Level 2 Strategic Flood Risk Assessment: Volume 3 Southampton Key Development Sites Flood Risk Management Options and Feasibility Study
August 2010 Southampton Level 2 SFRA: Volume 3
Southampton City Council Strategic Flood Risk Assessment: Volume 3
Report REV 2.0 / August 2010
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Job Number: CS/038822
PD: James Reddish
PM: Paul Hlinovsky
Report status: REV 2.0
Date of issue: August 2010
CSL Main author(s): Paul Hlinovsky
CSL Checked: James Reddish
Client Approval: Rob Crighton (SCC)
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PREFACE It is accepted that the technical content of the Southampton SFRA will need to be reviewed and amended as new information becomes available. The policy basis for this version is PPS 25 (CLG, March 2010) and the PPS 25 Practice Guide (CLG, December 2009).
Although there is no statutory consultation requirement at this stage, the nature of the intended end use for the information makes it appropriate to obtain feedback relating to the report in order to contribute to the overall robustness and credibility of this work. This information will also be an aid when formulating the necessary next steps in engaging those parties who will be involved in the future.
It is the responsibility of the reader to be satisfied that they are using the most up to date information and that this has been included within the Southampton SFRA.
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FOREWORD
The South East Plan promotes major development growth in South Hampshire. A Strategic Flood Risk Assessment (SFRA) has been completed by the Partnership for Urban South Hampshire (PUSH) which provides information on flood risk, taking account of climate change to provide a greater understanding of the risks across the sub-region and individual local authority area to allow the appropriate application of the Sequential Test.
The City Council Core Strategy was submitted in July 2009 and adopted in January 2010. In compliance with the South East Plan, it proposes major new development within the city (approximately 16,300 dwellings, a minimum of 130,000 square metres of retailing floor space, and 322,000 square metres of office floor space). The office and retail development, along with approximately 5,500 dwellings, will be located within the city centre. A significant element of the city’s development targets will need to be delivered in areas at risk of flooding, particularly on development sites in the central area, such as the major development quarter in the west of the city centre. Core Strategy Policy CS23 sets a strategic approach to flood risk and the need for further assessment (with reference to the Core Strategy’s flood risk background paper).
The Council is now in the process of preparing two subsequent “allocation plans” to identify specific development sites: the City Centre Action Plan and the Sites and Policies Plan (for the balance of the city). The further assessment of flood risk required to inform these plans are detailed in this scope. It is currently estimated that these action plans will be adopted in January 2013. Strategic Aims and Objectives The Practice Guide (CLG, December 2009) identifies a strategic approach to the preparation of SFRAs so that the level of details of assessment matches the significance of the risk. This SFRA describes the outcome of a ‘Level 2’ assessment, in accordance with paragraph E6 of PPS 25 and Section 3.59 of the PPS 25 Practice Guide. It contains a general assessment of risk from all sources over the whole study area and also detailed analysis for locations where flood risk is a significant issue (i.e. key development sites).
The principle objective of the Level 2 SFRA is to provide the appropriate level of information to facilitate the application of the Sequential and Exception Tests for development within the city of Southampton. More detailed information is required where there is deemed to be development pressure in areas that are at medium or high probability of flooding and where there are no other suitable alternative areas for development after applying the Sequential Test. This more detailed study considers the detailed nature of the flood hazard, taking account of the presence of flood risk management measures. This will allow a sequential approach to site allocation to be adopted within a Flood Zone (paragraphs 17 and D4 of PPS25), taking into consideration the wider planning context of Southampton. It will also provide a level of detail suitable for informing part C of the Exception Test at the ‘local’ scale, and informs the relevant Local Development Documents of the Local Development Framework.
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The aims of this Level 2 SFRA are to: a) Inform policies and plans to ensure future developments, where appropriate, have been subjected rigorously to the applications of the Sequential and Exception Tests, satisfying PPS 25; b) Identify strategies to limit flood risks and adapt to climate change; and c) Ensure the safety of new development.
The Level 2 SFRA has been provided as three distinct ‘volumes’: Stage 1: o Volume 1 – Strategic Assessment: the preparation of baseline information on flood risk, including guidance on using the SFRA and recommendations on policy development; o Volume 2 – Technical Appendices and Mapping; Details- how the data informing the Level 2 SFRA has been prepared; and, Stage 2: o Volume 3 – Flood Risk Mitigation Options and Feasibility Study : Building on the data in Vol. 1 and Vol. 2 to prepare conceptual measures to manage flood risk at key development sites in Southampton.
This report is Volume 3 of the assessment and the report structure is shown overleaf.
The SFRA is a live document that is intended to be updated as new information and guidance becomes available. The outcomes and conclusions of the SFRA may not be valid in the event of future changes to legislation, policy, revised government guidance on flood risk, the data or the baseline flooding situations. Decisions also require the inclusive assessment of wider planning issues and the user should be aware that changes to decision making principles affecting other planning issues can potentially affect the outcome of the risk based Sequential Test. The contents of this document are also dependent on the outcomes of the Regional Flood Risk appraisal. It is the responsibility of the user to ensure they are using the best available information.
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Diagram 1 – Proposed Report Structure for Level 2 SFRA
Volume 1: Main body of Level 2 SFRA Report Volume 2: Technical Appendices Chapter 1 Overview: Flood Risk in Southampton
Source Data Chapter 2 Applying the Sequential & Exception Tests E.g. topography Historic flood data Flood defence data, etc Chapter 3 Informing Core Strategy, City Centre Action Plan and Sites and Policies Plan Analysis E.g. flood modelling Chapter 4 Development Control: Site Specific Flood Risk Pluvial flooding Assessments assessment, etc
Chapter 5 Emergency Planning Background Summary of national, Chapter 7 Flood Risk Management Measures regional flood risk policy context Chapter 8 SFRA Management and Maintenance
Mapping Chapter 9 Glossary and Notation E.g. flood depth mapping Flood velocity mapping Chapter 10 References Speed of onset mapping etc
Volume 3: Flood Risk Management Key Development Sites
Site Proposals Site Specific
Technical Assessment of Flood Risk Environmental Impacts
Flood Risk Management: Economic Appraisal Strategic Precautionary Approach Site Specific Measures Managed Adaptive Approach Key Development Site Summary
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CONTENTS
Foreword ...... iii
1. INTRODUCTION ...... 1-1
1.2 Assessment Methodology ...... 1-2
1.3 Flood Risk Management Option Assessment ...... 1-2
1.4 Environmental Impacts ...... 1-4
2. STRATEGIC FLOOD RISK MANAGEMENT ...... 2-1
2.1 Background ...... 2-1
2.2 Key Strategic Flood Risk Management Assumptions ...... 2-2
2.3 Environmental Impacts ...... 2-5
2.4 Economic Appraisal ...... 2-6
3. KEY SITE 1: CENTRAL STATION ...... 3-1
3.1 Site Proposal ...... 3-1
3.2 Technical Assessment: Flood Risk ...... 3-2
3.3 General Flood Risk Management ...... 3-6
