should development proceed within flood affected areas in accordance with the NPPF
Figure (i) SFRA Process & Report Structure
59. It is important to recognise that all of the rivers that affect the Royal Borough of Windsor & Maidenhead flow into, or from, adjoining authorities within the Thames Valley. Future development within the Borough, if not carefully managed, can influence the risk of flooding posed to residents within neighbouring areas. Conversely, careless planning decisions within adjacent districts can also impact adversely upon flooding within the Borough.
60. All authorities within England are required to carry out similar strategic flood risk investigations. Whilst the delivery teams and programmes underpinning these studies vary from one district to the next, all should be developed in close liaison with the Environment Agency. Consistency in adopted approach and decision making with respect to the effective management of flood risk throughout the Thames system is imperative. Regular discussions with the Environment Agency, Communities and Local Government (CLG)7 and other interest groups have been carried out throughout the original SFRA process. This revision has been completed by continued discussion by seeking clarity and consistency where needed.
SFRA Review (2013)
61. This report reflects a detailed review of the Windsor & Maidenhead SFRA that was originally updated in April 2009 (see below). The review was focussed upon updating the hydraulic modelling of the River Thames and the update to National Planning Policy.
7 Authors of NPPF
November 2013 3 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 62. This review also includes the updated SHLAA (2011) and ELR (2009) sites in relation to potential site allocations.
SFRA Review (2008)
63. The previous SFRA Report (April 2009) reflected a detailed review of the May 2008 SFRA. This review of the SFRA was focussed upon updating both the appraisal of flood risk, and recommended Council policy responses, in light of the following:
¾ The Environment Agency release of updated flood zones for the River Thames and the River Colne, based upon detailed hydraulic modelling (Jacobs, 2008); ¾ The publication of the PPS25 Practice Guide (CLG, June 2008); ¾ The surface water flooding experienced in the Borough in July 2007; ¾ The publication of the Pitt Review following the widespread flooding of summer 2007 (June 2008); ¾ The analysis of flood risk in light of potential defence breach failure (Jacobs, October 2008);
November 2013 4 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 3 Policy Framework
3.1 Introduction
64. This section provides a brief overview of the strategy and policy context relevant to flood risk in the Royal Borough of Windsor and Maidenhead.
65. The success of the SFRA is heavily dependent upon the Council’s ability to implement the recommendations put forward for future sustainable flood risk management, both with respect to planning and development control decisions. A framework of national and regional policy directive is in place, providing guidance and direction to local planning authorities. Ultimately however, it is the responsibility of the Council to establish robust policies that will ensure future sustainability with respect to flood risk.
3.2 National Policy
3.2.1 Overview
66. National planning policy is set out through the National Planning Policy Framework (NPPF) and the Technical Guidance Note.
67. 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.
3.2.2 Flood and Water Management Act 2010
68. 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.
69. 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.
¾ "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)."
70. 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).
71. Further information on the act with regard to SuDS is listed in section 6.6.
72. The Act received Royal Assent on the 8th April 2010. The Act is being put through commencement in parts and not as a whole document and therefore some parts of the Act have already been implemented
3.2.3 National Planning Policy Framework (NPPF)
73. 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:
November 2013 5 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
¾ “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:
x applying the Sequential Test; x if necessary, applying the Exception Test; x safeguarding land from development that is required for current and future flood management; x using opportunities offered by new development to reduce the causes and impacts of flooding; and x 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.”
74. 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.
75. 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.
76. It is important to reiterate 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.
3.2.4 Technical Guidance to the National Planning Policy Framework
77. In March 2012 the Technical Guidance was published alongside 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 a helpful indication of the ways in which the principles of the NPPF might be applied in practice.
3.3 Regional Planning Guidance
3.3.1 South East Plan
78. 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 Borough.
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3.4 Local Development Plan
79. The RBWM Local Development Plan consists of:
x Adopted Local Plan (adopted in 1999 and incorporating alterations 2003) x Replacement Minerals Local Plan (Incorporating the Alterations Adopted in December 1997 and May 2001) x Waste Local Plan for Berkshire (adopted December 1998) x Saved South East Plan policy NRM6 Thames Basin Heaths SPA x Maidenhead Town Centre Area Action Plan (adopted September 2011)
3.4.1 Royal Borough of Windsor & Maidenhead Local Plan
80. The Royal Borough of Windsor and Maidenhead Local Plan sets out the Council’s proposals for the development and use of land in the Borough. The Plan was formally adopted in July 1999 with alterations adopted in June 2003. The Local Plan is currently under review and will be replaced by the Borough Local Plan. With respect to flooding, saved Policy F1 states:
“Within the area liable to flood as shown on the proposals maps, or within other areas subject to flooding, development will not be permitted for new residential or non- residential development, including extensions in excess of 30 square metres, unless it can be demonstrated to the satisfaction of the Borough Council, that the proposal would not of itself, or cumulatively in conjunction with other development: ¾ impede the flow of flood water; or ¾ reduce the capacity of the floodplain to store flood water; or ¾ increase the number of people or properties at risk from flooding.”
81. The adopted policy broadly encapsulates the key underlying principles set out in NPPF, and is considered robust in its approach8. Policy F1 is supported by supplementary planning guidance (SPG) that provides more specific recommendations with respect to future development within flood affected areas. This SPG9 is linked to Policy F1, and is therefore a material consideration.
82. Regarding the Council’s emerging Borough Local Plan, it is recommended that the SPG is reviewed in light of the Windsor & Maidenhead SFRA and within the context of the emerging Flood Risk and Water Management policy in the Borough Local Plan. Reference should be made to the suggested development control recommendations set out in Section 6.4 of this report. These have been identified and agreed in close consultation with the Environment Agency and the Council. They represent the minimum criteria that will be expected by the Environment Agency should development be permitted to proceed.
3.4.2 Emerging Borough Local Plan and its Context
83. The Council prepared and submitted a Core Strategy for examination in November 2006. The report of the examination was published in October 2007. The Inspector concluded that the Core Strategy was unsound, and the Core Strategy was subsequently withdrawn.
84. As indicated above, the Royal Borough of Windsor and Maidenhead is currently preparing its Borough Local Plan in accordance with the 2012 Town and Country Planning (Local Planning) (England) Regulations.
85. The Borough Local Plan will set an overarching spatial development strategy for the Borough including site allocations and detailed development management and delivery policies.
8 Under a Direction issued by the Government Office for the South East in September 2007, Policy F1 was confirmed as a ‘saved’ policy. It therefore continues to be included in the Development Plan for the Royal Borough. 9 Supplementary Planning Guidance on the Interpretation of Policy F1 (Area Liable to Flooding) (Adopted May 2004)
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86. The emerging Borough Local Plan is being accompanied by a Sustainability Appraisal. One of the SA objectives that will assess the performance and delivery of the policies contained in the SA is to “reduce and manage the risk of flooding and the resulting detriment to people, property and the environment.” The SA process will test whether the policies contained in the Borough Local Plan meet this objective.
3.4.3 Proposals Map
87. The Environment Agency updates its adopted Flood Zone Map, defining the extent of the area liable to flooding, on a quarterly basis as new data for various areas becomes available. This will supersede the currently adopted proposals map and any future maps.
88. It is highlighted that, whilst the information provided within the SFRA is robust and should be adopted as the evidence to support the development of the BLP, it does represent the best available information at the time of writing (November 2013). It is inevitable that this information will be superseded over time as the Environment Agency provides further investment in detailed modelling of the River Thames and its tributaries. For this reason, it is recommended that the Environment Agency Flood Zone Maps are retained as the ‘first pass’ filter at the development application stage, triggering (or otherwise) the need for a more detailed site-based investigation. The recommendations provided by the SFRA with respect to development within the functional floodplain, high, medium or low probability zones will not be affected.
3.4.4 Neighbourhood Plans
89. Under the new planning system through the Localism Act10, local areas are able to create a Neighbourhood Plan to guide development in their area. The Borough Council is a vanguard authority on this project, and is helping communities prepare Neighbourhood Plans. Funding has been secured from central government for most of the Neighbourhood Plan areas. The Council has 11 Neighbourhood Plans and Neighbourhood Plan Areas can be viewed on the RBWM website.
10 The Localism Act (November 2010) is an Act of Parliament that devolves greater powers to councils and neighbourhoods and gives local communities more control over housing and planning decisions.
November 2013 8 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 4 Data Collection
4.1 Overview
90. As indicated earlier, a considerable amount of knowledge exists with respect to flood risk within the Royal Borough of Windsor & Maidenhead, including (but not limited to):
¾ Historical river flooding information; ¾ Information relating to localised flooding issues (surface water, groundwater and/or sewer related), collated in consultation with the Council and the Environment Agency; ¾ Detailed flood risk mapping; ¾ Environment Agency Flood Zone Maps; ¾ Topography (LiDAR).
91. All of this data has been sourced from the Council and the Environment Agency, forming the core dataset that has informed the SFRA process. The application of this data in the delineation of the ‘high’, ‘medium’ and ‘low’ risk flood zones, and the formulation of planning and development control recommendations, is explained in Section 5. An overview of the core datasets, including their source and their applicability to the SFRA process, is outlined below.
4.2 Consultation
92. Consultation has formed a key part of the data collation phase for the Windsor & Maidenhead SFRA. The following key stakeholders have been consulted to inform the current investigation:
Royal Borough of Windsor & Maidenhead 93. Planning Consulted to identify areas under pressure from development and/or regeneration
Drainage Consulted to identify areas potentially at risk from river flooding and/or urban drainage
Emergency Planning Consulted to identify issues that may be of relevance to the planning of an emergency response to a flooding event within the Borough
Environment Agency 94. The Environment Agency has been consulted to source specific flood risk information to inform the development of the SFRA. In addition, the Environment Agency is a statutory consultee under the Duty to Co-operate and therefore must be satisfied with the findings and recommendations for sustainable flood risk management into the future. For this reason, the Environment Agency has been consulted during the development of the SFRA to discuss potential flood risk mitigation measures and planning recommendations. The EA has also formed a key part of the SFRA project management team in the production of this updated SFRA (2013).
Thames Water 95. Thames Water is responsible for the management of urban drainage (surface water) and sewerage within the Royal Borough.
It is recognised that issues associated with failures of the underground drainage/sewer systems are often relatively localised, and should not preclude development. Notwithstanding this however, specific related problems have been highlighted by the SFRA process (refer adjoining flood maps) through consultation with the Council and the Environment Agency.
November 2013 9 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan Thames Water is a statutory consultee as part of the emerging Borough Local Plan and has been consulted on the previous SFRAs. Responsibility for management of local flood risk (including surface water) has been transferred to the Borough Council as the Lead Local Flood Authority under the Flood and Water Management Act (FWMA) 2010. As a statutory undertaker for water supply and sewerage in the Borough, Thames Water has also been involved in other parts of the evidence base related to the 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.
Community Stakeholders
96. A small number of community representatives were consulted as part of the earlier SFRA process to seek their contribution, both in terms of local knowledge with respect to flood risk within the Borough, and with respect to emerging policy and development control recommendations.
4.3 Environment Agency Flood Zone Maps
97. The Environment Agency’s Flood Map shows the natural floodplain, ignoring the presence of defences, and therefore identifies areas potentially at risk of flooding from rivers or the sea. The Flood Map shows the area that has a 1% or greater chance of flooding occurring from rivers in any one year (1 in 100). It also indicates the area that has between a 1% and 0.1% chance of flooding occurring from rivers and/or the sea in any given year (1 in 100 to 1 in 1000). This is also known as the Extreme Flood Outline. The Extreme Flood Outline also includes historic flooding from fluvial sources.
98. The Flood Map outlines have been produced from a combination of a national generalised computer model, more detailed local modelling (if available), and some historic fluvial flood event outlines. The availability of detailed modelling for the Windsor and Maidenhead area is further discussed in Section 4.5. The Environment Agency’s Flood Map provides a consistent picture of flood risk for England and Wales.
99. 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. They have an on-going programme of improvement, and updates are made on a quarterly basis.
100. An overview of the Flood Map in the Royal Borough of Windsor & Maidenhead is provided in Figure A, showing a considerable proportion of the area being at risk from river flooding. This is not surprising given the relatively low lying topography of much of the Borough, particularly adjoining the River Thames and its tributaries. Indeed this area is historically natural floodplain that has been ‘reclaimed’ over centuries for the building of towns adjacent to crucially important transport corridors.
4.4 Historical Flooding
101. The Royal Borough of Windsor and Maidenhead has a history of flooding with significant flooding from the River Thames recorded no less than nine times within the past 100 years11, often having a devastating effect upon homes and livelihood.
102. In January 2003 flows in the River Thames triggered the operation of the recently constructed Jubilee River which comprises part of the Maidenhead, Windsor and Eton Flood Alleviation Scheme (MWEFAS). Approximately 1,000 properties in the Maidenhead, Windsor and Eton area were estimated to have been protected from flooding however 128 houses were affected downstream of the Jubilee River, many of which were located in the Ham Island and Wraysbury areas of the Royal Borough.
103. During the summer of 2007, widespread flooding occurred throughout England affecting literally thousands of homes, businesses and livelihoods. River flooding was predicted
11 RBWM Sustainability Appraisal Background Paper B – Baseline Data (1894, 1947, 1954, 1959, 1974, 1981,1990, 2000 & 2002/2003)
November 2013 10 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan within the lower reaches of the River Thames, however floodwaters subsided and fortunately no significant river flooding was reported within the Borough. The Environment Agency issued a flood alert for the Maidenhead area in the 2007 event. However a flood warning was not issued because the MWEFAS was working well, at the standard it was designed to.