3.4 Option 1: Strategic Precautionary Approach ...... 3-7
3.5 Option 2: Site Specific Measures ...... 3-7
3.6 Option 3: Management Adaptive Approach ...... 3-10
3.7 Environmental Impacts ...... 3-11
3.8 Site Summary ...... 3-11
4. KEY SITE 2: MAJOR DEVELOPMENT QUARTER ...... 4-1
4.1 Site Proposal ...... 4-1
4.2 Technical Assessment: Flood Risk ...... 4-2
4.3 General Flood Risk Management ...... 4-2
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4.4 Option 1: Strategic Precautionary Approach ...... 4-3
4.5 Option 2: Site Specific Measures ...... 4-4
4.6 Option 3: Management Adaptive Approach ...... 4-8
4.7 Environmental Impacts ...... 4-9
4.8 Site Summary ...... 4-10
5. KEY SITE 3: ROYAL PIER ...... 5-1
5.1 Site Proposal ...... 5-1
5.2 Technical Assessment: Flood Risk ...... 5-2
5.3 General Flood Risk Management ...... 5-7
5.4 Option 1: Strategic Precautionary Approach ...... 5-8
5.5 Option 2: Site Specific Measures ...... 5-8
5.6 Option 3: Management Adaptive Approach ...... 5-14
5.7 Environmental Impacts ...... 5-15
5.8 Site Summary ...... 5-15
6. KEY SITE 4: FRUIT AND VEGETABLE MARKET ...... 6-1
6.1 Site Proposal ...... 6-1
6.2 Technical Assessment: Flood Risk ...... 6-2
6.3 General Flood Risk Management ...... 6-5
6.4 Option 1: Strategic Precautionary Approach ...... 6-6
6.5 Option 2: Site Specific Measures ...... 6-6
6.6 Option 3: Management Adaptive Approach ...... 6-7
6.7 Environmental Impacts ...... 6-8
6.8 Site Summary ...... 6-8
7. KEY SITE 5: TOWN DEPOT...... 7-1
7.1 Site Proposal ...... 7-1
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7.2 Technical Assessment: Flood Risk ...... 7-2
7.3 General Flood Risk Management ...... 7-7
7.4 Option 1: Strategic Precautionary Approach ...... 7-8
7.5 Option 2: Site Specific Measures ...... 7-9
7.6 Option 3: Management Adaptive Approach ...... 7-11
7.7 Strategic Flood Risk Management Solution by 2055 ...... 7-12
7.8 Strategic Flood Risk Management Solution by 2070 ...... 7-13
7.9 Environmental Impacts ...... 7-14
7.10 Site Summary ...... 7-15
8. KEY SITES 6 AND 7: MERIDIAN AND DRIVERS WHARF ...... 8-1
8.1 Site Proposal ...... 8-1
8.2 Technical Assessment: Flood Risk ...... 8-2
8.3 General Flood Risk Management ...... 8-7
8.4 Option 1: Strategic Precautionary Approach ...... 8-8
8.5 Option 2: Site Specific Measures ...... 8-9
8.6 Option 3: Management Adaptive Approach ...... 8-12
8.7 Strategic Flood Risk Management Solution by 2055 ...... 8-12
8.8 Strategic Flood Risk Management Solution by 2070 ...... 8-13
8.9 Environmental Impacts ...... 8-14
8.10 Site Summary ...... 8-15
9. KEY SITE 8: LOWER HIGH STREET ...... 9-1
9.1 Site Proposal ...... 9-1
9.2 Technical Assessment: Flood Risk ...... 9-2
9.3 General Flood Risk Management ...... 9-5
9.4 Option 1: Strategic Precautionary Approach ...... 9-5
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9.5 Option 2: Site Specific Mitigation ...... 9-6
9.6 Option 3: Management Adaptive Approach ...... 9-8
9.7 Environmental Impacts ...... 9-9
9.8 Site Summary ...... 9-9
10. KEY SITE 9: COLLEGE STREET ...... 10-1
10.1 Site Proposal ...... 10-1
10.2 Technical Assessment: Flood Risk ...... 10-1
10.3 General Flood Risk Management ...... 10-2
10.4 Option 1: Strategic Precautionary Approach ...... 10-2
10.5 Option 2: Site Specific Mitigation ...... 10-3
10.6 Option 3: Management Adaptive Approach ...... 10-6
10.7 Strategic Flood Risk Management by 2055 ...... 10-6
10.8 Strategic Flood Risk Management by 2070 ...... 10-7
10.9 Environmental Impacts ...... 10-7
10.10 Site Summary ...... 10-8
11. KEY SITE 10: SOLENT SKY AREA ...... 11-1
11.1 Site Proposal ...... 11-1
11.2 Technical Assessment: Flood Risk ...... 11-2
11.3 General Flood Risk Management ...... 11-6
11.4 Option 1: Strategic Precautionary Approach ...... 11-7
11.5 Option 2: Site Specific Measures ...... 11-7
11.6 Option 3: Management Adaptive Approach ...... 11-10
11.7 Environmental Impacts ...... 11-11
11.8 Site Summary ...... 11-11
APPENDIX A: ENVIRONMENTAL APPRAISAL ...... A1
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1. Introduction
1.1.1 Volume 1 and Volume 2 (Technical Appendices and Mapping) of the Level 2 SFRA are compiled to provide a baseline assessment of the current and future flood risk to Southampton from potential sources of flooding in accordance with Planning Policy Statement 25 (PPS 25).
1.1.2 This report, Volume 3, aims to produce a robust evidence base to demonstrate that development required in areas identified at high or medium probability of flooding, either now or in the future, can be made ‘safe’ in accordance with PPS 25, and that the necessary flood risk management infrastructure has a reasonable prospect of delivery. It aims to identify options for managing each of the key development sites as well as provide recommendations for inclusion in the Site Allocations DPD and City Centre Action Plan. It also provides a guide for developers when preparing Masterplans on the type of options and potential ‘abnormal’ costs associated with flood risk management at the development sites.
1.1.3 The flood risk management options presented here have been prepared to demonstrate there is a reasonable prospect of delivering development. There may be other suitable options to manage flood risk at the development sites, however these should be discussed and agreed with SCC and the Environment Agency early in the pre-application consultation process. It is recommended that a degree of flexibility is retained in defining an appropriate level of ‘safety’ for individual developments.
1.1.4 The assessment focuses on nine key development sites in Southampton, however it is envisaged that the contents of Volume 3, along with the guidance and recommendations in Volume 1, can be used to support other proposed development, including windfall sites.
1.1.5 As identified in Chapter 7 of Volume 1, the SFRA is a “live” document. It is expected that the contents of all three Volumes of the SFRA are updated as improved information becomes available.
1.1.6 The contents of Volume 3 do not remove the need for developers to prepare detailed Flood Risk Assessments (FRAs) to support planning applications. Developers should always ensure that FRAs are based on the most up to date information and agree the scope of the FRA with both SCC and the Environment Agency (refer to Chapter 5 in Volume 1 for guidance on preparing FRAs in Southampton).
1.1.7 The nine key development sites that have been identified by Southampton City Council (SCC) and assessed in this report are:
Central Station (Chapter 3); Fruit and Vegetable Market Major Development Quarter (Queens Way) (Chapter 8); (Chapter 4); College Street (Chapter 9); Royal Pier (Chapter 5); Lower High Street (Chapter 10); Town Depot (Chapter 6); and Meridian/ Drivers Wharf (Chapter Solent Sky Area Site (Chapter 7); 11).
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1.1.8 All nine of the key development sites are located in or around Southampton city centre (refer Figure 1.1 in Volume 2). As discussed in Chapter 1 of Volume 1, the main source of flood risk to Southampton is from the sea (tidal flooding). The city centre area does not currently benefit from raised flood defences that provide protection from tidal flooding. Although the hazard from tidal flooding in the city centre is relatively low at present, it is expected that over the lifetime of proposed development sea level rise will significantly increase both the extent and depth of flooding and the associated hazard. For this reason the focus of this Volume 3 has been to demonstrate how development can remain ‘safe’ from tidal flooding for the lifetime of development. Where this SFRA has identified other sources of flooding (in particular surface water) as a risk to development, these have been identified and discussed.
1.1.9 The format of this report has been divided into a number of easy to refer to chapters, with a chapter dedicated to each of the development sites. This format will enable this volume of the SFRA to be easily updated if additional information or options become available for one of the development sites. All figures referred to in this SFRA are contained within Appendix 10 of Volume 2. Where appropriate diagrams identifying specific flood risk management options are located throughout the Volume.
1.2 Assessment Methodology
1.2.1 The assessment of each site follows an identical format, providing the following information:
Site Proposal – A brief description of the site location, current development proposals and spatial planning requirements based on the level of detail available for each site;
Technical Assessment: Flood Risk – a ‘baseline’ discussion of the current and future probability of flooding to the site from tidal, fluvial, surface water and sewer, groundwater, and artificial sources using data and guidance in Volume 1 and Volume 2 of the SFRA;
Risk Assessment – a summary of the existing and future flood risk to the site, considering both the probability and consequences of flooding;
Flood Risk Management – discussion on the flood risk management options for the site based on the three broad flood risk management ‘options’ discussed below (Section 1.3), as well as identifying reliance on critical infrastructure; ‘general’ flood risk management requirements that apply independent of the management option;
Environmental Impacts – a screening of potential environmental impacts that may result from implementing the identified flood risk management options (Section 1.4);
Economic Appraisal – a summary of the economic implications of the potential flood risk management options (Section 1.5);
Site Summary – headline points for consideration by Southampton City Council in determining whether there is a reasonable prospect of delivering the development site.
1.3 Flood Risk Management Option Assessment
1.3.1 The study seeks to establish a consistent basis for the management of tidal flooding in Southampton. Table 1.1 identifies the three broad approaches that have been applied for developing flood risk management options for the key development sites.
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Table 1.1: Flood Risk Management Options in Southampton
Flood Risk Management Details Option
This refers to a city-wide solution that would positively benefit the development sites as well as Southampton as a whole. As identified in Chapter 1 and 6 of Volume 1, SCC will soon begin the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane), referred to in this document as the Strategic Southampton FCERM Strategy, which is expected to detail how Precautionary protection can be provided to the city centre. A broad assessment of Approach this option and benefits to the development sites is considered, however the SFRA does not seek to replace the need to undertake the study. This option and key assumptions are discussed further in Chapter 2 of this Volume. How the option impacts each site is discussed in Chapters 3 -11.
This approach refers to development of individual flood risk management measures at each of the development sites, as might be Site-specific Flood expected if development is brought forward in isolation. This could Risk Management include measures such as land raising, raised floor levels, and restrictions on ground floor uses.. This option and key assumptions are discussed under each key development site (Chapter 3 – Chapter 11).
The management adaptive option is a more flexible approach, adapting as flood risk changes over the lifetime of development and may include a combination of strategic and site specific options. This approach looks at the potential implementation of a strategic flood risk management solution which would protect the development sites (but will not be constructed until some time into the future), and therefore, interim (site specific) measures are necessary (phasing of development, partial land raising, defence measures to protect against Managed Adaptive climate change etc). Approach It is expected that the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane) will consider where and when a strategic flood risk management solution can economically be brought forward. For the purposes of this SFRA the assessment of this option is made at four intervals - 2010, 2055, 2070 and 2115. This option and key assumptions are discussed in Chapter 2 of this volume. How the option impacts each site is discussed in Chapters 3 - 11.
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1.4 Environmental Impacts 1.4.1 An environmental “sieve analysis” has been undertaken for the identified flood risk management options to inform stakeholders on the potential environmental constraints and opportunities. The “sieve analysis” adopts a ‘tick’/’cross’ methodology thus enabling the identification of potential constraints and opportunities of each flood risk management option against each screening criteria. The assessment, whilst not enabling the options to be ranked, will support the choice of one option over another, supported by a series of conclusions stating, for example, where a ‘cross’ represents a potential ‘showstopper’ in terms of the environment, or where a ‘cross’ can potentially be overcome through mitigation at the detailed design and implementation stage.
1.4.2 The screening criteria have been established based on the Environmental Impact Assessment Regulations (1999) and have considered:
Population; Fauna; Flora; Soil; Water; Air; Climatic factors; Material assets – Including the architectural and archaeological heritage; Landscape; and The inter-relationship between the above topic areas.
1.4.3 The sieve analysis has assessed options such as strategic land raising or defences, but not ‘building level’ flood risk management such as ‘more vulnerable’ development on first floor or the use of flood resilience or resistance measures.
1.4.4 The identified screening criteria have been set out within an assessment table (Appendix A) which also identifies the options for the nine key development sites.
1.5 Economic Appraisal 1.5.1 A high level appraisal of the potential ‘abnormal’ costs associated with implementing each flood risk management option has been made. The costs are considered precautionary and based on broad assumptions with respect to the scale of potential development within each site.
1.5.2 A range of sources have been used to estimate potential flood risk management costs including:
The Environment Agency database of unit costs;
Abnormal cost estimates for recent and similar developments in Southampton (provided by Southampton City Council);
Recent flood risk management works in Southampton (e.g. road raising);
Spon’s Architects and Builders price Book 2010.
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1.5.3 A brief overview of the economic implications is discussed for each site, with details of costs and key assumptions provided separately to this document.
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2. Strategic Flood Risk Management
2.1 Background
2.1.1 The probability of tidal flooding in Southampton is expected to increase significantly over the next 100 years as a result of climate change. A strategic solution to manage the increasing tidal flood risk has the potential to benefit new development, as well as the existing community. The recently commissioned Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane) is expected to identify measures that manage tidal flood risk to the city centre, including the areas occupied by the key development sites assessed in this document.
2.1.2 There are, two broad approaches to strategic flood risk management in Southampton:
1. Strategic Precautionary Approach – a strategic solution is brought forward now as a one-off intervention and is suitable for the lifetime of proposed development (i.e. precautionary approach advocated by PPS 25). This approach is most suitable where it isn’t possible to adapt over time with multiple interventions; and
2. Managed Adaptive Approach – a strategic solution is brought forward over time with regular improvements based on the risk of tidal flooding which has been informed by the best available information at that time.
2.1.3 The differences in the two approaches are demonstrated in Diagram 2.1 below (source: Figure 4.2 in Defra Appraisal of Flood and Coastal Erosion Risk Management, June 2009). The precautionary approach creates an immediate and significant decrease in risk, which reduces over time. The managed adaptive approach has an initial, smaller decrease in risk, which decreases, and is then improved at regular intervals.
Diagram 2.1: Precautionary and Managed Adaptive Approach
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2.1.4 A comparison of the key differences between the two approaches is presented in Table 2.1, overleaf. Table 2.1: Comparison of strategic flood risk management approach
Issue Precautionary Approach Managed Adaptive Approach
Policy Support PPS 25 Making Space for Water
Suitable where ongoing responsibility Suitable where ongoing responsibility Responsibility can be assigned to operating is unclear or complex to administer authorities
Suitable where it is not technically Suitable where flexibility is able to be Deliverability feasible to upgrade the standard of incorporated into construction for protection in the future future upgrading.
Based on current and future Based on current understanding of understanding of flood risk, and best Climate Change flood risk and climate change available data on climate change in the future
2.1.5 The Southampton FCERM Strategy is expected to determine whether a precautionary or managed adaptive approach is most appropriate in Southampton (or separate approaches in different areas).
2.2 Key Strategic Flood Risk Management Assumptions
2.2.1 A number of assumptions have been made in assessing the potential costs and benefit to the key development sites of a strategic flood risk management solution (Table 2.2). Although the Southampton FCERM Strategy will not focus on the benefits to new development, it is expected that many of these assumptions in this SFRA can be clarified through completion of the FCERM strategy.
2.2.2 The assumptions are based on national, regional and local policy and guidance as well as consultation with key stakeholders, including the Environment Agency, SCC and Associated British Ports (ABP). Where there is significant uncertainty, or significantly more information would be required to improve uncertainty, a precautionary approach has been adopted in assumptions.
2.2.3 Table 2.2 identifies whether assumptions apply to the Strategic Precautionary Approach or the Managed Adaptive Approach.