104. It is important to highlight however that a large proportion of the flooding that occurred throughout the country during the 2007 event was as a result of surface water flooding (i.e. intense rainfall that exceeded the capacity of the existing drainage system). The Royal Borough of Windsor & Maidenhead was no exception, with flash flooding affecting town centres throughout the Borough as explained in Section 5.4.
105. Two River Thames flooding events were also seen recently in November and December 2012 partly as a result of saturated ground leading to greater runoff into rivers. The Environment Agency produced a report into the events which indicated that six properties in the Royal Borough of Windsor and Maidenhead experienced internal flooding during the November event and none in the December event. The December event was a bigger event in Windsor and Maidenhead in terms of river levels12, however, and it is likely that a similar number of properties experienced internal flooding.
106. A number of properties have also been affected historically as a result of groundwater flooding and/or failure of the underground 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 (for example) 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 (1 in 100).
107. Historical information has also been captured through the Preliminary Flood Risk Assessment (PFRA) process13. This information identifies locations of flood events that have occurred in the Borough from local sources of flooding.
108. Those areas known to have been susceptible to localised flooding in recent years (and not subsequently mitigated through the provision of an improvement scheme) have been highlighted in the adjoining flood risk maps (refer Figure B). It is important to highlight these areas as part of the SFRA as a number of these properties are situated outside of the delineated flood risk zones. These are an important reminder that the risk of flooding is not restricted purely to fluvial (river) flooding.
4.5 Detailed Hydraulic Modelling
109. A number of detailed flooding investigations have been carried out by the Environment Agency throughout the Royal Borough of Windsor & Maidenhead, including the River Thames (incorporating the Jubilee River), the River Colne, the Lower Loddon (incorporating the Lower Loddon, Old Loddon, St. Patricks Stream, River Thames (from Sonning to Marsh Lock), Twyford Brook, Billingbear Brook and Emm Brook (from Toutley Bridge to Loddon) and The Cut. 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.
110. The modelling of the River Thames was updated in January 2008 following a detailed review of the operation of the Jubilee River, the outcomes of which are now reflected in the adjoining flood maps (November 2013). The Lower Thames within Reach 3 was also updated in 2009 following reports of discrepancies in some locations of the River Thames floodplain (refer Figures A and F). The SFRA mapping also takes account of the Henley to Hurley (Thames (Henley – Hurley) Flood Mapping Study completed in September 2002 and updated with climate change runs in 2006.
12 Environment Agency Flood Event Report – Winter 2012/13, West Thames Area (February 2013) 13 RBWM Preliminary Flood Risk Assessment (May 2011)
November 2013 11 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 111. The Lower Loddon model was updated in 2009. Modelling of The Cut, which is located towards the south west of the Borough, has recently been completed and has been included within this SFRA (refer Figures A and F).
112. It should be noted that these detailed hydraulic models assume ‘typical’ conditions within the respective river systems that are being analysed. 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.
4.6 Flood Defences
113. Flood defences are typically raised structures that alter natural flow patterns and prevent floodwater from entering property in times of flooding. They are generally categorised as either ‘formal’ or ‘de facto’ defences. A ‘formal’ flood defence is a structure that is maintained by its respective owner, regardless of whether it is owned by the Environment Agency. 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.
114. Formal raised flood defences have been identified in consultation with the Environment Agency. The defences identified are located mainly on the River Thames, as highlighted in the adjoining flood maps. The main formal raised defences are as follows:
¾ Cookham Bund; ¾ North Maidenhead Bund; ¾ Datchet Golf Course Bund (PNEU School Bund); ¾ Battle Bourne; ¾ Myrke Embankments; ¾ Cookham Flood Wall
115. No de facto raised flood defences providing protection from flooding have been specifically identified in the Royal Borough of Windsor & Maidenhead as part of the SFRA process.
116. The Environment Agency has no statutory responsibility to maintain flood defences within the UK. This remains the responsibility of the riparian land owner. The EA retain ‘permissive powers’ however, and using these powers the EA carry out a programme of capital works (construction of new schemes), and the monitoring and maintenance of existing defences. Government funding is clearly finite however, and the long term structural integrity of the defences can never be fully guaranteed.
117. Within defended areas there will always be a residual risk of flooding. This may be due to an extreme event that overtops the design ‘height’ of the defence, changing climatic conditions that increases 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 incumbent on both the Council and developers to ensure that the level and integrity of defence provided within developing areas can be assured for the lifetime of the development.
4.7 Geology & Topography
118. Topographic information has been provided by the Environment Agency in the form of LiDAR. LiDAR is a detailed Digital Elevation Model (DEM) that, in simple terms, offers a three dimensional representation of the local topography. Whilst the vertical accuracy of LiDAR data is generally very good (within ±250mm in many cases), the urbanised nature of the river valleys within urban centres of the Borough may adversely affect this in Windsor & Maidenhead. Geological information has been retrieved from the British Geological Society (BGS), providing an overview of soils and substrate.
November 2013 12 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan Geology
119. The solid geology in the Royal Borough of Windsor & Maidenhead is characterised by Chalk to the north (underlying Maidenhead and surrounds), and London Clay to the south (underlying Windsor and the surrounding area). These two distinct geological features are separated by a wedge of Oldhaven, Blackheath, Woolwich and Reading and Thanet beds that broadly follows the corridor of the M4 motorway through the Borough.
120. The geology of the Borough is a very important consideration when establishing both the potential risk of groundwater flooding to, and the design of sustainable drainage systems within, a site. This is discussed further in the sections below. An overview of the Borough’s geology is provided in Figure C.
Topography
121. The topography of the Royal Borough is generally undulating with no particularly steep slopes and/or distinct river valleys. A large proportion of the Borough falls gently in a northerly direction towards the River Thames.
122. A large proportion of the properties that are at risk of flooding within the Borough are situated towards the east, within the valley of the River Thames. The undulating nature of the Borough however does introduce a potential risk of surface water flooding due to localised ponding virtually anywhere, and it is important that this is captured (and mitigated) at an early stage in the design process.
123. An overview of the Borough topography is provided in Figure D.
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5 Flood Risk in Windsor & Maidenhead
5.1 Overview
124. 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 Borough are situated along the length of the river, and not surprisingly a considerable proportion of the Borough is affected by flooding. An overview of flood risk is provided in Appendix A (SFRA flood maps) and Appendix E (key locations).
125. Significant flooding from the River Thames has affected the Borough nine times within the past 100 years, most recently in 2003. Substantial investment has been made in recent years in an endeavour to alleviate the risk of flooding, including (but not limited to) the Maidenhead, Windsor & Eton Flood Alleviation Scheme14 and the Cookham Flood Alleviation Scheme remedial works, improving the standard of protection provided to properties within the Borough.
126. It is essential to recognise that the River Thames flood defences provided do not fully remove the risk of flooding to properties within the Borough. In many areas, the standard of protection provided by the defences is less than 1% AEP (1 in 100). More importantly however, it is recognised that there is a risk to properties situated behind the defences as a result of groundwater flooding, exacerbated by high river levels. 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 Borough, including defended areas, evidenced very clearly by the events of the summer of 2007.
127. In addition to the River Thames, a risk of flooding has also been identified in association with watercourses including Colne Brook, the River Colne, The Cut, Strand Water, and White Brook (refer adjoining flood zone maps). A number of smaller local watercourses also pose a potential risk of flooding, including Bourne Ditch, the Battle Bourne, Wraysbury Drain, and Horton Drain.
128. These rivers and drains affect fewer properties within the Borough than the River Thames. They are far more susceptible to flash flooding as a result of localised intense rainfall, however, and with changing climate patterns it is expected that storms of this nature will become increasingly common. 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.
129. Finally a number of key water supply reservoirs are situated within, or immediately adjoining, the Royal Borough of Windsor and Maidenhead. These reservoirs are situated above ground, and a sudden failure of the embankments retaining the stored water would have a catastrophic affect on properties situated in the path of the resulting flood wave.
130. The reservoirs are, however, very stringently managed and monitored by Thames Water and the Environment Agency (Spitafield Reservoir), and the potential risk of failure is considered very small15. The possible failure of the underground system is in contrast a very real risk, as experienced within the 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.
131. 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 areas.
14 Incorporating the Jubilee River, the North Maidenhead Bund, and the Cookham Bund 15 Estimated by Defra to be in the order of 2 x 10-5%AEP
November 2013 14 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 132. 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 3 High Probability (i.e. at risk of river flooding), many more are potentially at risk of 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 Borough within which the safety of residents cannot be assured in times of flood.
5.2 Fluvial Flooding - Delineation of the NPPF Flood Risk Zones
133. 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 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.
134. 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 (Page 23) of the NPPF and Paragraph 7 of the Technical guidance to the NPPF. To inform the planning process, it is necessary to delineate the area into zones that depict the likelihood (or probability) that flooding will occur.
135. The Borough has been delineated into the flood zones summarised below:
Zone 3b Functional Floodplain Areas of the region susceptible to flooding within which “water has to flow or be stored in times of flood” (NPPF Technical Guidance Table 1) Zone 3a High Probability Land assessed as having a 1% (1 in 100) or greater annual probability of river flooding in any year Zone 2 Medium Probability Land assessed as having between a 1% AEP (1 in 100) and 0.1% AEP (1 in 1000) annual probability of river flooding in any year Zone 1 Low Probability Land assessed as having a less than 0.1% (1 in 1000) annual probability of river flooding in any year
136. The delineation of the NPPF flood zones is discussed below, and presented in the adjoining Flood Risk Maps.
5.2.1 Delineation of Zone 3b Functional Floodplain
137. Zone 3b Functional Floodplain is defined as those areas in which “water has to flow or be stored in times of flood”. The definition of functional floodplain remains somewhat open to subjective interpretation. The NPPF16 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 of 1 in 20 (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 Windsor & Maidenhead SFRA, Zone 3b has been defined in the following manner:
¾ land subject to flooding in the 5% AEP (20 year) flood event; ¾ land which provides a function of flood conveyance (i.e. free flow) or flood storage,
16 Table 1: Flood Zones NPPF Technical guidance
November 2013 15 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan either through natural processes, or by design (e.g. washlands and flood storage areas); ¾ 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;
138. Detailed modelled flood extents for the 5% (1 in 20) 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. In these areas where the 5% (1 in 20 years) flood event has not been identified, a precautionary approach should be applied; assuming that all of the 1% (1 in 100 years) flood extent is the functional floodplain until the extent of the 5% flood extent is adequately demonstrated.
5.2.2 Existing Development Affected by the 5% (1 in 20) Design Event - Zone 3b Developed
139. 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 however, and these must be protected.
140. It is also important to recognise that all areas within Zone 3b Developed are subject to relatively frequent flooding – on average, flooding once in every 20 years. 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.
5.2.3 Delineation of Zone 3a High Probability
141. Zone 3a High Probability is defined as those areas of the Borough that are situated below (or within) the 1% AEP (1 in 100) fluvial flood extent
142. The detailed modelling outputs developed by the Environment Agency, where available (i.e. River Thames, Henley to Hurley, River Colne, Lower Loddon and The Cut. Refer Section 4), 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.
143. The Environment Agency also produces mapping to show the areas within Flood Zone 3a that are benefitting from current defences.
144. In other areas where detailed modelling has not been completed, the delineation of the Flood Zone Maps is explained in Section 4.3 above. At these locations, 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.
5.2.4 Delineation of Zone 2 Medium Probability
145. Zone 2 Medium Probability is defined as those areas of the Borough that are situated between the 0.1% AEP (1 in 1000) and the 1% AEP (1 in 100) flood extents. In this instance, Zone 2 Medium Probability is defined in accordance with the Environment Agency Flood Zone Map.
146. 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 3 and Zone 2 flood extents is limited.
November 2013 16 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 5.2.5 Delineation of Zone 1 Low Probability
147. Zone 1 Low Probability is defined as those areas of the Borough that are situated outside of the 0.1% AEP (1 in 1000) flood extent. For SFRA purposes, this incorporates all land that is outside of the shaded Zone 2 and Zone 3 flood risk areas (as defined above).
148. It is highlighted that a small number of areas within the Borough that fall into Zone 1 Low Probability are in fact ‘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 access to medicines, food, water and utilities (i.e. electricity and telephone). It is essential that any future development within these areas considers carefully the emergency response in times of flood. To ensure that this occurs, it is recommended that all ‘dry islands’ are categorised as falling within the flood zone that encircles it for planning purposes. By way of updating the SFRA, at the earliest opportunity, the Council will produce an addendum map to the SFRA which will form an Appendix J and clearly identify the large islands that are considered to be capable of providing enough support and facilities in times of an emergency. This work will also clearly identify those smaller dry islands which should be categorised as falling within the flood zone that encircles it for planning purposes.
5.3 Assessment of Risk to Life (Flood Hazard)
5.3.1 Definition of Flood Hazard
149. The assessment of flood risk has thus far considered the maximum extent to which flooding will occur during a particular flood event. This provides the basis for assessing broadly the areas potentially impacted by flooding. Of equal importance however 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.
150. 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 directly to the depth of flow, and the speed with which the water is flowing.
151. To ensure that the risk posed by floodwaters is assessed consistently, Defra (in collaboration with the Environment Agency) has 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. The 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 additional risk identified incorporated into a ‘debris factor’ (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 2.5 then the hazard posed to life is extreme with “deep fast flowing water”, representing a danger to all.