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Table 2.2: Key Strategic Flood Risk Management Assumptions
Applied to Applied to Managed Strategic No. Assumption Adaptive Precautionary Approach Approach
Strategic flood risk management will be predominantly an engineering intervention that reduces the probability of flooding. It is 1 expected the Southampton FCERM Strategy will also consider ‘softer’ approaches to strategic flood risk management within the context of the Shoreline Management ‘Hold the Line’ policy. 2 The engineering intervention is a flood defence 'sea wall' throughout the river length. A managed adaptive approach will adopt the same overall measures for managing tidal flood risk as the Strategic Precautionary 3 Approach. The difference between the strategic flood risk management and managed adaptive approach is where (i.e. the extent) and when (i.e. 2010, 2055 or 2070) the works are undertaken. x Flood defence works are undertaken before they are required to maintain the required standard of protection. Based on the available model results for climate change in 2010, 2055, 2070 and 2115 the following lengths of flood defence have been estimated: 8 x o In 2010 7.2km are required to manage flooding up to 2055. o In 2055 8.8km are required to manage flooding up to 2070 o In 2070 7.6km are required to manage flooding up to 2115. 4 A 1 in 200 year ARI standard of protection from time of construction for the lifetime of development is provided. 5 All works are undertaken at the same time (i.e. not based on the condition and existing standard of protection). x
The coastal defences from Redbridge to Woodmill Lane will be completely replaced (i.e. existing defences cannot be retained or 6 built upon) – 23km length Land within the 1 in 1000 year return period flood extent does not benefit from defences (i.e. only a 1 in 200 year standard of 7 protection is provided and overtopping would occur in a 1 in 1000 year return period event, resulting in flooding as depicted by Flood Zone 2) Works can be undertaken along the entire existing river frontage. It is recognised that port (and other) operation and access 11 requirements may be a significant constraint. Alternative ‘defence lines’ around ABP land have been considered in the assessment
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Applied to Applied to Managed Strategic No. Assumption Adaptive Precautionary Approach Approach
for the Major Development Quarter. A strategic flood risk management solution is constructed ‘today’ – i.e. as a minimum an agreed and fully funded strategy is in place. 12 This is not currently the case, but can be used to understand the implications of strategic flood risk management, and how it relates x to a ‘managed adaptive’ approach.
Residual risks exist from a breach in proposed defence. A consistent breach ‘level’ is assumed based on tidal water levels in the 13 river. Where appropriate breach modelling is likely to lower than this level for sites located at distance from the river frontage. Existing drainage outfalls through walls will be retained. Adaption of surface water management to climate change will be considered 14 as part of the Southampton SWMP. In accordance with the recommendations in Volume 1 of this SFRA, the Environment Agency and SCC working with Hampshire 15 County Council to develop a strategy for the management of change in groundwater levels in Southampton as a result of climate change. The environmental ‘sieve’ analysis is based on the environmental setting and information available in early 2010. As the environment 16 changes it would be necessary to review the proposed defence interventions associated with the Managed adaptive approach at that juncture. 17 The environmental impacts are assumed to be the same for the Strategic Precautionary and Managed Adaptive Approach.
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2.3 Environmental Impacts
2.3.1 Key environmental impacts associated with this strategic defence are likely to change over time - the impacts associated with constructing a strategic defence scheme now (Strategic Precautionary Approach), might be different if defences are constructed in the future (Managed Adaptive Approach) as the environment, both built and natural may change. This assessment of ‘strategic flood risk management’ is for both the Strategic Precautionary and Managed Adaptive approaches. Further details and background to the environmental sieve analysis is located in Appendix A.
SSSI’s/SINCs
2.3.2 There are a number of key SSSIs and one Site of Important Nature Conservation Interest (SINC) within close proximity to the assumed defence line that can be considered as sensitive receptors. The Lee-on-Solent to Itchen Estuary SSSI has previously been subject to a decline in the value of the site due to the implementation of coastal flood defences in other areas. There is a potential risk that this decline may be exacerbated by the installation of the new flood defences in Southampton. However, due to the predominantly industrialised nature of the Southampton frontage and the ‘hard edge’ of the existing river banks in the vicinity of these designations, it is unlikely that there will be a significant impact on the condition of the designations.
Ecology
2.3.3 There is the potential for impacts on both the aquatic and terrestrial ecology as part of the implementation and operation of new defences. Accounting for the limited ecological information regarding the areas that may be affected by the defences it is difficult to ascertain the level of impact. There may be the potential to impact on aquatic ecology as part of the installation of the defences arising from sediment loading or disturbance of habitat. Terrestrial ecology located close to the water’s edge may also be affected by the installation of the defences through the potential disturbance and loss of habitat.
2.3.4 Certain sections of the River Itchen are bounded by relatively mature trees that may have the potential to provide habitat for bats (or other protected species), as well as other avifauna that may be disturbed as part of construction works. There is the also potential to locally improve habitats through the use of ‘softer’ approaches along the waterfront, whilst still maintaining the required standard of protection.
Archaeology
2.3.5 Much of the dockland area is designated as having archaeological potential on the SCC Local Plan. However, it is not anticipated that the proposed works would culminate in a significant impact in this area due to the industrialised nature of the area, and previous disturbances.
Heritage
2.3.6 There may be a limited potential for the temporary impact on the setting of a number of listed buildings within close proximity to the proposed defences associated with the construction activities. Notably work in the vicinity of Royal Pier will require consultation with English Heritage, with particular consideration given to a design that minimises any impacts on local heritage values.
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Townscape
2.3.7 There may be some temporary impacts to the townscape of the area affected by the construction phase of the works (as a result of the presence of heavy machinery and earth works), however this is not anticipated to be significant in nature.
Contamination
2.3.8 Due to the urbanised nature of the dockland area, there may be the potential of disturbance/exposure of previously unrecorded areas of contaminated land arising from the historical and present day industrial use of the area. Further investigation is required as part of the Southampton FCERM Strategy. Mitigation may be required during construction phase to avoid the detriment impacts from this material.
2.4 Economic Appraisal
2.4.1 Detailed costings are expected to be prepared as part of the Southampton FCFRM Strategy as well as the Southampton Developer Contribution Guidance Document. For the purposes of this SFRA a broad scale cost for the Strategic Precautionary and Managed Adaptive approaches have been developed to understand the relative potential difference in whole life cost. These costs are based on a series of costing assumptions, identified in Table 2.4 (overleaf).
2.4.2 Table 2.3 provides a summary of the potential broad scale costs associated with a Strategic Precautionary and Managed Adaptive Approaches. Actual values should be used as a guide as costs are based on assumptions that may be considered precautionary. Notably adopting a Managed Adaptive approach is likely to result in a considerably lower overall cost compared to a Strategic Precautionary approach. Further details on the costings are provided separate to this report.
Table 2.3: Summary Strategic Flood Risk Management Costs Approach Payment Method Cost Assumption Net Present Value (NPV) Strategic Plan Period pays Cost based on ‘Average’ Rates £35.8M Precautionary for all works to Cost based on ‘Maximum’ Rates £87.8M Approach 2115 Plan period pays Cost based on ‘Average’ Rates £15.8M for all works to provide protection Cost based on ‘Maximum’ Rates £38.5M Managed to 2115) Adaptive Plan period pays Cost based on ‘Average’ Rates £11.0M Approach for works to provide protection Cost based on ‘Maximum’ Rates £26.8M to 2055
Southampton City Council 2-6 August 2010
Southampton Level 2 SFRA: Volume 3
Table 2.4: Key Economic Assumptions Applied to Applied to Managed Strategic No. Assumption adaptive Precautionary Approach Approach 1 Present Value estimates assume funding is available and set x aside in 2010.
2 The unit cost for a new sea wall has been taken from the Environment Agency unit Cost Database. Two cost scenarios have been reviewed: ‘Average’ cost of £1,522/m; and ‘Maximum’ cost of £3,736/m.
3 For a strategic flood risk management approach the contribution made by proposed housing during the plan period should also consider the benefit to existing housing in x the future, as well as proposed housing in Flood Zone 2/3 in the future; i.e. consider existing and currently proposed housing in Flood Zone 3 in 2115.
4 A Strategic Precautionary approach is funding solely within the plan period (2007-2026). Residential after 2026 could be used to fund defence maintenance works, however as there x is no certainty on future residential development, this has been ignored at present.
5 Two scenarios for funding a Managed Adaptive Approach have been considered: Housing during the plan period (2007-2026) contributes for all works required to provide protection over the lifetime of the development. Housing during the plan period only contributes for works required to protect up to 2055 (closest match to plan period), with future housing after 2026 paying for works required from 2055.
6 For the latter scenario it is assumed flood risk remains approximately the same (based on current prediction of tidal flood levels).