152. This guidance should be used as part of the design process for all site based Flood Risk Assessments to ensure that the proposed development is safe under all flooding conditions. When considering safe access and egress for more vulnerable developments in Flood Zone 3a and highly vulnerable development in Flood Zone 2 a hazard rating of ‘very low’ should apply along the entirety of the route. The route should not end within a dry island area (defined within NPPF guidance) but should be located outside the 1 in 100 year with an allowance for climate change. The route should be on publicly accessible/permissible land to allow safe access and egress along the entirety of the route. The delineation of flood hazard should also be used to inform a sequential approach to the siting of development within an area, guiding vulnerable uses away from areas most at risk. If safe access and egress cannot be achieved in accordance with NPPF guidelines and a low hazard rating cannot be achieved, then new development will not be permitted unless a Flood Evacuaton Plan is submitted to and approved by the Local Planning Authority that shows the danger to site users can be appropriately managed.
November 2013 17 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 153. The SFRA has broadly considered the risks associated with river flooding throughout the Borough, and the possibility of failure of water storage facilities, as described below.
5.3.2 Flood Hazard due to River Thames Flooding
154. The speed and depth of the flow of floodwater from the River Thames in the Royal Borough of Windsor & Maidenhead is an important consideration. Deep, fast flowing water may potentially pose risk to life. This must be considered when planning future development.
155. The modelling of the River Thames throughout a relatively large proportion of Windsor & Maidenhead (i.e. upstream of Windsor) has been undertaken in one dimension, based on representative cross sections taken at defined locations across the stream. This form of modelling provides a single ‘average’ flow velocity across the river and its floodplain, and therefore is unsuitable 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% (1 in 100) design event has been carried out. This is presented in Figure F, providing a clear depiction of areas within which the risk to life may be greater than others (i.e. as a result of much deeper flood waters). The Defra guidance FD2321 referenced in Section 5.3.1 above indicates that flood depths exceeding 1.25m may represent a danger to some. This only assumes standing water, however, and this risk will increase where the water is flowing and where a debris factor should be applied. FD2320 considers ‘people vulnerability’ and reclassified still water with depths between 0.25 – 1.25m as danger to some. Whilst from a NPPF perspective these areas are all delineated as Zone 3 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.
156. It is worth noting that the likelihood of a rapid river level rise within the River Thames, resulting in the rapid inundation of urban areas within the Royal Borough of Windsor & Maidenhead is considered to be small. This is primarily due to the large River Thames system and its substantial upper contributing catchment area. It should be possible to provide timely flood warnings to those who have signed up to receive them. Whilst the Environment Agency aims to provide a warning of at least two hours, there is no guarantee that a warning will be issued with more time.
5.3.3 Flood Hazard due to Flood Defence Failure
157. A small number of raised defences have been identified within the Royal Borough of Windsor & Maidenhead, indicated on the flood maps in Appendix A, providing localised protection against fluvial flooding. Flood defences are typically raised structures that alter natural flow patterns and prevent floodwater from entering property in times of flooding.
158. 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 overbank areas behind the defence, posing a potential risk to residents, pedestrians and property that may be in the path of the floodwaters.
159. An assessment of the potential risk to life within defended areas has been undertaken. The locations adopted for analysis were selected largely on the basis of perceived risk to life should a catastrophic failure of a raised flood defence occur, i.e. focussing on urbanised areas situated immediately behind raised flood defences. In the initial instance a qualitative approach was adopted using the topography and the response of the detailed analyses as the basis for considering the approximate ‘high hazard zone’ for planning purposes.
160. The raised defences highlighted in the flood maps typically all exceed 1m in height. Given that this is the case, should (in a worst case scenario) a catastrophic structural failure of one of these raised defences occur during high water levels within the river, then a wave of flood water will rapidly inundate the area immediately behind the location of the breach. This may pose a risk to life to those who happen to be standing immediately behind the defence at the time of failure.
November 2013 18 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 161. It is highlighted that a breach failure of the River Thames defences will, 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 being considered in the current context, potentially knocking pedestrians off their feet.
162. To provide an indication of areas that may be at risk as a result of catastrophic defence failure within the Borough, however, a series of ‘danger envelopes’ have been established using the principles of FD2320 (i.e. adopting the Simple Approach17). 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 H.
163. The structural integrity of the existing flood defences is absolutely integral to the sustainability of both existing and future development in Cookham, Maidenhead, Windsor and Eton. Without the raised defences, the severity and frequency 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 likelihood and consequence of defence failure in the vicinity of the site.
5.4 Local Drainage Issues
164. 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 and 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.
165. A considerable number of known localised problems have been identified throughout the Borough, as indicated in Appendix A, highlighted as an outcome of flooding experienced by local residents or businesses. It is important to note that many have either subsequently been addressed through investment in localised flood mitigation measures to rectify the problem (e.g. culvert and/or channel improvements), or they fall within the ‘high’ risk flood zone identified in the adjoining maps. As a result, the management of localised flooding will be an integral requirement of the detailed Flood Risk Assessment (to be completed by the developer).
166. Widespread flooding affected large areas of England during the summer of 2007. Whilst there was very little river flooding within the Royal Borough of Windsor and Maidenhead during this event, there is clear anecdotal 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 (Nicholson’s Walk) was closed due to storm water flooding.
167. Within the urban centres of the Borough, it is inevitable that localised flooding problems arising from under capacity drainage and/or sewer systems will occur. Information provided by Thames Water has been included in the Borough’s PFRA setting out known and/or perceived problem areas. This information has been used for the update of the SFRA maps. The information provided is very general, and specific flooding events should be confirmed by the developer as part of a Flood Risk Assessment. Issues of this nature, in addition to those outlined above, are generally localised problems that can be addressed as part of the design process. However, careful consideration should be given to the potential impact of future intensification and/or redevelopment. It is essential to ensure that future development does not exacerbate known existing problems. Planning decisions should be made with due consideration to potential sewer capacity problems (to
17 FD2320 Flood Risk Assessment Guidance for New Development
November 2013 19 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan be advised by Thames Water as part of the statutory Borough Local Plan consultation process), and conditions should be placed upon future development to ensure that these capacity issues are rectified before development is permitted to proceed. The Infrastructure Needs Assessment prepared as evidence for the emerging Borough Local Plan also assesses likely water and sewerage demands in the Borough and examines future supply and planned investment by the statutory water and sewerage undertakers.
168. It is essential to ensure that future development does not exacerbate existing flooding problems. Strict planning conditions should be placed upon developers to ensure that best practice measures are implemented to mitigate any potential increase in loading upon existing drainage system(s).
169. The Environment Agency strongly advocates the use of Sustainable Drainage Systems (SuDS). A wide variety of SuDS techniques are available (refer Section 6.6.3), potentially providing water quality and quantity improvement as well as habitat and amenity benefits on a site by site basis throughout the Borough. Wherever possible within brownfield18 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). Collectively, the effective application of SuDS as part of all future development will assist in reducing the risk of flooding to the Borough.
170. Under the Flood and Water Management Act 2010 the Lead Local Flood Authority will be responsible for setting up the SuDS Approving Body (SAB). The SAB will be responsible for approving, adopting and maintaining Sustainable Drainage Systems. The exact requirements and detail of this is not yet known, however draft National Standards19 have been produced by Defra. It is currently anticipated that the National Standards will be finalised by April 2014 when the sustainable drainage provisions of the Flood and Water Management Act will be commenced.
5.5 Groundwater Issues
171. There is a known risk of groundwater flooding within the River Thames catchment due to the presence of ‘Thames Gravels’. This is a term commonly used to describe the highly permeable soils beneath the historical floodplain of the River Thames. During periods of high water levels in the river, the local water table within this gravel layer rises, often resulting in localised groundwater flooding to properties situated away from the direct influence of the river.
172. Equally, where flood defences have been constructed to mitigate the risk of fluvial flooding, a residual risk of groundwater flooding may remain. Groundwater could move through the Thames Gravels, driven by high water levels in the river, flooding land 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.
173. The impact of groundwater flooding can be quite severe, and it is important that planning decisions are taken in an informed manner within areas that may be susceptible. Some key characteristics of groundwater flooding are set out below20:
¾ During groundwater flooding events, property, land, roads and services are “under water” for long periods of time with significant physical, economic and social consequences. Groundwater flooding can be present for periods of up to many months. This long duration of inundation increases the level of impact compared to fluvial flooding.
¾ The damage to property is of a different type to that normally associated with fluvial flooding. Properties that are subjected to groundwater inundation for long periods can suffer damp penetration to the extent that they become structurally unsafe and
18 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. 19 National Standards for sustainable drainage systems (December 2011) 20 Strategy for Flood and Coastal Erosion Risk Management: Groundwater Flooding Scoping Study (Defra), Jacobs 2004
November 2013 20 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan require significant structural repairs or even demolition.
¾ Where groundwater flooding affects roads, it can cause deterioration to the road top surface and sub-base due to erosion and hydrostatic uplift pressures.
¾ The surcharging of sewers by groundwater can cause backwater flows of untreated sewage into properties. In homes and businesses that have cellars, this impact can be particularly damaging and unpleasant.
¾ Groundwater can exacerbate the potential risk of river flooding, increasing the base flow within the river channel, and reducing the capacity of the river to capture overland flow.
¾ With the long duration of groundwater flooding houses can become uninhabitable for long periods of time without such ground-floor facilities as kitchen, toilets, power and heating. Roads can be closed for several weeks, which prevents people from leaving their properties. Workplaces, shops, and schools may become inaccessible thereby affecting the normal way of life and means of employment.
174. The risk of groundwater flooding is highly variable and heavily dependent upon local conditions at any particular time. The Environment Agency have 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 actually likely to suffer the consequences of groundwater flooding. The data should not be interpreted as identifying areas where groundwater is actually 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 Borough is presented in Figure H. 175. It is recognised that the risks associated with groundwater flooding are not well understood, and it is important to ensure that future development is not placed at unnecessary risk. In accordance with the NPPF, all future development will require an appropriate Flood Risk Assessment (FRA) at the planning application stage, commensurate with the level of flood risk posed to the site. In those areas where a possible risk of groundwater flooding has been identified, i.e. within the Areas Susceptible to Groundwater Flooding (AStGWF) (Figure H), the FRA should consider more explicitly the localised risk of flooding to the site due to groundwater. 176. The adopted development will need to mitigate both the risk of groundwater flooding to the development itself, and the potential increase in flood risk posed to adjoining properties. The raising of thresholds and the provision of high-level access may be required to minimise the potential risk to tenants within the development. It is also important to consider the impact that groundwater may have upon the effectiveness of any adopted SuDS system.
5.6 Water Supply Infrastructure Failure
177. Thames Water21 is responsible for water supply infrastructure located within (and adjacent to) the Royal Borough of Windsor & Maidenhead which could cause flooding should any of the infrastructure fail. This infrastructure includes large water supply reservoirs and several deep large diameter pipes.
178. Thames Water was previously consulted22 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 was understood that the risk of flooding associated with
21 Three Valleys Water and South East Water also provide clean water to the Borough. 22 Thames Water was consulted during the drafting of the April 2009 Level 1 SFRA
November 2013 21 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan the failure of such reservoirs has been determined under a study sponsored by Defra. Unfortunately, the water supply reservoirs are part of the national critical infrastructure and therefore due to security reasons no specific information can be provided.
179. It is noted that the Water Act 2003 amended the Reservoirs Act 1975, requiring the preparation of dedicated Flood Plans for reservoirs. A Flood Plan is a set of documents that describe the arrangements to be put into operation in response to 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.
180. 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 Forums23 and reservoir operators to prepare reservoir flood emergency plans.
181. Dedicated Flood Plans will be required for all reservoirs that may pose a risk to the Borough of Windsor and Maidenhead within the next few years. In the interim, Thames Water has provided assurance that the water supply reservoirs are actively managed and that all required safety standards are met. Thames Water previously confirmed that there is a Reservoir Surveillance Management Process that is externally accredited via the ISO 9000 accreditation. 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 engineers. In addition to the statutory requirement, Thames Water state that it undertakes inspections of the reservoirs by trained individuals at a frequency agreed by the Supervising Panel Engineer. On this basis the possible risk of failure of these reservoirs is considered to be minimal.
182. The Flood and Water Management Act (FWMA 2010) was given Royal assent on 8 April 2010. When fully implemented it will update the Reservoirs Act 1975 and will reflect a more risk-based approach to reservoir regulation. Phase 1 of the FWMA 2010 was brought into affect on 30 July 2013. Phase 1 applies to reservoirs with a volume above 25,000 cubic metres only. The Government has not yet confirmed if the current capacity will be changed to 10,000 cubic meters or another level agreed by Ministers. Defra is likely to provide confirmation in 2015/6.
183. Issues associated with failures of large diameter underground pipe systems could result in flooding to localised areas. It is important to emphasise however that issues of this nature should not preclude development, and therefore do not affect the outcomes and/or recommendations of the SFRA process.
5.7 Climate Change
184. 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. The Technical Guidance for the NPPF states that a 10% increase in the 1% AEP (1 in 100) peak river flow can be expected within the next 10 years, increasing to 20% within the next 40 to 100 years. The recommended sensitivity ranges also include an allowance for peak rainfall intensity of 5% within the next 10 years and a 30% rise in the next 40 to 100 years.
185. Whilst present day flood extents should be used to establish flood zones at a development site (refer Appendix A) it is essential 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 EA as outlined above.
23 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.
November 2013 22 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
186. The detailed modelling of the River Thames has considered the potential impact of climate change over the next 100 years. Peak predicted water levels for the period from 2025 to 2115 are depicted as Zone 3aCC in the flood maps provided in Appendix F. In other areas (i.e. in which detailed modelling is not available), experience has shown that, in simple terms, the anticipated extent of the 1% AEP (1 in 100) flood affected area for the period from 2025 to 2115 can be approximated by the current 0.1% AEP (1 in 1000) flood outline, i.e. Zone 2 Medium Probability. Within the Royal Borough of Windsor and Maidenhead, this indicates a relatively small increase in the number of properties at risk of flooding.
187. In planning terms, present day flood maps (refer Appendix A) should form the basis of planning decisions and the application of the sequential test. However, it is essential that the Council consider their response to the potential impacts of climate change within the Borough. It is clear that climate change will not markedly increase the extent of flooding, and therefore few areas that are currently situated outside of Zone 3 High Probability will be at risk of flooding in future years. This is an important conclusion from a spatial planning perspective. Notwithstanding this, however, 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 Borough) carefully mitigates against the potential impact that climate change may have upon the risk of flooding to the property.
188. For this reason, all of the development control recommendations set out in Section 6.4 below require all floor levels, access routes, drainage systems and flood mitigation measures to 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 Borough over the next 100 years, ensuring that future development is considered in light of the possible increases in flood risk over time.
189. It is emphasised that 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 Borough. This may exacerbate localised drainage problems. The localised flooding that occurred in the summer of 2007 is an example. It is important therefore that any site based detailed Flood Risk Assessment (i.e. prepared by the developer at the planning application stage) takes due consideration of climate change (refer Appendix F) to ensure that the development does not increase, and remains protected against, flood risk, over its lifetime.
5.8 Residual Risk of Flooding
190. It is essential that the risk of flooding is minimised over the lifetime of the development in all instances. It is important to recognise however that flood risk can never be fully mitigated, and there will always be a residual risk of flooding.
191. This residual risk is associated with a number of potential risk factors including (but not limited to): ¾ a flooding event that exceeds that for which the flood risk management measures have been designed; ¾ the structural deterioration of flood defence structures (including de facto structures acting as a flood defence) over time; and/or ¾ general uncertainties inherent in the prediction of flooding.
192. The SFRA process has carried out a review of flood risk within the Borough of Windsor & Maidenhead in accordance with the NPPF Sequential Test, identifying a number of areas that fall within Zone 3a High Probability. The modelling of flood flows and flood levels is not an exact science. There are limitations in the methodologies used for prediction, and the models developed are reliant upon observed flow data for calibration, much of which is often of questionable quality. For this reason, there are inherent uncertainties in the prediction of flood levels used in the assessment and management of flood risk.
November 2013 23 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 193. It is difficult to quantify uncertainty. The adopted flood zones underpinning the Windsor & Maidenhead SFRA are based upon the detailed flood mapping within the area adjoining the River Thames. Whilst these provide a robust 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.
194. 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 clearly 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. Wherever possible floor levels should be situated a minimum of 300mm above the 1% AEP (100 year) plus climate change flood level, determined as an outcome of the site based FRA, or 600mm above the 1% AEP (100 year) flood level if no climate change data is available.
195. It is incumbent on developers to carry out a detailed Flood Risk Assessment as part of the design process. A review of uncertainty should be undertaken as an integral outcome of this more detailed investigation.
November 2013 24 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 6 Sustainable Management of Flood Risk
6.1 Overview
196. An ability to demonstrate ‘sustainability’ is a primary government objective for future development within the UK. The definition of ‘sustainability’ encompasses a number of important issues ranging broadly from the environment (i.e. minimising the impact upon the natural environment) to energy consumption (i.e. seeking alternative sources of energy to avoid the depletion of natural resources). Of particular importance however is sustainable development within flood affected areas.
197. A considerable number of people live and work within areas that are susceptible to flooding, and ideally development should be moved away from these areas over time. It is recognised however that this is often not a practicable solution. For this reason, careful consideration must be taken of the measures that can be put into place to minimise the risk to property and life posed by flooding. These should address the flood risk not only in the short term, but throughout the lifetime of the proposed development. This is a requirement of the NPPF.
198. The primary purpose of the SFRA is to inform decision making as part of the planning and development control process, taking due consideration of the scale and nature of flood risk affecting the Borough. Responsibility for flood risk management resides with all tiers of government, and indeed individual landowners.
6.2 Responsibility for Flood Risk Management
6.2.1 Responsibilities
199. There is no statutory requirement for the Government to protect property against the risk of flooding. Notwithstanding this however, the Government recognises the importance of safeguarding the wider community, and in doing so the economic and social well-being of the nation. An overview of key responsibilities with respect to flood risk management is provided below.
200. The Environment Agency has a statutory responsibility for flood management and defence in England. It assists the planning and development control process through the provision of information and advice regarding flood risk and flooding related issues.
201. The Local Planning Authority is responsible for carrying out a Strategic Flood Risk Assessment. The SFRA should consider the risk of flooding throughout the district and should inform the allocation of land for future development, development control policies and sustainability appraisals. Local Planning Authorities have a responsibility to consult with the Environment Agency when making planning decisions.
202. The Lead Local Flood Authority (LLFA) was introduced through the Flood and Water Management Act (FWMA) 2010 and is defined as the Unitary or County Council. LLFAs are responsible for managing local flood risk and have been given new duties under the FWMA which include the development of a Local Flood Risk Management Strategy and the Preliminary Flood Risk Assessment.
203. Landowners & Developers24 have the primary responsibility for protecting their land against the risk of flooding. They are also responsible for managing the drainage of their land such that they do not adversely impact upon adjoining properties.
204. The Environment Agency has developed a guide entitled “Living on the Edge” (2013, 4th edition) that provides specific advice regarding the rights and responsibilities of property owners, the Environment Agency and other bodies. The guide is targeted at owners of land situated alongside rivers or other watercourses, and is a useful reference point outlining who is responsible for flood defence, and what this means in practical terms. It
24 Referred to also as ‘landowners’ within NPPF
November 2013 25 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan also discusses how stakeholders can work collaboratively to protect and enhance the natural environment of our rivers and streams. This guide can be found on the Environment Agency’s website at www.environment-agency.gov.uk.
6.3 Strategic Flood Risk Management - The Environment Agency
6.3.1 Overview
205. With the progressive development of urban areas along river corridors, particularly during the industrial era, a reactive approach to flood risk management evolved. As flooding occurred, walls or embankments were built to prevent inundation to developing areas. Needless to say, construction of such walls should be carefully assessed so that it does not result in the redistribution of floodwater, inadvertently increasing the risk of flooding elsewhere.
206. The Environment Agency (EA) in more recent years has taken a strategic approach to flood risk management. The assessment and management of flood risk is carried out on a ‘whole of catchment’ basis. This enables the Environment Agency to review the impact that proposed defence works at a particular location may have upon flooding at other locations throughout the catchment.
207. A number of flood risk management strategies are underway within the region, encompassing many of the large river systems that influence flood risk within the Royal Borough of Windsor & Maidenhead. A brief overview of these investigations is provided below.
6.3.2 Catchment Flood Management Plan (CFMP)
208. “One of the Environment Agency’s main goals is to reduce flood risk from rivers and the sea to people, property and the natural environment by supporting and implementing government policies.
209. Flooding is a natural process – we can never stop it happening altogether. So tackling flooding is more than just defending against floods. It means understanding the complex causes of flooding and taking co-ordinated action on every front in partnership with others to reduce flood risk by:
¾ Understanding current and future flood risk; ¾ Planning for the likely impacts of climate change; ¾ Preventing inappropriate development in flood risk areas; ¾ Delivering more sustainable measures to reduce flood risk; ¾ Exploring the wider opportunities to reduce the sources of flood risk, including changes in land use and land management practices and the use of sustainable drainage systems.
210. Catchment Flood Management Plans (CFMPs) are a planning tool through which the Agency aims to work in partnership with other key decision-makers within a river catchment to explore and define long term sustainable policies for flood risk management. CFMPs are a learning process to support an integrated approach to land use planning and management, and also River Basin Management Plans under the Water Framework Directive.”25
211. The flood risk regime within the Royal Borough of Windsor & Maidenhead is heavily influenced by the River Thames. The Thames system is under careful consideration by the Environment Agency, and resources are currently being targeted at a strategic level to ensure that the nature and severity of flood risk throughout the wider greater London area is broadly understood. This will enable the Environment Agency, responsible for the future management of flood risk within the area, to target future activities in a cost effective and sustainable manner.
25 Catchment Flood Management Plans – Volume 1 (Guidance), Version 1.0, July 2004, Preface
November 2013 26 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 212. A CFMP has been developed for the River Thames catchment. There are two main policy units relevant to the Royal Borough:
x Windsor and Maidenhead policy unit which covers the Thames floodplain from Cookham to Windsor, x the Addlestone Bourne, Cut and Emm Brook policy unit which relates to The Cut which joins the Thames in the Royal Borough.
The key messages applicable to the Royal Borough from the Windsor and Maidenhead CFMP policy unit are:
x “Redevelopment rates in some areas are very high and offer the opportunity to reduce the risk and the current reliance on flood defences. This includes making the urban environment more resilient and with a layout that offers more options for managing future flood risk and the impacts of climate change. x Generally the existing river corridors in these areas provide an opportunity to be able to adapt to the impacts of climate change and we are seeking to safeguard them from inappropriate development. We are seeking to maintain existing assets at least until redevelopment takes place. x Climate change will mean that we need to adapt the existing defences over time. Rather than replacing them like for like, we will be seeking a different combination of flood storage, river defences and floodplain attenuation. Some of these areas are susceptible to rapid flooding from thunderstorms. Emergency response and flood awareness are particularly important.”26
The implications of this approach in the Royal Borough are:
x “Maintain the integrity of the Maidenhead, Windsor and Eton Flood Alleviation Scheme. x Implement measures to manage the possible consequences of flooding. This extends beyond preventing inappropriate development in the floodplain, to adaptation and resilience measures. x Preserve and manage the undeveloped functioning floodplain for the benefit of existing areas of development. x Consider the future maintenance of the watercourses and alternative options to restore the river and floodplain to a sustainable and more natural state. This supports Water Framework Directive objectives and the strategic opportunities that have been identified to improve both the channel and surrounding floodplain environment.”27
The key messages applicable to the Borough from the Addlestone Bourne, Cut and Emm Brook CFMP policy unit are:
x “The location, layout and design of developments – in that order – are the most vital factors in managing future flood risk so that past mistakes are not repeated. Regeneration and re-development of some areas offers an opportunity to reduce flood risk; for example re-establishing river corridors and more effective management of run-off x We want the rivers to be part of the urban landscape in these areas – at present they are often culverted and hidden away. Previous modifications to these watercourses now cause some flooding. We want to learn from previous decisions x Flooding in these locations tends to arise from a number of sources and can be quite localised. Engineering interventions will tend to rely on opportunity; either to increase the conveyance of the watercourses by modifying or removing obstructions to flow or attenuating water at a local scale x These areas are susceptible to rapid flooding from thunderstorms.”28
26 Thames Catchment Flood Management Plan (July 2008), Chapter 6, p446 27 Thames Catchment Flood Management Plan (July 2008), Chapter 6, p450-1 28 Thames Catchment Flood Management Plan (July 2008), Chapter 6, p409
November 2013 27 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 213. The CFMP encourages local authorities (and indeed developers) to strive for a positive reduction in flood risk through future development and regeneration. This is striving to ensure that collectively decisions taken not only avoid the creation of a future legacy of new development at risk of flooding, but also progressively reduces the risk of flooding to existing development. This is a key objective of the NPPF.
6.3.3 The Lower Thames Flood Alleviation Scheme
214. The Lower Thames Flood Alleviation Scheme29, previously known as the Lower Thames Strategy, is being carried out by Halcrow and Jacobs on behalf of the EA, triggered as an outcome of the widespread flooding experienced within the catchment in 2003. The initial phase of the investigation was completed in 2005, considering the management of flood risk from the River Thames between Datchet and Walton Bridge. A subsequent phase has since been considered, reviewing the reach extending from Walton Bridge to Teddington.
215. The Scheme has investigated a number of large-scale engineering solutions, community based measures and non-structural options to mitigate the risk to urban areas as a result of flooding from the River Thames. The engineering solutions considered included flood walls, flood storage, channel improvements (i.e. widening and/or deepening of the river channel), and the construction of new flood relief channels.
216. A public consultation of the Lower Thames Strategy was undertaken from September 2009 to December 2009 which included the formal consultation and public exhibitions of the options. Following this consultation the Lower Thames Strategic Appraisal Report was published providing information on the options considered and information on the recommended strategy. This strategy comprises a number of non-technical community based projects such as enhanced mapping for emergencies, community defences and property level protection.
217. To ensure that there is no increase in downstream flood risk during construction of the channel, enabling works will be required to improve capacity. A small increase in capacity is required to Sunbury, Teddington and Mosely weirs. An increase in capacity at Desborough Cut is also required which will be achieved by channel widening or deepening.
218. The structural elements of the Scheme comprise three channels between Datchet and Shepperton and the widening of the Desborough Cut. The whole of channel 1 is located within the Royal Borough of Windsor and Maidenhead. The scheme also includes for the on-going maintenance and replacement of the Thames Weirs.
219. There are two phases of scheme delivery; phase 1 includes the non-structural element and the undertaking of further survey and investigations. These are currently being undertaken as is the Property Level Protection part of the project. The next phase includes the engineering components of the project which includes all three channels. This has yet to be developed or undertaken.
220. The final stage of the study will be to prepare a final strategy document recommending the preferred options to manage flood risk in the study area. The timing of the Scheme is unclear at the time of writing (November 2013) due to funding limitations. It is important to recognise, however, that the intention of the study is not to reduce flood risk in order to make way for future development. It is also unlikely that the physical management measures identified will be in operation within foreseeable planning timeframes.