7 A consistent level of flood risk is applied to all locations and properties (i.e. all properties in the Flood Zones benefit equally from defences).
Southampton City Council 2-7 August 2010
Southampton Level 2 SFRA: Volume 3
3. Key Site 1: Central Station
3.1 Site Proposal
3.1.1 The proposed development envelope encompasses an area of approximately 5.5 ha and is located at National Grid Reference 441350 112160. The topography of the site varies from below 2.5mAOD in south-west up to 4.2mAOD in the north-west.
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Diagram 3.1: Site Location
3.1.2 Key spatial requirements for the site include:
Proposed transport interchange which includes pedestrian links and high density office/hotel buildings (with the potential for residential development).
3.1.3 In accordance with Table D.2 of PPS 25, the proposed residential and hotel areas are classified as a ‘more vulnerable’, the office areas as ‘less vulnerable’ and the Transport as ‘essential infrastructure’.
3.1.4 The lifetime of development for residential development and essential infrastructure has been assumed as 100 years and 60 years for non-residential development.
3.1.5 Ground levels vary across the site:
North of the Railway Station - 3.5 – 10mAOD
Along the railway line – 2.5-3mAOD
In the south west corner – 5 – 7mAOD
Along Western Esplanade – 2.5 – 3.5mAOD
Southampton City Council 3-1 August 2010
Southampton Level 2 SFRA: Volume 3
3.2 Technical Assessment: Flood Risk
Summary of Flood Risk
3.2.1 The site is currently at a ‘low’ probability of tidal flooding, however after 2070 this is expected to be ‘high’ with a ‘significant’ flood hazard predicted within the boundaries of the site.
3.2.2 The site is currently assessed as being susceptible to surface water flooding as it is located near the bottom of a local ‘basin’.
3.2.3 Groundwater flooding may become an issue in the future and should be assessed and managed as part of a detailed site FRA. Future regional groundwater management should be considered by HCC, SCC, EA.
3.2.4 Further detail on the existing and future flood risk at the site are provided in the following sections.
3.2.5 Southampton Central Station (essential infrastructure) and associated transport interchange are unlikely to be reasonably located elsewhere within the city. Southampton City Council is required to consider the Sequential Test.
Historic Flooding
3.2.6 Environment Agency Record – 26 May 2008: Problem at West Quay culvert; Tracks flooded to a depth of 60cm-1m. Adjacent highways were also flooded during this event.
3.2.7 SCC Record – 27 May 2008: South-eastern area of the site, on the Western Esplanade, was subject to flooding following heavy rain. The cause of the flooding was concluded to be from groundwater sources however this could also be caused by surface water exceedance flooding.
Flood Zones
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Legend: Flood Zone 3
Flood Zone 2
Diagram 3.2: Environment Agency Flood Zones
Southampton City Council 3-2 August 2010
Southampton Level 2 SFRA: Volume 3
3.2.8 The site is located within Flood Zones 1, 2 and 3. Current development proposals are compatible in this location, subject to the Sequential and (potentially) Exception Test.
3.2.9 If the predicted effects of climate change are accurate then the majority of the site could be located within Flood Zone 3 in the future (refer to Diagram 3.3) The ‘future’ flood zones should be considered when undertaking the Sequential test and Sequential Approach.
3.2.10 PPS25 advises essential transport infrastructure is compatible in Flood Zone 2 and requires the Exception Test to be passed if located in Flood Zone 3.
Tidal Flood Risk and Hazard
3.2.11 The site is not currently located in an area at tidal risk of flooding due to higher ground levels between the site and the River Test.
3.2.12 Hydraulic modelling of the site indicates that it is predicted to remain outside the 1 in 200 year ARI (0.5%AEP) flood event until at least 2070. By 2115 flood water passes through the Major Development Quarter to the south, over Western Esplanade and floods Southampton Central Station (0.7-1.2m deep), refer to Diagram 3.3.
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Legend (m)
Diagram 3.3 Flood Depth 2115 - 1 in 200 year ARI Tidal Flood Event
Southampton City Council 3-3 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend: Low Moderate Significant Extreme
Diagram 3.4 Flood Hazard 2115 - 1 in 200 year ARI Tidal Flood Event
3.2.13 A review of the model indicates that it would take about 2 hours from when tidal overtopping occurs before the floodwaters reach the site.
3.2.14 The PUSH SFRA outputs adopted in this SFRA for the Western Docks indicate flooding may occur onto the railway line further west and result in a flood flow pathway along the mainline railway and into the Central Station. This data is precautionary and there this additional flowpath should be investigated in any future FRAs proposed for the site.
3.2.15 Significant flood hazard is also predicted across part of Western Esplanade (Diagram 3.4). A flood flow pathway could also potentially develop through the railway tunnel beneath the Civic Centre and should be assessed as part of a detailed FRA.
Residual Tidal Risk
3.2.16 The site is partially located within the ‘extreme’ 1 in 1000 year ARI (0.1% AEP) tidal flood event (Flood Zone 2). By 2115 the majority of the site could be located within the ‘extreme’ 1 in 1000 year tidal flood event – refer to Figure 2.4 in Volume 2.
Fluvial Flood Risk
3.2.17 The site is located within an area identified at low probability of fluvial flooding.
Surface Water Flood Risk
3.2.18 The site is identified as being susceptible to surface water flooding (refer to Diagram 3.5). Broad-scale surface water modelling indicates the site is located on an overland flow route from a local catchment located to the north. This overland flowpath is associated with the Rolles Brook. This watercourse becomes culverted to the north, and passes beneath the railway. The overland flowpath and flooding depicted is unlikely to be an accurate representation of the ‘actual’ risk of surface water flooding, as it ignores the capacity of the culvert beneath the railway and is not modelled to specifically represent the catchment.
Southampton City Council 3-4 August 2010
Southampton Level 2 SFRA: Volume 3
3.2.19 However, during extreme events when the Rolles Brook and drainage network is exceeded surface water flooding could occur. In May 2008 flooding occurred at Southampton Central Station, and although the cause of the flooding has not been clearly established, it is likely to have been a combination of under capacity drainage network, partial culvert blockage, saturated ground conditions and the local topography (overland flow route).
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Diagram 3.5 Susceptibility to Surface Water Flooding
Groundwater Flood Risk
3.2.20 The site is located in an area identified as having a ‘Moderate Risk’ to groundwater flooding. This classification has been provided based on the permeability of the underlying geology and historical flood incident near the boundary of the site. The Watermark West Quay Flood Risk Assessment (November 2008) identified that within the MDQ south of Central Station ‘groundwater was seen to be flowing under artesian conditions from standpipes up to 1994. In addition to this, artesian water was found to be issuing from a deep standpipe installed to the west of the Site in 2000. The Land Quality Assessment notes that groundwater was encountered at approximately 2m below ground level (bgl) indicating that there may be groundwater in the River Terrace Deposits which were found at depths of between 1.7m bgl and 4.3m bgl.’
3.2.21 Sea level rise as a result of climate change is likely to also have an effect on the water table level on the low lying ground in Southampton. The available data indicates that if an associated 1m rise in the groundwater table occurs, groundwater levels at Central Station could approach the surface level during periods of prolonged rainfall. This could result in shallow, but long lasting flooding.
Southampton City Council 3-5 August 2010
Southampton Level 2 SFRA: Volume 3
3.2.22 Development proposals on the Central Station site will need to consider the existing site ground conditions (based on site specific data) and groundwater levels over the lifetime of the development - in particular the design of any underground structures or services and foundations. Future regional groundwater management should be considered by Hampshire County Council, SCC and the EA.
Flood Risk from Artificial Sources
3.2.23 No artificial sources are located near the site. 3.3 General Flood Risk Management
3.3.1 The following guidance applies independent of the overall approach to managing flood risk on site.
3.3.2 There is the opportunity to adopt the sequential approach to layout on site. If residential or hotel development (‘more vulnerable’) is proposed, this should be located on higher ground to the north of the railway station, or above land uses more appropriate for areas of the identified flood risk.