221. For this reason, the SFRA has not taken the potential flood risk reduction measures into account in this instance. Within future planning horizons, however, the revision of the SFRA should review the status of schemes recommended as an outcome of The Lower Thames Flood Alleviation Scheme, and consider the potential impact that these may have had upon flood risk within the Borough.
29 http://www.environment-agency.gov.uk/homeandleisure/floods/123097.aspx
November 2013 28 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
6.3.4 The Jubilee River
222. The Jubilee River is a flood alleviation channel built as part of the Maidenhead, Windsor & Eton Flood Alleviation Scheme (MWEFAS). The river runs for a length of 11.6km, leaving the River Thames upstream of Boulters weir, controlled by a new structure (Taplow weir), and re-joining the natural Thames immediately upstream of Datchet. Construction was undertaken between 1996 and 2002, and the channel was first used to alleviate flooding during the 2002-2003 flood event.
223. During the 2002-03 flood event, severe erosion of the Jubilee River banks was experienced. Manor Farm weir suffered structural failures whilst Slough Road weir also suffered damage. Subsequent improved modelling of the Jubilee River and the River Thames indicated that the maximum capacity of the channel was less than the original design capacity. Subsequent repair and improvement works have since been undertaken.
224. The completion of these works has resulted in the current maximum capacity of the Jubilee River being estimated at 180m3/s. Collectively therefore (i.e. incorporating also the capacity of the River Thames main channel), the MWEFAS system therefore offers a total capacity that is broadly equivalent to the 4% (25 year) fluvial flood event. In other words, the MWEFAS provides a 4% AEP (25 year) standard of protection to properties situated within Maidenhead, Windsor and Eton.
6.3.5 Cookham Flood Alleviation Scheme
225. This scheme was completed by the Environment Agency in 2009 and includes a raised bund across Cookham Marsh, and the lowering of the sill levels at the Cookham (Odney) weir. This alleviates flooding in the Strande Lane area of Cookham.
6.3.6 Other Schemes
226. Capital works were recently completed by the EA as part of the Lower Colne Flood Alleviation Scheme, reducing the risk of flooding to Horton.
6.3.7 Green Infrastructure
227. The concept of Green Infrastructure applies to areas of green open space that can be defined as a multifunctional resource with the potential to be used for flood storage purposes as well as amenity benefits within urban areas. Extensive green areas can also assist in mitigating against the effects of climate change, through reducing temperatures within localised micro climates. Green Infrastructure should be viewed as a positive solution to mitigating against flood risk in areas that are prone to extensive fluvial and tidal risk.
228. Green Infrastructure can provide an effective means of mitigating against the risk of flooding and making space for water. These areas of green open space should be linked to other areas of green open space where possible to increase biodiversity opportunities and provide green corridors within urban environments.
229. Flood management infrastructure, including SuDS, should be retained and maintained primarily for the purpose for which they were designed, whilst being sensitive to the multi- functional benefits they can provide.
6.4 Application of the NPPF within Windsor & Maidenhead
6.4.1 Planning Solutions to Flood Risk Management The Sequential Test
230. Historically, urbanisation has evolved along river corridors due to the rivers providing a critical source of water, food and energy. This leaves many areas of England with a
November 2013 29 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan legacy of urban centres that, because of their close proximity to rivers, are at risk of flooding. 231. The ideal solution to effective and sustainable flood risk management is a planning led one, i.e. steer urban 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 seek 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 Technical Guidance to the NPPF (March 2012).
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 Technical Guidance to the NPPF, 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 Royal Borough has undertaken a separate Sequential Testing Report in 2013 that outlines the methodology and presents the results of their Sequential Test.
232. 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 must restrict development to the permissible land uses summarised in the Technical Guidance Note to the NPPF (Table 2), duplicated in Appendix G for ease of reference. This may involve seeking opportunities to ‘swap’ more vulnerable allocations at risk of flooding with areas of lesser vulnerability that are situated on higher ground. 233. 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. The detailed FRA will be required to demonstrate the careful and measured consideration of whether indeed there is an alternative site available within an area of lesser flood risk, in accordance with the NPPF Sequential Test30. 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. 234. The Royal Borough of Windsor & Maidenhead SFRA has been developed to inform the Sequential Test. The Royal Borough has prepared a Sequential Testing report on the basis of the information contained in the updated 2013 SFRA, allocating potential sites for future development accordingly. Developers proposing sites in Zone 3 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’. 235. 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
236. A relatively large proportion of the Borough is affected by Zone 3, including the major urban and retail centres of Maidenhead and Windsor. Prohibiting future development
30 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
November 2013 30 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan 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. It may be however that pressing planning arguments (that outweigh flood risk) remain, putting into place a requirement to investigate further the possibility of regeneration and/or future development within areas at risk of flooding. 237. 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: ¾ “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 ¾ 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
238. 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 in the initial instance, 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. 239. The management of flood risk throughout the 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 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. 240. An overview of flood risk throughout the district has been provided in Appendix A and Appendix F. Future planning decisions should consider the spatial variation in flood risk across the district, 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 Appendix F) to ensure that the development does not increase, and remains protected against, flood risk, over its lifetime.
6.4.2 A Proactive Approach – Positive Reduction of Flood Risk through Development
241. It is crucial to reiterate that the NPPF considers not only the risk of flooding posed to new development. It also seeks to positively reduce the risk of flooding posed to existing properties within the district. It is strongly recommended that this principle be adopted as the underlying ‘goal’ for developers and Council development control teams within the Royal Borough of Windsor & Maidenhead. 242. Developers should be encouraged to demonstrate that their proposal will deliver a positive reduction in flood risk to the Borough, 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). This should not be seen as an onerous requirement, and indeed if integrated into the design at the conceptual stage, will place no added demands upon the development and/or planning application process. 243. Possible risk reduction measures for consideration may include the following: ¾ The integration of SuDS to reduce the runoff volume and rate from the site; ¾ A change in land use to reduce the vulnerability of the proposed development; ¾ A reduction in the building platform area and intensity of use. This is to prevent
November 2013 31 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan intensification through the addition of storeys (or other conversion) within the same footprint; ¾ 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; ¾ The rearrangement of buildings within the site to remove obstructions to overland flow paths. This is to ensure that water does not pond and cause localised flooding; ¾ 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; and ¾ Risk reduction should also be linked to the specific messages and objectives that have emerged from the Thames CFMP.
244. 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. 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.
6.4.3 Localised Flood Risk within the Planning Process
245. The NPPF states that Local Planning Authorities should, when allocating land, apply a sequential approach, taking into consideration all sources of flooding. The local drainage related problems identified within the Borough are generally localised, and relate to historical incidents, the source of which is often somewhat uncertain. It is important to recognise that these are not a measure of ‘risk’, but rather problems that may have occurred due to a particular set of local circumstances in the past (for example, the blockage of a local gully inlet). These may or may not reoccur in future years. 246. From a spatial planning perspective, it is considered unreasonable to restrict future development within areas that may have suffered a localised flooding incident in years past. It is essential, though, not to overlook the potential risk of localised flooding during the design process. Whilst the incidents that have been identified may typically not result in widespread damage or disruption, a proactive approach to risk reduction through design can mitigate the potential for damage, both to the development itself and elsewhere. The NPPF, and advice from the Environment Agency, says it is for the site-specific FRA to demonstrate whether a site is acceptable or not within a localised flood area. Specific development control recommendations have been provided accordingly.
November 2013 32 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
6.4.4 Spatial Planning & Development Control Recommendations
NPPF Flood Zone
Zone 3a High Probability NPPF Requirement
Zone 3b Developed Zone 3b Functional Floodplain Defended Areas (highlighted as 'Areas Zone 2 Medium Probability Zone 1Low Probability Undefended Areas Benefitting from Defences' in the adjoining Flood Zone Maps)
SPATIAL PLANNING RECOMMENDATIONS
It is important to recognise that Zone 3b Developed relates solely to the footprint of existing solid buildings that are impermeable to flood water. The land surrounding these buildings are important flow paths and/or flood storage areas that must be retained. Future compatible development within this Zone can only be considered following the It is important to recognise that sites within undertaking, and passing, of the Sequential Test and where necessary the Exception Test. Developers must seek to Future development within Zone 2 Medium Zone 1may be susceptible to flooding from proactively reduce the risk of flooding within Zone 3b Developed by (for example) reducing the building footprint, Future development within Zone 3a High Probability can only be considered following the Probability can only be considered following Important Considerations other sources. Development may contribute increasing the resilience of buildings to flooding, and using Sustainable Drainage techniques. application of the Sequential Test, and where necessary the Exception Test application of the Sequential Test, and where to an increase in flood risk elsewhere if not necessary the Exception Test It should be recognised that property situated within this zone will be subject to frequent flooding, on average, no less carefully mitigated than once in every 20 years. There are clear sustainability implications to be considered in this regard, and it is highly questionable whether insurance against flooding related damages will be available in the longer term.
Proactively seek a reduction in risk by reducing the vulnerability of the existing land use. Water-compatible development is compatible with this Land use should be restricted to Water Water-compatible development is Zone subject to passing the Sequential Test. Essential infrastructure is Compatible, Essential Infrastructure, Less compatible with this Zone if it passes the Land use should be restricted to Water Compatible or Less Vulnerable development. Land Use (refer Table 3 in the compatible if it passes the Sequential Test and Exception Test. Less Vulnerable or More Vulnerable development. Sequential Test. Essential infrastructure is Essential Infrastructure and More Vulnerable development may only be considered if the No restrictions Technical Guidance to NPPF) vulnerable and More Vulnerable development is not appropriate in this Highly Vulnerable development may only be compatible if it passes the Sequential Test Sequential Test and Exception Test can be passed. Zone unless it utilises the footprint of existing solid buildings that are considered if the Sequential Test and and Exception Test. impermeable to flood water and is not an increase in vulnerability over the Exception Test can be passed existing use.
Permitted Development & Property Property subdivision may increase the intensity of development and the popultation at risk, and should not be permitted N/A N/A Subdivision
DEVELOPMENT CONTROL RECOMMENDATIONS
Required for all sites greater than 1ha in area, and where the site may be subject to other Detailed Flood Risk Assessment (FRA) Required (refer Section 6.6.1) Required (refer Section 6.6.1) Required (refer Section 6.6.1) Required (refer Section 6.6.1) Required (refer Section 6.6.1) non-fluvial sources of flooding or where there may be drainage problems.
Floor Level To be situated a minimum of 300mm above the 1in 100 year river flood level, including climate change No minimum level stipulated by NPPF
For residential property, dry access is to be provided in the 1 in 100 year river To ensure the safety of residents and employees during a flood, access and egress routes must be designed to meet Environment Agency flood with an appropriate allowance for the potential impacts of climate Site Access & Egress N/A defined criteria, as set out in Appendix C. It is essential to ensure that the nominated evacuation route does not divert evacuees onto a ‘dry No minimum level stipulated by NPPF change. For commercial property, access must be 'safe' in accordance island’ upon which essential supplies (i.e. food, shelter and medical treatment) will not be available for the duration of the flood event. with Defra "Flood Risk to People" (FD2320 & FD2321)
No habitable uses permitted at basement level. All basements must have an access point that Basements Not permitted No restrictions No restrictions is above the 1in 100 year river flood level, including climate change
Implement SuDS to ensure that the rate and volume of runoff from the site (post redevelopment) is equivalent to Greenfield conditions. Where it is adequately demonstrated that this is not feasible on Brownfield sites, the Greenfield conditions should be used as the starting point for identifying the next best option Site Runoff (ensuring no increase over existing conditions). Any SuDS design must take due account of groundwater and geological conditions (refer Section 6.6.3)
A minimum 8m buffer zone must be provided to ‘top of bank’ within sites immediately adjoining a river corridor. This relates to both open waterways and culverted waterway corridors. Reference should be made to the Environment Agency's "Living on the Edge" guide (www.environment-agency.gov.uk) that Buffer Zone discusses any development situated in, over, under or adjacent to rivers and/or streams.
Ensure that the proposed development does not result in an increase in maximum flood levels within adjoining properties. This may be achieved by ensuring (for example) that the existing building footprint is not increased, that overland flow routes are not truncated by buildings and/or infrastructure, or hydraulically linked compensatory flood storage is provided within the site (or upstream)
Where existing buildings are being retained, seek that their refurbishment increases their resilience / resistance to flooding
Other Identify opportunities to recreate river corridors and wetland habitats in urban areas. Encourage new development and any redevelopment of these areas to acknowledge these opportunities in their site layouts and set development back, allowing space for water, habitat, wildlife and recreation. Encourage partners to assess the viability of future land swapping opportunities in those areas where there is a risk of flooding
As an integral part of the government’s “Making Space for Water” agenda, the Environment Agency is actively seeking the renaturalisation of culverted watercourses as part of any future development. Realistic opportunities to reinstate the natural open waterway within existing culverted reaches of the river(s) should be promoted.
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6.5 SFRA Interpretation
247. The spatial variation in flood risk across the district is depicted in Appendix A and Appendix F, and described in Section 5. The Windsor & Maidenhead SFRA should be used by both the Council and prospective developers to assist them to meet their obligations under the NPPF throughout the planning cycle, including the delivery of a detailed site-based FRA. Instructions for use are provided below:
Royal Borough of Windsor & Maidenhead Council (Forward Planning)
248. The SFRA flood maps in Appendix A provide an overview of the spatial variation in fluvial flood risk throughout the Borough (i.e. the risk of flooding from rivers), based upon current climate predictions. It is necessary to adopt a sequential approach when considering where land should be allocated for future development, and this is described in Section 6.4. These Figures should be used to inform this sequential approach. Furthermore, the NPPF provides clear guidance on compatible land use within areas potentially at risk from flooding, and this too is discussed in Section 6.4. 249. To further inform the allocation of sites within areas affected by flooding from the River Thames, an appraisal of flood depth in the 1% (1 in 100) design event has been carried out. This is provided in Figure F. The Council should use this information to apply a sequential approach for sites located entirely within Zone 3a, steering development towards areas of lesser depth (and hence lesser risk). 250. Whilst there is no particular constraint placed upon land use within areas of Zone 1 Low Probability within the Borough, it is strongly recommended that the Council takes due consideration of flooding from other sources (i.e. non fluvial). Observed incidents of localised flooding are provided in Appendix A, and these should be used to inform design to ensure that future development does not exacerbate these existing problems. 251. Many of these localised sources of flooding can be effectively managed through the design process (see Section 6.4.4). However, it is recommended that advice is taken from the Environment Agency to ensure that the severity of the local issue that may affect (or be exacerbated by) the proposed allocation is fully appreciated. 252. RBWM in its role as Lead Local Flood Authority (LLFA) has carried out a Preliminary Flood Risk Assessment (PFRA) and reviewed the surface water flood mapping produced by the EA. These maps show the areas that are susceptible to surface water flooding and should be considered within the preparation of any site-specific FRA and the design of proposed development. These maps are known as the Flood Map for Surface Water (FMfSW) (Refer Figure I).
RBWM Council (Development Control) & Developers
253. All development applications should consider the need for a further, more detailed assessment, of flood risk. In accordance with Section 6.6.1 of this report, all sites situated within Zone 2 or Zone 3, and sites greater than 1ha within Zone 1, require a detailed Flood Risk Assessment. A Flood Risk Assessment will also be required where the proposed development or change of use to a more vulnerable class may be subject to other sources of flooding, such as surface water flooding, or where there may be drainage problems. 254. The SFRA flood maps provided in Appendix A and Appendix F summarise the extent of flooding (from rivers) across the site, highlighting the zone within which the proposed development site will fall. These should be used to trigger a more detailed assessment of flood risk related issues within the site, as described in Section 6.4 and Section 6.6. 255. The assessment of flood related issues is imperative for all proposed development, irrespective of its location and/or scale within the district, and the SFRA provides some helpful tools to assist in this regard. It is imperative that the information outlined below is used with careful reference to the discussion and guidance provided in Sections 5 and 6 of this report. ¾ The risk of flooding to the site from rivers is summarised in the SFRA flood
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maps in Appendix A, and Appendix F. ¾ The locality of formal flood defences, and the areas that benefit from the presence of these defences, are presented in Appendix A. 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. Available flood warning services throughout the district are described in Section 6.8. ¾ Figure D offers a broad indication of the topography of the 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. Areas that may be susceptible to localised ponding are also evident. ¾ The SFRA flood maps in Appendix A, and the discussions in Appendix E, 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(s). 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 the NPPF flood zones. Recorded accounts of historical flooding from all sources are provided in Figure B. Areas that may be acceptable to surface water flooding can be reviewed in the Royal Borough’s PFRA, with the data available from the EA. ¾ Within all areas of the 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 district is provided in Figure C. ¾ Appendix D provides the current Interim National Guidance for developers for Rainfall Runoff Management. This guidance has been provided by the Environment Agency (June 2008) and will assist developers to design the drainage system for their site. ¾ Appendix C provides clear guidance for developers to ensure that safe access and egress can be provided to/from the site to address the residual risk of flooding
6.5.1 The NPPF Constraints upon Potential Site Allocations
Housing and Employment
256. A review of potential sites which may be considered for housing and employment allocations in the emerging Borough Local Plan has been carried out. There are currently 89 in total. The sites within Strategic Housing Land Availability Assessment (SHLAA) dated 2011 and the Employment Land Review (ELR) dated 2009 have also been reviewed to understand which flood zone each site falls within.
257. A ‘traffic light’ system has been adopted to identify, in accordance with the NPPF, suitable land uses (on purely flood risk related grounds) for future development. Should the proposed land use differ from that identified, then careful consideration must be given to either deleting the potential allocation or modifying the proposed land use to reduce the vulnerability of the development to flood risk.
Minerals and Waste
258. For information, existing sites allocated in either the Berkshire Minerals and Waste Local Plans have been overlaid onto the adopted NPPF flood zones to identify any flood risk related constraints that may influence future planning decisions (i.e. with due consideration to Table 1 and 2 of the NPPF). A summary of findings is provided in the table below.
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Site NPPF Flood Zone NPPF Constraint
Hindhay Quarry Zone 1 Low Probability No flooding related constraints (Pinkneys Green) No flooding related constraints, however Zone 2 Medium Braywick (Maidenhead) installations requiring hazardous substances Probability consent should not be permitted Small area within flood zone 3b: Suitable for sand & gravel workings only. Other minerals working & processing, and waste management (and/or Zone 3b Functional Monkey Island (Bray) storage) facilities, should not be permitted within Floodplain this flood zone. However the majority of the site is within Flood zone 2.
Partly in Flood Zone 3a, 2 and 1. The site is mostly in Flood Zone 1 and 2. In FZ3a installations Riding Court Farm Zone 3a Medium requiring hazardous substance consent are not (Datchet) Probability permitted. Waste management facilities and for hazardous waste require the exception test. The site is mostly in Flood Zone 1/2. In FZ3a installations requiring hazardous substance consent Site north of Horton Zone 3a Low Probability are not permitted. Waste management facilities and for hazardous waste require the exception test. Suitable for sand & gravel workings only. Other Railways land, Flood Zone 3b Functional minerals working & processing, and waste Kingsmead, Horton Floodplain management (and/or storage) facilities, should not be permitted
6.6 Detailed Flood Risk Assessment (FRA) – The Developer
6.6.1 Scope of the Detailed Flood Risk Assessment
259. Once the Sequential Test has been applied in accordance with Section 6.4 and the table within Section 6.4.4 to determine the allocation of sites for future development, it is imperative that a site-based Flood Risk Assessment (FRA) is carried out by the developer for all proposed developments. This should be submitted as an integral part of the planning application. It is emphasised that, for windfall sites (and any allocated sites that have not been sequentially tested), it will be necessary for the developer to demonstrate that the Sequential Test has been applied (in accordance with the NPPF) within the detailed FRA. 260. 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 (e.g. Zone 1 Low Probability; the site is not at risk of other sources of flooding; and there are no drainage problems identified), 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. The particular requirements for FRAs within each delineated flood zone are outlined below.
The detailed FRA should utilise the background information provided within this SFRA, as explained in Section 6.5. It is important to reiterate that the SFRA provides the best available information at the time of writing (November 2013). As
highlighted below, the Environment Agency is an excellent source of information to inform the development of the detailed FRA, and they should be contacted as early as possible to source additional (more recent) information as appropriate.
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Proposed Development within Zone 3a High Probability, Zone 3b Developed & Zone 3b Functional Floodplain
261. All FRAs supporting proposed development within Zone 3b Functional Floodplain, Zone 3b Developed and Zone 3a High Probability should include an assessment to meet the requirements set out in Table 6.4.4 including an assessment of the following. ¾ The vulnerability of the development to flooding from other sources (e.g. surface water drainage) as well as from river flooding. In addition to the use of information provided within the SFRA, this will involve discussion with the Council and the Environment Agency to confirm whether a localised risk of flooding exists at the proposed site. Specific guidance is provided in Section 6 for the assessment of flood risk from other sources. ¾ The vulnerability of the development to flooding over the lifetime of the development (including the potential impacts of climate change) for all sources of flooding, i.e. maximum water levels, flow paths and flood extents within the property and surrounding area. The Environment Agency may have carried out detailed flood risk mapping (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. Where detailed modelling is not available, modelling by suitably qualified engineers will be required, proportionate to the nature and scale of the development, to determine the risk of flooding to the site. Residual risks such as the propensity of culverted systems to block, increasing the risk of flooding, should be considered. ¾ The presence of both formal and de-facto (including, for example, local road and/or rail embankments) flood defences within the proximity of the site must be considered. Flood defences may alter the risk of flooding within the site, and it is imperative that any change in the flooding regime as a result of a flood defence is thoroughly understood. The integrity of the defence must be assessed to ensure that the defence will be structurally sound throughout the lifetime of the proposed development. The potential impact of a defence failure must be considered. It should be remembered that a flood defence that provides protection against a small event, say 1 in 20, does not necessarily also provide protection against a larger event. ¾ 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 Borough. Future development within the district may adversely affect sites within adjoining boroughs, and it is essential that this is mitigated. ¾ A demonstration that residual risks of flooding (after existing and proposed flood management and mitigation measures are taken into account) are acceptable. Measures may include flood defences, flood resistant and resilient design, provision for escape/evacuation (refer Appendix C), effective flood warning and emergency planning. ¾ Details of existing site levels, proposed site levels and proposed ground floor levels. All levels should be stated relevant to Ordnance Datum. ¾ Details of proposed sustainable drainage systems (SuDS) that will be implemented to ensure that runoff from the site (post redevelopment) does not exceed greenfield runoff rates and volumes. Where it is adequately demonstrated that this is not feasible on brownfield sites, the greenfield conditions should be used as the starting point for identifying the next best option (ensuring no increase over existing conditions). Any SuDS design must take due account of topographical, groundwater and geological conditions (refer Section 6.6.3). ¾ The developer must provide a clear and concise statement summarising how the proposed (re)development has contributed to a positive reduction in flood risk
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within the district;
Proposed Development within Zone 2 Medium Probability
262. For all sites within Zone 2 Medium Probability, a high level FRA commensurate with the level of risk posed to the site should be prepared based upon readily available existing flooding information, sourced from the EA. All FRAs supporting proposed development within Zone 2 Medium Probability should include an assessment to meet the requirements set out in Table 6.4.4 including an assessment of the following. ¾ It will be necessary to demonstrate that the residual risk of flooding to the property is effectively managed through, for example, the provision of raised floor levels (refer Section 6.6.2) and the provision of a planned evacuation route and/or safe haven (refer Appendix C). ¾ The risk of alternative sources of flooding (e.g. urban drainage and/or groundwater) must be considered, and sustainable drainage techniques must be employed to ensure no worsening to existing flooding problems elsewhere within the area. Once again, it is reiterated that future development within the district may adversely affect sites within adjoining boroughs, and it is essential that this is mitigated. ¾ As part of the high level FRA, the developer must provide a clear and concise statement summarising how the proposed (re)development has contributed to a positive reduction in flood risk within the district. ¾ Details of proposed sustainable drainage systems (SuDS) that will be implemented to ensure that runoff from the site (post redevelopment) does not exceed greenfield runoff rates and volumes. Where it is adequately demonstrated that this is not feasible on brownfield sites, the greenfield conditions should be used as the starting point for identifying the next best option (ensuring no increase over existing conditions). Any SuDS design must take due account of topographical, groundwater and geological conditions (refer Section 6.6.3).
Proposed Development within Zone 1 Low Probability
263. For all sites greater than 1 hectare in area, and sites that may be at risk of other non- main river flooding or where there may be drainage problems, a simple Flood Risk Assessment must be prepared: ¾ The risk of alternative sources of flooding (e.g. urban drainage and/or groundwater) must be considered, and sustainable drainage techniques must be employed to ensure no worsening to existing flooding problems elsewhere within the area. Once again, it is reiterated that future development within the district may adversely affect sites within adjoining boroughs, and it is essential that this is mitigated. ¾ As part of the high level FRA, the developer must provide a clear and concise statement summarising how the proposed (re)development has contributed to a positive reduction in flood risk within the Borough. ¾ Details of proposed sustainable drainage systems (SuDS) that will be implemented to ensure that runoff from the site (post redevelopment) does not exceed greenfield runoff rates and volumes. Any SuDS design must take due account of topographical, groundwater and geological conditions (refer Section 6.6.3). Liaison with the Environment Agency
264. 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.environment-agency.gov.uk. 265. 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
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possible to source information relating to (for example) historical flooding, hydraulic modelling and topography (LiDAR). It is emphasised that the information provided within the SFRA is the best available at the time of writing (November 2013). More up to date information may be available, and contact should always be made with the EA at an early stage to ensure that the detailed site based FRA is using the most current datasets, avoiding unnecessary re-work. 266. It is strongly recommended that a draft of the detailed FRA is provided to the EA for review and comment before being submitted with the planning application, thereby reducing potentially costly delays to the planning process.
6.6.2 Raised Floor Levels & Basements (Freeboard)
267. The raising of floor levels above the 1% AEP (100 year) fluvial flood level will ensure that the damage to property is minimised. Given the anticipated increase in flood levels due to climate change, the adopted floor level should be raised above the 1% AEP (100 year) predicted flood level assuming a 20% increase in flow.
268. Wherever possible, floor levels should be situated a minimum of 300mm above the 1% AEP (100 year) plus climate change flood level, determined as an outcome of the site based FRA, or 600mm above the 1% AEP (100 year) flood level if no climate change data is available. The height that the floor level is raised above flood level is referred to as the ‘freeboard’, and is determined as a measure of the residual risks.
269. The use of basements within flood affected areas should be discouraged. Where basement uses are permitted however, it is necessary to ensure that the basement access points are situated 300mm above the 1% AEP (100 year) flood level plus climate change. The basement must be of a waterproof construction to avoid seepage during flooding conditions. Habitable uses of basements within flood affected areas should not be permitted.
270. Where existing buildings are being retained, seek that their refurbishment increases their resilience / resistance to flooding (refer Section 6.7 below),
6.6.3 Sustainable Drainage Systems (SuDS)
271. SuDS is a term used to describe the various approaches that can be 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 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.