Surface Water Flooding
3.3.3 The Southampton Surface Water Management Plan should investigate the causes of flooding in May 2008 and identify if remedial work is required to the drainage network to reduce the probability of flooding. It is recommended that either as part of the SWMP, or a detailed FRA, that hydraulic modelling of the local surface water catchment is undertaken which considers the Rolles Brook, as well as ‘direct rainfall’ on the local catchment to more accurately quantify the risk of surface water flooding.
3.3.4 Until more detailed information on the risks of surface water flooding is made available, it is recommended that:
The site layout locate ‘less vulnerable’ or ‘water compatible’ uses, in areas identified as at ‘high’ susceptibility to surface water flooding (e.g. in the railway station carpark);
Provision is made within the site layout for additional surface water attenuation (e.g. pond or wetland) in the public open space to assist in managing runoff from offsite areas. This has the potential to reduce the overall flood risk;
At source’ control measures such as green roofs, rainwater harvesting and water butts should be included in the design. Permeable pavement should be considered in surface carparking areas, but may not be appropriate for the transport interchange due to vehicle loading. Public gardens and open space should consider benefits of including swales and ‘rain gardens’ (bioretention devices) for both surface water management and biodiversity. Development should seek to reduce the rate and volume of water discharging from the site to reduce the load on the local surface water network and ABP Pump Station; and
Finished floor levels should be raised a minimum of 300mm above local road and carparking levels to manage the risk of surface water flooding.
3.3.5 These measures are likely to reduce flood risk overall, and assist in demonstrating the site passes part (c) of the Exception Test.
3.3.6 There is likely to be limited benefit at present in placing site users on the EA Flood Warning Register, as the site is currently outside an area of ‘high’ probability tidal or fluvial flooding.
Southampton City Council 3-6 August 2010
Southampton Level 2 SFRA: Volume 3
3.3.7 A site specific flood emergency plan should be prepared, in consultation with Council emergency planners, emergency services, and with reference to Multi Agency Flood Plan to evacuate site users out of the floodplain in an ‘emergency’ flood event in the future.
3.3.8 Emergency services access should be available for the site via West Park Road, Commercial Road and Central Station Bridge for the lifetime of the development. 3.4 Option 1: Strategic Precautionary Approach
3.4.1 The site would benefit from strategic flood risk management infrastructure on the River Itchen and the River Test to a specified standard of protection informed by the Southampton FCERM Strategy (e.g. 1 in 200 year ARI tidal event in 2115 of 4.2m AOD + freeboard).
3.4.2 Development, including Southampton Central Station, should contribute to the overall delivery of strategic flood risk management. This has the benefit of protecting Southampton Central Station and the mainline railway to the east and west
3.4.3 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD to manage tidal residual risk over the lifetime of the development. Although they do not necessarily need to remain dry, buildings south of the railway must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of approximately 0.9m.
Essential Infrastructure: Southampton Central Station and mainline railway
3.4.4 Network Rail should consider whether a higher standard of protection is required as part of their business continuity and emergency planning. This may mean either a local, or general increase in SoP in Southampton that would benefit the wider community as well as the Central Station site specifically.
Basements
3.4.5 If necessary, basement carpark access off Western Esplanade would be acceptable, if accompanied by appropriate flood warning and signage to advise on flood risk in this area. 3.5 Option 2: Site Specific Measures
3.5.1 Conceptual site specific flood risk management measures have been identified for each development type currently proposed:
Essential Infrastructure: Southampton Central Station and mainline railway
3.5.2 Based on currently available information, maintaining a ‘site specific’ 1 in 200 year (or higher) standard of protection to the railway station for the next 100 years will be difficult to achieve without raising the railway line to the east and the west of Central Station, which is not considered a practical solution.
3.5.3 Three sub-options have been assessed for managing flood risk to the ‘essential’ infrastructure of the Central Station:
Southampton City Council 3-7 August 2010
Southampton Level 2 SFRA: Volume 3
3.5.4 Option 1 (CS_SSM_St_O1): The infrastructure associated with the railway station, such as platforms, controls and control rooms, can be designed to be resilient to flooding at the site specific. This will also assist in managing surface water flood risk during high intensity rainfall events that exceed the capacity of the drainage system. This would reduce the existing flood risk, however the essential infrastructure would not remain ‘dry’ for the lifetime of development.
3.5.5 Option 2 (CS_SSM_St_O2): Building a small wall along the site boundary with Western Esplanade would potentially cut off the overland flow route from the Major Development Quarter onto the Central Station. The solid wall with no openings to allow the flow of water would need to be approximately 320m long and to a level of approximately 4.25mAOD (this includes a 300mm freeboard). At the lowest point on Western Esplanade the top of the wall would be approximately 1.5m higher than the current road level. This wall would only need construction after 2070, however whether it acted as an effective flood defence would depend on whether flooding further to the west resulted in flooding on the railway line, conveying floodwaters towards Central Station. Based on currently available information this solution is not likely to be practical, however could be reassessed if improved flood modelling data in the west becomes available.
3.5.6 Option 3 (CS_SSM_St_O3): Raising Western Esplanade would have a similar effect for the Central Station as constructing a wall along the boundary. Raising the road would have the benefit of making one of the key transport routes in Southampton more resilient to flooding, however is likely to have an impact on off-site vehicle entrances (e.g. the Toys R’ Us Superstore) and be more expensive than the construction of a wall (refer to Diagram 3.6).
Legend: Wall (to 4.25m AOD) N Road Raising
Diagram 3.6: Option 2 and Option 3
Southampton City Council 3-8 August 2010
Southampton Level 2 SFRA: Volume 3
More Vulnerable: Residential
3.5.7 If residential development is required, adopting a sequential approach to layout by locating it in the north of the site will reduce the scale of mitigation required. Residential development in the north of the railway station and outside of the railway carpark susceptible to surface water flooding is expected to remain ‘safe’, and is likely to be able to include ground floor habitable rooms, with safe ‘dry’ access via Commercial Road and Central Station Bridge.
3.5.8 Where ground floor residential development is proposed, including property entrances, these should be set at a minimum of 300mm above surrounding road levels north of the railway to manage the risk of surface water flooding. This will also locate properties approximately 500- 600mm above the predicted tidal flood levels.
3.5.9 If residential development is proposed on upper storeys south of the railway, these should have ‘safe’ access available out of the building either:
In a northerly direction onto Blechynden Terrace or,
To the west onto Central Station Bridge, not onto Western Esplanade which could be at potentially significant flood hazard. This access can be a ‘secondary’ or ‘emergency’ exit (above 3.8mAOD), with the main entrance located on Western Esplanade if necessary. It is expected this safe access will be achievable through careful development design.
More Vulnerable: Hotel
3.5.10 Similar to residential development, two sub options have been considered based on the potential development life of the hotel: if possible this use should be located on higher ground in the north of the site.
3.5.11 Option 1 (CS_SSM_H_O1): 60 year lifetime of development. On the basis of a lifetime of development for the hotel of approximately 60 years, there is no restriction on finished floor levels based on tidal flood risk. It is recommended that floor levels are a minimum of 300mm above road and carpark levels to reduce the risk of surface water flooding.
3.5.12 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD (1 in 1000 year ARI tidal flood event plus freeboard to 2070).
3.5.13 Option 2 (CS_SSM_H_O2): 100 year lifetime of development - If the hotel is likely to have a lifetime of development of up to 100 years, the ground floor should be located at a minimum of 4.1mAOD (approximately 1.1-1.3m above the low ground in the south of the site). Guest sleeping accommodation should be located above the ground floor.
3.5.14 Similar to residential development, if a hotel is proposed south of the railway it should have ‘safe’ access available out of the building either in a northerly direction over the railway onto Blechynden Terrace or to the west onto Central Station Bridge. This access can be a ‘secondary’ or ‘emergency’ exit (above 3.8mAOD), with the main entrance on Western Esplanade if necessary. It is not recommended that Western Esplanade be utilised as a primary escape route due to its potentially significant flood hazard
Southampton City Council 3-9 August 2010
Southampton Level 2 SFRA: Volume 3
3.5.15 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders, and raised internal services above 4.7mAOD (1 in 1000 year ARI tidal flood event plus freeboard to 2115).
Less Vulnerable: Commercial
3.5.16 Wherever possible, finished floor levels for property should be 300mm above road and carparking levels to manage surface water flood risk.
3.5.17 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD (1 in 1000 year ARI tidal flood event plus freeboard for the year 2070).
3.5.18 Basement carparking has not currently been indicated as part of development proposals. However, if proposed, it is recommended entrances are not located off Western Esplanade, to avoid potentially deep water flowing into the carpark. Entrance levels for basement carparking access off Western Esplanade, will be difficult to deliver due to existing flood depth and road levels. 3.6 Option 3: Management Adaptive Approach
3.6.1 If a strategic flood risk management solution is brought forward before 2070, the benefits will be of most significance to the railway station (essential infrastructure) and reduce the scale of flood risk management measures required for residential/hotel accommodation.
3.6.2 Development, including railway infrastructure, should contribute to the overall delivery of strategic flood risk management.
3.6.3 Phasing of development could be used to fund either the contribution to strategic flood risk management, or contribution to site specific measures (e.g. a wall along the southern boundary). For example, bring forward development north of the railway station first, where less tidal flood risk mitigation is required. However, this would need to be informed by a more detailed assessment of the surface water flood risk to the site and the outcomes of the Southampton FCERM Strategy. If the timeframes for development cannot be phased to coincide with the implementation of the strategic flood risk management measure and more vulnerable landuses are located in areas of flood risk, then the site should consider adopting site specific measures to allow development to occur in a timely manner.
Essential Infrastructure: Southampton Central Station and mainline railway
3.6.4 Under this approach the railway station would be expected to be protected to a 1 in 200 year standard for the next 100 years, however Network Rail should still consider whether a higher standard is required for business continuity and emergency planning.
3.6.5 If a 1 in 200 year ARI standard of protection is appropriate, flood resilient and resistance measures should be included in the station redevelopment, such as external/internal wall renders, and raised internal services above 4.7mAOD (1 in 1000 year ARI tidal flood event plus freeboard for the year 2115).
More Vulnerable: Residential/Hotel
3.6.6 Residential development could be located south of the railway station, however residential in the north should still be favoured (sequential approach) to assist with managing the ‘residual’ risk of tidal flooding from an extreme flood event.
Southampton City Council 3-10 August 2010
Southampton Level 2 SFRA: Volume 3
3.6.7 Ground floor residential levels should be set 300mm above the surrounding road and carpark levels to minimise the risk of surface water flooding.
3.6.8 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.7mAOD.
Less Vulnerable: Commercial:
3.6.9 There is limited benefit to commercial development applying the managed adaptive approach, on the basis of a lifetime of development of 60 years, as the site does not flood during the 1 in 200 year ARI tidal flood event before 2070.
3.6.10 Wherever possible, finished floor levels for property should be 300mm above road and carparking levels to manage surface water flood risk.
3.6.11 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
Less Vulnerable: Basements
3.6.12 If necessary, basement carpark access off Western Esplanade would be feasible, if accompanied by appropriate flood warning and signage to advise on risk. 3.7 Environmental Impacts
3.7.1 Key potential environmental impacts associated with the site specific mitigation may include an impact on the townscape of the area due to the raised floor or ground levels. This may be especially prominent where the residential elements of the development are proposed.
3.7.2 Views of the site may also be impacted by the installation of a small wall along the site boundary with Western Esplanade which would be 320m long and approximately 4.25m AOD. Views may also be adversely impacted by the proposed option to raise Western Esplanade Road.
3.7.3 There may be the potential for disturbance to unknown archaeological remains as part of associated earthworks. However, significant impact is not anticipated due to the urbanised nature of the area and that much of the ground is likely to have been disturbed previously.
3.7.4 The Listed Wyndam Court building is in close proximity to the site and is located on Commercial Road. There may be a slight, temporary disruption to the setting of this building whilst construction works are carried out; however, this is not anticipated to be a significant impact. 3.8 Site Summary
3.8.1 The site is currently located partially within Flood Zone 2 and 3. The Sequential (and likely Exception) Test will need to be demonstrated for development proposed within this site.
3.8.2 The Sequential Approach should be applied to the development of this site, favouring development in the north first.
3.8.3 Tidal flooding (after 2070) and surface water flooding are the main risks to the site. Groundwater flooding may also be an issue and should be assessed through further site investigations.
Southampton City Council 3-11 August 2010
Southampton Level 2 SFRA: Volume 3
3.8.4 Flood resilience and resistance techniques are likely to be required throughout much of the ground floor of new development, the scale and associated cost will be dependent on the development proposals.
3.8.5 A Strategic Flood Management solution constructed prior to 2070 will benefit the site, in particular the existing rail infrastructure and more vulnerable development (residential/hotel).
3.8.6 Site specific raising of Western Esplanade to protect against tidal flooding in the future is unlikely to be practical due to impacts on site access of properties located on the road and could be expensive. A creation of a water tight wall/building facade (which can withstand the lateral hydrostatic pressure from tidal flood waters) may be more feasible due to the location of the site and the existing developments nearby.
3.8.7 No specific flood risk management measure for the tidal flood risk are required for commercial or less vulnerable land uses due to the estimated lifetime of development (60 years) indicating that the tidal flood risk is low for the site.
3.8.8 It is recommended that SUDS are incorporated into the final layout to assist in reducing the volume of runoff entering the existing drainage infrastructure during the 1 in 100 year plus climate change event. As a minimum it is recommended that flood levels are raised 300mm above the existing ground level.
3.8.9 All site users are to receive an ‘information pack’ from developers identifying, as a minimum; the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
3.8.10 More detailed assessment of surface water flooding should be undertaken as part of either the Southampton SWMP, or detailed FRA to inform the site masterplanning, or a precautionary approach taken avoiding ‘more vulnerable’ or essential infrastructure in areas identified as susceptible to surface water flooding.
Southampton City Council 3-12 August 2010
Southampton Level 2 SFRA: Volume 3
4. Key Site 2: Major Development Quarter
4.1 Site Proposal
4.1.1 The proposed development envelope encompasses an area of approximately 56.7 ha and is located at National Grid Reference 441450 111780. The topography of the site varies from 2.8mAOD south-east, 3mAOD north-east, 2.8mAOD south-west and 4.2mAOD north-west.
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Diagram 4.1: Site Location
4.1.2 Key spatial requirements for the site include:
A mixed use shopping centre (potentially including residential development) and West Quay Retail Park (also with the potential for residential development).
The eastern areas closest to West Quay shopping mall are likely to be developed for retail purposes. Residential land uses are considered on upper floors across the entire site (issues relating to safe access will need to be addressed) whilst areas south of West Key Road will be utilised for office and leisure land uses.
Southampton City Council 4-1 August 2010
Southampton Level 2 SFRA: Volume 3
The Combined Heat and Power (CHP) plant will be retained within the boundaries of the site.
It is understood the ’vision’ is to create a street scene with access to commercial/retail properties at ground floor.
4.1.3 Given the size of the development, a site specific flood risk management option has the potential to form part of the strategic solution for Southampton.
4.1.4 The proposed retail, commercial and leisure areas are classified as a ‘less vulnerable’, the residential units as ‘more vulnerable’ and the CHP Plant as ‘essential infrastructure’ in accordance with Table D.2 of PPS 25.
4.1.5 PPS 25 requires that ‘highly vulnerable’ development in Flood Zone 2 must additionally pass the Exception Test. Similarly ‘more vulnerable’ development or ‘essential infrastructure’ in Flood Zone 3 will also need to pass the Exception Test. ‘Highly vulnerable’ development should not be located in Flood Zone 3 on site. 4.2 Technical Assessment: Flood Risk
Summary of Flood Risk
4.2.1 The site is currently at a ‘low’ probability of tidal flooding, however by 2070 this is expected to be ‘medium’ and by 2115 a ‘high’ probability with a ‘significant’ flood hazard predicted over the majority of the site. The topography means that flooding may occur for extended periods as water is unable to recede from the central low-lying parts of the site.