272. SuDS may improve the sustainable management of water for a site by31:
¾ reducing peak flows to watercourses or sewers and potentially reducing the risk of flooding downstream; ¾ reducing volumes and the frequency of water flowing directly to watercourses or sewers from developed sites; ¾ improving water quality over conventional surface water sewers by removing pollutants from diffuse pollutant sources; ¾ reducing potable water demand through rainwater harvesting; ¾ improving amenity through the provision of public open space and wildlife habitat; and ¾ replicating natural drainage patterns, including the recharge of groundwater so that base flows are maintained.
31 Interim Code of Practice for Sustainable Drainage Systems National SuDS Working Group, 2004
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In addition, SuDS can:
¾ contribute to the enhanced amenity and aesthetic value of developed area; and ¾ provide habitats for wildlife in urban areas and opportunities for biodiversity enhancement.
273. 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.
274. There are numerous different ways that SuDS can be incorporated into a development and the most commonly found components of a SuDS system are described in the following table32. The SuDS techniques may be introduced simply to slow discharge from impermeable surfaces, or to capture and store rainfall on site for non-potable uses (i.e. rainwater harvesting). 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.
Pervious surfaces Surfaces that allow inflow of rainwater into the underlying construction or soil.
Vegetated roofs that reduce the volume and rate of runoff and remove pollution. Green roofs Ecological, aesthetic and amenity benefits. Linear drains consisting of trenches filled with a permeable material, often with a Filter drain perforated pipe in the base of the trench to assist drainage, to store and conduct water; they may also permit infiltration.
Vegetated areas of gently sloping ground designed to drain water evenly off Filter strips impermeable areas and to filter out silt and other particulates.
Shallow vegetated channels that conduct and retain water, and may also permit Swales infiltration; the vegetation filters particulate matter.
Basins, Ponds and Areas that may be utilised for surface runoff storage. High potential for ecological, Wetlands aesthetic and amenity benefits.
Sub-surface structures to promote the infiltration of surface water to ground. They can Infiltration Devices be trenches, basins or soakaways.
Vegetated areas designed to collect and treat water before discharge via a piped Bio-retention areas system or infiltration to the ground.
275. 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. The geology of the Borough is summarised in Figure C. 276. 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 upon the surface. An overview of the topography of the Borough is included in Figure D to assist in this regard. 277. 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. 278. 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. 279. The adoption and future maintenance of sustainable drainage systems is a crucial consideration when implementing SuDS. Two possible options available to ensure that the SuDS are properly implemented and maintained, and the arrangement to be adopted
32 Interim Code of Practice for Sustainable Drainage Systems National SuDS Working Group, 2004
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will be dictated by the Royal Borough of Windsor & Maidenhead Council. These include an agreement under Section 106 of the Town and Country Planning Act, or by a condition to planning permission. Further information relating to the adoption and maintenance of SuDS within the Borough should be sought from the Council. 280. The Flood and Water Management Act (2010) has included a requirement for SuDS measures to be approved, adopted and maintained by the SAB, in this instance RBWM as the LLFA. Although Defra have published Draft National Standards (see reference below) in the design and approval of SuDS measures this part of the Act has not yet been commenced and further guidance is awaited from Defra on its implementation. There may in future be further requirements in respect of the design and information required for the use of sustainable drainage systems, but at the time of writing this report (November 2013) this information is not available. 281. 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 5 of the CIRIA document C697 ‘The SuDS Manual’ (which should normally be the first point of reference for SuDS). 282. For more guidance on SuDS, the following documents and websites are recommended as a starting point:
¾ C697 The SuDS manual (Woods Ballard B; Kellagher R et al, 2007 – available from the CIRIA bookshop www.ciria.org)
¾ Preliminary rainfall runoff management for developments (R & D Technical Report W5-074/A Revision E, Environment Agency and Kellagher R, 2005 – Free download from the Environment Agency web site www.environment-agency.gov.uk)
¾ Interim code of practice for sustainable drainage (National SuDS Working Group, 2004) - Free download from CIRIA web site www.ciria.org or Environment Agency web site www.environment-agency.gov.uk)
¾ 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)
¾ 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)
¾ C625 Model agreements for sustainable drainage systems (Shaffer et al, 2004 - available from the CIRIA bookshop www.ciria.org)
¾ Report 156 Infiltration drainage – manual of good practice (Bettess R, 1996 - available from the CIRIA bookshop www.ciria.org)
¾ 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)
¾ W12 Sustainable water management in schools (Duggin & Reed, 2006 – Free download from CIRIA web site www.ciria.org)
¾ Harvesting rainwater for domestic uses: an information guide (Environment Agency, 2003 - Free download from Environment Agency web site www.environment- agency.gov.uk)
¾ National Planning Policy Framework (NPPF) and Technical Guidance Note (Department for Communities and Local Government, 2012 - Free download from Government web site http://www.gov.uk)
November 2013 41 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
¾ The Building Regulations: Approved document H – Drainage and waste disposal (Office of the Deputy Prime Minister, 2010 - Free download from CLG web site http://www.communities.com)
¾ National Standards for sustainable drainage systems – Designing, constructing, operating and maintaining drainage for surface runoff (Defra, 2011 – Free download from Defra website http://www.defra.gov.uk)
¾ www.ciria.org.uk/SuDS/
283. Developers should also utilise the guidance within the document ‘Rainfall Runoff Management for Developments - Interim National Procedure’, which can be seen in Appendix D of this report.
6.7 Local Community Actions to Reduce Flood Damage 284. There will always be a residual risk of flooding, whether that be (for example) from an event that is more extreme than that considered, or whether as a result of a flood defence system that fails unexpectedly. Flood resistance and flood resilience may need to be incorporated into the design of buildings for this reason. 285. In all areas at risk of flooding, a basic level of flood resistance and resilience will be achieved by following good building practice and complying with the requirements of the Building Regulations 201033. The difference between ‘resilience’ and ‘resistance’ is explained below:
¾ Flood resistance, or ‘dry proofing’, where flood water is prevented from entering the building. For example using flood barriers across doorways and airbricks, or raising floor levels. ¾ Flood resilience, or ‘wet proofing’, accepts that flood water will enter the building and allows for this situation through careful internal design for example raising electrical sockets and fitting tiled floors. The finishes and services are such that the building can quickly be returned to use after the flood. 286. Examples of both flood-resistant and flood resilient design are given in Improving the Flood Performance of New Buildings (Flood Resilient Construction), CLG (2007). 287. A number of properties within the Borough are potentially at risk of flooding. It is essential therefore to ensure a broad awareness with respect to flood risk, providing the community with the knowledge (and tools) that will enable them to help themselves should a flood event occur. 288. The Civil Contingencies Act 2004 (CCA) places a legal duty on responders to undertake risk assessments and maintain them in a Community Risk Register, and publish this register. The Community Risk Register should be approved by the Local Resilience Forum (LRF). The LRF usually consists of Category One responders; these are the Police, Fire, Ambulance, Environment Agency and Health Authorities. Their role is to ensure that there is integrated emergency management for major incidents. The Community Risk Register is the first step in the emergency planning process. The Thames Valley LRF Risk Register is in place and can be accessed on their website34. 289. The following ‘community based measures’ are cost effective solutions that local communities may introduce to minimise the damage sustained to their own homes in the case of flooding. Further guidance is provided by the EA, Defra and CLG35 (refer the National Flood Forum (www. floodforum.gov.uk)).
33 Department for Communities & Local Government (CLG) and Building Regulations Advisory 34 http://thamesvalleylrf.org.uk/useful-links/publications/risk-register.ashx 35 Improving the Flood Performance of New Buildings – Flood Resilient Construction (May 2007)
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6.7.1 Flood Proofing
290. The ‘flood proofing’ of a property may take a variety of forms:
For new homes and/or during redevelopment
¾ Raising of floor levels The raising of floor levels above the anticipated maximum flood level ensures that the interior of the property is not directly affected by flooding, avoiding damage to furnishings, wiring and interior walls. It is highlighted that plumbing may still be impacted as a result of mains sewer failure.
¾ Raising of electrical wiring The raising of electrical wiring and sockets within flood affected buildings reduces the risks to health and safety, and reduces the time required after a flood to rectify the damages sustained.
For existing homes ¾ Flood boards
The placement of a temporary watertight seal across doors, windows and air bricks to avoid inundation of the building interior. This may be suitable for relatively short periods of flooding; however the porosity of brickwork may result in damage being sustained should water levels remain elevated for an extended period of time. This may lessen the effectiveness of flood proofing to existing properties affected by flooding from larger river systems such as the Thames.
6.8 Emergency Planning
291. The SFRA provides a concise summary of the possible sources of flooding within the Borough, and may be used by the Council’s Emergency Planning Unit as an additional guide to inform the assessment of flood risk. The Royal Borough of Windsor & Maidenhead Generic Flood Plan was published in September 2008, and is available online at http://www.rbwm.gov.uk/public/sdop_rbwm_flood_plan_2011-03-16_361.pdf. The flood plan deals specifically with the Borough’s preparedness and response in relation to fluvial flooding36. The plan should be read in conjunction with the Borough’s Generic Emergency Plan which provides full information on standard recognised emergency planning systems and procedures. The plan is part of on-going policy aimed at:
¾ Informing & educating the public; ¾ Warning the public; ¾ Pre planning for floods; ¾ Responding to floods; ¾ Recovering from floods.
292. The Council and its partner agencies continue to work closely in developing and improving local plans. The Council’s priorities are:
¾ To save life ¾ To prevent escalation of an emergency ¾ To relieve suffering ¾ To safeguard the environment ¾ To protect property ¾ To continue to maintain services at an appropriate level ¾ To inform the public ¾ To promote self-help & recovery ¾ To restore normality as soon as possible ¾ To evaluate the response and identify lessons to be learnt ¾ To cooperate with criminal investigations or other inquires
36 Note that other types of flooding are difficult to plan for, but a Generic Emergency Plan also exists to activate a response as required
November 2013 43 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
293. It is important to recognise that Emergency Planning guidelines and training highlights the need for people and responders to avoid entering water at all (due to flow, obstacles and contamination). Planning considerations should consider the practicalities of safety to residents and responders prior to, during and after flooding. During a major flood event resources will be limited. Emergency Planning issues can be mitigated through good planning including infrastructure (safe, dry access & egress), utility resilience, the protection of key sites, and the effective management of potential contamination and pollution issues. These are reflected in the development control recommendations set out in Section 6.4.4.
294. The Environment Agency monitor river levels within the main rivers affecting the Royal Borough of Windsor & Maidenhead, and based upon weather predictions provided by The Met Office, make an assessment of the anticipated maximum water level that is likely to be reached within the proceeding hours (and/or days). Where these predicted water levels are expected to result in the inundation of populated areas37, the Environment Agency will issue a series of flood warnings within defined flood warning areas, encouraging residents to take action to avoid damage to property in the first instance.
295. Warnings are issued using a set of three easily recognisable codes which indicate the level of danger associated with the warning. The code definitions are as follows:
Flooding is possible. Be Prepared
Flood is expected. Immediate Action Required
Severe Flooding Danger to life
296. Nationally, the Environment Agency aims to give a two-hour warning in advance of any flooding taking place. However in certain cases this may not always be possible.
297. All warnings issued are disseminated to professional partners, the media and the public. Once issued, all warnings are available via the internet through the EA website, and Facebook and Floodline. Warnings are disseminated using the Floodline Warnings Direct Service. This service exploits current and emerging technologies to deliver warnings simultaneously via, telephone, mobiles, pager, fax, email and SMS text messaging. All alerts come to the Royal Borough’s 24/7 Control Room and Duty Emergency Planning Officers. The Royal Borough also has Flood Wardens & Flood Boards operating throughout the Borough which are activated when alerts come in.
298. As water levels rise and begin to pose a risk to life and/or livelihood, it is the responsibility of the emergency services to coordinate the evacuation of residents. Coordination between the emergency services, local authority and the Environment Agency is imperative to ensure the safety of residents in time of flood. It is essential that a robust plan is in place that clearly sets out (as a minimum):
¾ roles and responsibilities; ¾ paths of communication; ¾ evacuation routes;
37 Restricted to those urban areas situated within Environment Agency flood warning zones
November 2013 44 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
¾ community centres to house evacuated residents; ¾ contingency plans in case of loss of power and/or communication.
299. Flooding within the Borough is a result of two distinct types of rainfall (flooding) events, including ‘regional’ fluvial flooding, and localised ‘flashy’ storm events.
300. Widespread flooding throughout the region is a recognised risk associated with rising water levels within the River Thames and its tributaries. This event will occur due to long duration rainfall depressions situated over southern England, and forewarning will be provided to encourage preparation in an effort to minimise property damage and risk to life. The Environment Agency will issue warnings as early as possible using the information available at the time. Depending on the situation, warnings may be issued more than two hours in advance of any flooding but this may not always be possible. This applies to all rivers for which the Environment Agency provides a Flood Warning service including the River Thames.
301. Residents in areas affected by flooding in the 5% (20 year) event (i.e. with Zone 3b Functional Floodplain) are likely to be the most vulnerable as water levels rise. These areas will flood more frequently than other areas of the Borough, and are likely to be the first cut off from safe evacuation routes.
302. Other areas within the Borough are susceptible to ‘flashier’ flooding, associated with storm cells that pass over the district resulting in high intensity, often relatively localised, rainfall. It is anticipated that events of this nature will occur more often as a result of possible climate change over the coming decades. Events of this nature are difficult to predict accurately, and the rapid runoff that follows will often result in flooding that cannot be sensibly forewarned.
303. All urbanised areas are potentially at some degree risk of localised flooding due to heavy rainfall. The blockage of gullies and culverts as a result of litter and/or leaves is commonplace, and this will inevitably lead to localised problems that can only realistically be addressed by reactive maintenance.
304. It is noted that the River Thames CFMP highlights the critical importance of effective emergency planning for communities situated adjacent to Thames.
6.9 Insurance
305. Many residents and business owners perceive insurance to be a final safeguard should damage be sustained as a result of a natural disaster such as flooding. Considerable media interest followed the widespread flooding of 2000 when it became clear that the insurance industry were rigorously reviewing their approach to providing insurance protection to homes and businesses situated within flood affected areas. Not surprisingly, the widespread flooding of summer 2007 further exacerbated the discussion surrounding the future of insurance for householders and business owners situated within flood affected areas.
306. The Association of British Insurers (ABI) and the Government negotiated a statement of principles that was completed in July 2008. The principles discuss a long term approach to flood risk and insurance. Paragraph 4 of that statement is reproduced below: “The ABI and the Government have agreed to work together to provide a long-term solution that will enable flood insurance to continue to be as widely available as possible without distorting the market. To achieve this, we have identified the following measures that must be taken:
x improve our understanding of flood risk through assessing both the probability and consequences of flooding from all sources including surface water;
x put in place a long-term strategy to reduce flood risk; set out the Government’s short, medium and long term strategic flood prevention aims; assess funding needs; and ensure effective and prioritised allocation of resources and delivery over the medium and long term in line with future Government spending rounds;
November 2013 45 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
x ensure that the planning system prevents inappropriate development in flood risk areas, and that any essential new development in high flood-risk areas is flood resistant and/or resilient;
x raise awareness in areas where flood risks are significant, encourage actions to mitigate and minimise the risks and costs of being flooded; and provide information about how to obtain flood insurance; and
x promote access to insurance for low-income households.”
307. In June 2013 Government announced the future for flood insurance and the position of the insurance market after July 2013. The government has agreed a way forward with the ABI that focuses on delivering the ABI's proposed 'Flood Re' insurance approach for high risk households across the UK in a way that minimises additional costs for other policyholders. Flood Re secures affordable insurance for many of those most at risk by placing a limit on flood insurance premiums. The Flood Re pool will be funded through a new industry-backed levy that all UK household insurers will have to contribute to, thereby creating a fund that can be used to pay claims for people in high-risk homes.
308. In summary, for the time being, residents and business owners can be assured that insurance will be available to assist in recovery following a flood event. However, it would appear fair to say that the future availability of flood insurance within the UK will be heavily dependent upon commitment from the government to reduce the risk of flooding over time, particularly given the anticipated impacts of climate change. Investment is required in flood defence and improving the capacity of sewage and drainage infrastructure. It is also essential to ensure that spatial planning decisions do not place property within areas at risk of flooding.
November 2013 46 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
7 Conclusion & Recommendations
309. A considerable proportion of the Royal Borough of Windsor & Maidenhead is at risk of flooding. The risk of flooding posed to properties within the Borough arises from a number of sources including river flooding, localised runoff, sewer and groundwater flooding.
310. The River Thames represents the northern and eastern boundaries of the Borough and is a major source of flood risk to properties within the Borough, including core centres of future growth (i.e. Maidenhead and Windsor). 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 risk of flooding.
311. A collation of potential sources of flood risk has been carried out in accordance with the NPPF, developed in close consultation with both the Council and the Environment Agency. The Borough has been broken down into zones of ‘high’, ‘medium’ and ‘low’ probability of flooding, as well as areas of functional floodplain, in accordance with the NPPF, providing the basis for the application of the Sequential Test.
312. Substantial investment in flood defence has been delivered within the Borough, providing a degree of protection to existing property in the form of raised embankments, flood control structures and diversion channels (e.g. Jubilee River). A residual risk of flooding remains however, associated both with an event that may exceed the design capacity of the defences, and/or a structural failure.
313. 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 all urban centres within the Borough.
314. If after having undertaken the Sequential Test it has been identified that there are no reasonably available sites in areas not at risk of flooding, specific recommendations have been provided to assist the Council and the developer apply the Exception Test (refer Section 6.4). These should be considered when writing new policies as part of the Borough Local Plan, as well as in the determination of planning applications.
315. For existing buildings in the floodplain a key message of the River Thames CFMP in the Royal Borough is that refurbishment to achieve resilience / resistance, and redevelopment to make space for water, are a very important part of the planning solution needed to reduce flood risk.
316. Council policy is essential to ensure that the provided development control recommendations can be imposed consistently at the planning application stage. This is essential to achieve future sustainability within the Borough with respect to flood risk management
317. A review of potential sites which may be considered for housing and employment, and other uses has been carried out, identifying acceptable uses as set out in the NPPF (i.e. depending upon the flood zone within which the sites fall). The Windsor & Maidenhead Level 1 SFRA does not attempt, and indeed cannot, fully address the requirements of the Sequential Test. It is necessary for the Council to demonstrate that sites for future development have been sought within Zone 1 Low Probability (refer Increased Scope SFRA and Sequential Testing report). 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.
November 2013 47 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
318. It will be necessary for the Council (and the developer) to apply the Exception Test at some locations. This will involve adherence to development control recommendations as set out in Section 6.4, and the clear demonstration that the proposed development will provide sustainability benefits to the community that outweigh the identified flood risks. For this reason an ‘Increased Scope’ SFRA has been produced by the Royal Borough (November 2013).
319. Emergency planning is crucial for the minimisation to the risk to life posed by flooding within the Borough. The Council’s Emergency Planning Unit should use the SFRA as a guide for risk identification, supporting the generic Flood Plan that will guide the emergency response in case of flooding. A Living Document
320. The SFRA has been developed building heavily upon existing knowledge with respect to flood risk within the Borough. A rolling programme of detailed flood risk mapping within the Thames region is underway. This, in addition to observed flooding that may occur throughout a year, will improve the current knowledge of flood risk within the Borough and may marginally alter predicted flood extents within Windsor & Maidenhead. Given that this is the case, a periodic review of the Windsor & Maidenhead SFRA is imperative upon any emerging policy directives and significant hydraulic modelling updates or river flooding events.
321. It is recommended that the Windsor & Maidenhead SFRA is reviewed on a regular basis. The following key questions should be addressed as part of the SFRA review process:
Question 1 Has any flooding been observed within the Borough since the previous review? If so, the following information should be captured as an addendum to the SFRA:
¾ What was the mapped extent of the flooding? ¾ On what date did the flooding occur? ¾ What was the perceived cause of the flooding? ¾ If possible, what was the indicative statistical probability of the observed flooding event? (i.e. how often, on average, would an event of that magnitude be observed within the Borough?) ¾ If the flooding was caused by overtopping of the riverbanks, are the observed flood extents situated outside of the current Zone 3a? If it is estimated that the frequency of flooding does not exceed, on average, once in every 100 years then the flooded areas (from the river) should be incorporated into Zone 3a to inform future planning decision making.
Question 2 Have any amendments to the NPPF or the Technical Guidance Note to the NPPF been released since the previous review? If so, the following key questions should be tested:
¾ Does the revision to the policy guidance alter the definition of the NPPF Flood Zones presented within the SFRA? ¾ Does the revision to the policy guidance alter the decision making process required to satisfy the Sequential Test? ¾ Does the revision to the policy guidance alter the application of the Exception Test? ¾ Does the revision to the policy guidance alter the categorisation of land use vulnerability, presented within Table 2 in the Technical Guide to the NPPF (March 2012)? If the answer to any of these core questions is ‘yes’ then a review of the SFRA recommendations in light of the identified policy change should be carried out.
November 2013 48 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
Question 3 Has the Environment Agency issued any amendments to their flood risk mapping and/or standing guidance since the previous policy review? If so:
¾ Has any further detailed flood risk mapping been completed within the District, resulting in a change to the 20 year, 100 year or 1000 year flood outline? If yes, then the Zone 3b and Zone 3a flood outlines should be updated accordingly. ¾ Has the assessment of the impacts that climate change may have upon rainfall and/or river flows over time altered? If yes, then a review of the impacts that climate change may have upon the Borough is required. ¾ Do the development control recommendations provided in the SFRA in any way contradict emerging EA advice with respect to (for example) the provision of emergency access, the setting of floor levels and the integration of sustainable drainage techniques? If yes, then a discussion with the EA is required to ensure an agreed suite of development control requirements are in place.
It is highlighted that the Environment Agency reviews the Flood Zone Map on a quarterly basis. If this has been revised within the Borough, the updated Flood Zones will be automatically forwarded to the Council for their reference. It is recommended that only those areas that have been amended by the Environment Agency since the previous SFRA review are reflected in Zone 3 and Zone 2 of the SFRA flood maps. This ensures that the more rigorous analyses carried out as part of the SFRA process are not inadvertently lost by a simple global replacement of the SFRA flood maps with the Flood Zone Maps.
Question 4 Has the implementation of the SFRA within the spatial planning and/or development control functions of the Council raised any particular issues or concerns that need to be reviewed as part of the SFRA process?
SFRA Limitations
The Windsor & Maidenhead SFRA has been developed based upon the best available information at the time of publication (November 2013). 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.
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.
November 2013 49 STRATEGIC FLOOD RISK ASSESSMENT (SFRA) A Technical Report Supporting the Royal Borough of Windsor and Maidenhead Borough Local Plan
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Windsor and Maidenhead SFRA
Figure A Overview of Borough 24 25 ± Kilometers 0 1 2 This map is based upon Ordnance Survey material with the 26 permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationary Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. 27 Environment Agency Licence No 100026380 2006. River Thames
Strand Water
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t u C e h Legend Twyford Brook T
Main Rivers
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Unknown Flood Event from PFRA (2011)
Fluvial Windsor and Maidenhead SFRA Sewer Figure B Historic Flooding Groundwater
Surface Water Kilometers 0 1.5 3 Highway April 2013 This map is based upon Ordnance Survey material with the Combined permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationary Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. Unknown Environment Agency Licence No 100026380 2006. Superficial Geology Alluvium Clay with Flints Drift Geology Glacial Sand and Gravel River Terrace Deposits (Undifferentiated) Sand and Gravel Bedrock Geology Barton, Bracklesham and Bagshot Beds Chalk including Red Chalk London Clay Oldhaven, Blackheath, Woolwich, and Reading and Thanet bed Open Water
Windsor and Maidenhead SFRA
Figure C Geology Map
Kilometres 0 1.5 3
This map is based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationary Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. Environment Agency Licence No 100026380 2006. 480000 490000 500000 510000
ames River Th Strand Water 0 0 0 0 0 0 0 0 8 8 1 1
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t u C Twyford Brook River Thames e s h e T m a h T r e iv R
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0 SFRA 0 7 Elevation 7 1 1 < 10m 10m - 20m Figure D R iver B TOPOGRAPHY 20m - 30m ourne 30m - 40m (Overview) 40m - 50m 50m - 60m 60m - 70m 0 1 2 3 4 5 70m - 80m Kilometers 80m - 90m This map is based upon Ordnance Survey material with the 90m - 100m permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationary Office. Crown Copyright. 100m - 125m Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. > 125m Environment Agency Licence No 100026380 2006.
P:\Environment\B0265100 SFRA_Windsor & Maidenhead\200 GIS\ 480000 490000 500000 Projects\Windsor_Maidenhead_Top5og1ra0p0hy01.0mxd WM2
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Windsor and Maidenhead SFRA Figure E WM13 Index Map - Character Areas ±
Kilometers 0 1 2 3 4 5
This map is based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationary Office. Crown Copyright. Flood Zone 3 Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. RBWM Boundary Environment Agency Licence No 100026380 2006. Flood Depth (m)
0.00 - 0.25
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Windsor and Maidenhead SFRA Figure F River Thames and The Cut Flood Depth 1% (1 in 100) Event ± Kilometers 0 1 2
This map is based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationary Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. Environment Agency Licence No 100026380 2006. Extract from FDD2320 "Flood Risk Assessment Guidance for New Development (2008)
m s a e h c k n o e o f e C D
North Maidenhead Defences
t m n Eton W r ick e a F m Embankment
r k o n o a b M D m a E t c h e t
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O l E d m W i ba n n ds k o n t m r o t n e r e n o t m H k r n e a w b o m L E
Legend
Danger for all
Danger for most Staines By-pass Channel Embankment Danger for some Defence Elevation used (m)
0 - 0.5 Windsor and Maidenhead SFRA 0.5 - 1.0 Figure G 1.0 - 2.0 Hazard due to Defence Failure
2.0 - 5.0 Kilometers Unknown (assumed 1m) 0 1.5 3
Defence elevations which lay This map is based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller between bandings as within the of Her Majesty's Stationary Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and FDD2320 report were rounded may lead to prosecution or civil proceedings. up to be conservative. Environment Agency Licence No 100026380 2006. Legend
Areas Susceptible to Groundwater Flooding Risk
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< 25%
Windsor and Maidenhead SFRA Figure H Areas Susceptible to Groundwater Flooding
Kilometers 0 1 2
This map is based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationary Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. Environment Agency Licence No 100026380 2006. Key
Shallow (> 0.1m)
Deep (> 0.3m)
Windsor and Maidenhead SFRA Figure I-1 Flood Map for Surface Water 1 in 30 year ± Kilometers 0 1 2
This map is based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationary Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. Environment Agency Licence No 100026380 2006. Key
Shallow (> 0.1m)
Deep (> 0.3m)
Windsor and Maidenhead SFRA Figure I-2 Flood Map for Surface Water 1 in 200 year ± Kilometers 0 1 2
This map is based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationary Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. Environment Agency Licence No 100026380 2006.