4.2.2 Low lying land in the centre of the site may be susceptible to surface water flooding.
4.2.3 Groundwater flooding may become an issue in the future and should be assessed and managed as part of a detailed site FRA. Future regional groundwater management should be considered by the Environment Agency and SCC.
4.2.4 Further detail on the existing and future flood risk at the site are provided in the following sections.
Historic Flooding
4.2.5 SCC Record – 5 June 2008: A small area of land near the intersection of Southern Road and Western Quay Road (outside of site boundary) was subject to flooding after a period of heavy rainfall. Records indicate the cause of the flooding as from groundwater sources, however it is likely that surface water runoff/sewer flooding was also a contributing factor.
Flood Zones
4.2.6 Diagram 4.2 identifies that the site is located within Flood Zones 1, 2 and 3.
4.2.7 If the predicted effects of climate change are accurate then within the next 100 years the majority of the site could be located within Flood Zone 3 (Diagram 4.2).
Southampton City Council 4-2 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend: Flood Zone 3
Flood Zone 2
Diagram 4.2: Environment Agency Flood Zones
Tidal Flood Risk and Hazard
4.2.8 The site is not currently located in an area at ‘high’ probability of tidal flooding due to higher ground levels between the site and the River Test (currently ground levels within the site range between 2.75 to 4 mAOD). The area is generally outside the 1 in 200 year ARI tidal flood extent in 2055 (Diagram 4.3).
4.2.9 By 2070 flooding south of West Quay Road is predicted to occur due to the low lying ground levels of the site, (Diagram 4.4). The probability of tidal flooding up to 2055 is considered ‘low’ to ‘medium’ with some areas of low to significant flood hazard occurring in the south eastern corner of the site by 2070 (Diagram 4.4).
4.2.10 By 2115 the majority of the MDQ will be inundated during 1 in 200 year ARI tidal flood event (Diagram 4.5). Flood depth is generally 500-800mm, however up to 1m in the lowest parts of the site. Flood hazard is significant over much of the area.
4.2.11 By 2115 (Diagram 4.8) the majority of the site is modelled to be at a ‘significant’ risk of flooding from tidal sources.
Southampton City Council 4-3 August 2010
Southampton Level 2 SFRA: Volume 3
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N
Legend (m)
Diagram 4.3 Flood Depth 2055 - 1 in 200 year ARI Tidal Flood Event
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Legend (m)
Diagram 4.5 Flood Depth 2115 – 1 in 200 year ARI Tidal Flood Event
Legend (m)
Diagram 4.4 Flood Depth 2070 – 1 in 200 year ARI Tidal Flood Event
Southampton City Council 4-4 August 2010
Southampton Level 2 SFRA: Volume 3
Legend:
Low Moderate N Significant
Extreme
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Diagram 4.4 Flood Hazard 2055 – 1 in 200 year ARI Tidal Flood Event
Legend: Low N Moderate Significant Extreme
Legend: Low Moderate Significant Extreme
Diagram 4.8 Flood Hazard 2115 – 1 in 200 year ARI Tidal Flood Event
Diagram 4.7 Flood Hazard 2070 – 1 in 200 year ARI Tidal Flood Event
Southampton City Council 4-5 August 2010
Southampton Level 2 SFRA: Volume 3
Tidal Residual Risk
4.2.12 By 2115 the entire site could be located within the ‘extreme’ 1 in 1000 year ARI (0.1% AEP) tidal flood event.
Fluvial Flood Risk
4.2.13 The site is located within an area identified at low probability of fluvial flooding.
Surface Water Flood Risk
4.2.14 The site is particularly susceptible to surface water flooding (Diagram 4.9). Surface water modelling indicates ponding is likely to occur in low ground in the centre of the site as a result of overland flow from surrounding higher ground to the north and east. In June 2008 flooding occurred in the area, and although the cause of the flooding has not been clearly established, it is likely to have been a combination of under capacity drainage network, partial culvert blockage, and local topography.
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Diagram 4.9: Susceptibility to Surface Water Flooding
Southampton City Council 4-1 August 2010
Southampton Level 2 SFRA: Volume 3
Groundwater Flood Risk
4.2.15 The site is ‘Moderate Risk’ due to permeability of the underlying geology and historical flood incident near the boundary of the site. The Watermark West Quay Flood Risk Assessment (November 2008) identified that ‘according to the Land Quality Assessment, groundwater was seen to be flowing under artesian conditions from standpipes up to 1994. In addition to this, artesian water was found to be issuing from a deep standpipe installed to the west of the Site in 2000. The Land Quality Assessment notes that groundwater was encountered at approximately 2m below ground level (bgl) indicating that there may be groundwater in the River Terrace Deposits which were found at depths of between 1.7m bgl and 4.3m bgl.’
4.2.16 Sea level rise as a result of climate change is likely to also have an effect on the water table level in proximity to the river frontage. The available data indicates that if an associated 1m rise in the groundwater table occurs, groundwater levels in the MDQ could approach the surface level during periods of prolonged rainfall. This could result in shallow, but long lasting flooding.
4.2.17 Development proposals will need to consider site ground conditions and groundwater levels over the lifetime of the development. In particular the design of any underground structures or services and foundations. Future regional groundwater management should be considered by Hampshire County Council, SCC and the EA.
Flood Risk from Artificial Sources
4.2.18 No artificial sources are located near the site. 4.3 General Flood Risk Management
4.3.1 The following guidance applies independent of the overall approach to managing flood risk on site:
4.3.2 There is the opportunity to adopt the sequential approach to layout on site. If residential development (‘more vulnerable’) is proposed, this should be located on higher ground in the east of the MDQ, wherever possible. The low lying area in the centre of the site where water is likely to pond for extended periods should be the last area for development following the sequential approach as flood hazards will last longest.
Surface Water Flooding:
4.3.3 The Southampton Surface Water Management Plan should investigate the causes of flooding in June 2008 and identify if remedial work is required to the drainage network to reduce the probability of flooding. It is recommended that either as part of the SWMP, or a detailed FRA, that hydraulic modelling of the local surface water catchment is undertaken which considers the Rolles Brook, as well as ‘direct rainfall’ on the local catchment to quantify more accurately the risk of surface water flooding. This could be considered in conjunction with any future assessment undertaken for the Central Station redevelopment.
Southampton City Council 4-2 August 2010
Southampton Level 2 SFRA: Volume 3
4.3.4 Until more detailed information becomes available it is recommended that:
The site layout should attempt to locate ‘less vulnerable’ or ‘water compatible’ uses such as open space in areas of tidal and surface water flood risk, and avoid locating ‘more vulnerable’ land uses in these areas;
Provision is made within the site layout for overland flow routes from higher ground that can channel runoff towards the River Test, or additional surface water attenuation (e.g. pond or wetland) in public open space to assist in managing runoff from off site areas. This has the potential to reduce flood risk overall;
At source’ control measures such as green roofs, rainwater harvesting and water butts should be included in the design. Permeable pavement should be considered in surface carparking areas or hard landscape areas. Public gardens and open space should consider benefits of including swales and ‘rain gardens’ for both surface water management and biodiversity. Development should seek to reduce the rate and volume of water discharging from the site to reduce the load on the local surface water network and ABP Pump Station; and
Finished Floor levels should be raised a minimum of 300mm above local road and carparking levels to manage the risk of surface water flooding.
4.3.5 These measures will are likely to reduce flood risk overall, and assist in demonstrating the site passes part (c) of the Exception Test.
4.3.6 There is likely to be limited benefit at present in placing site users on the EA Flood Warning Register, as the site is currently outside an area of ‘high’ probability tidal or fluvial flooding.
4.3.7 Until the outcomes of the Southampton FCERM Strategy are known, it is recommended that the MDQ Masterplanning set aside a minimum 20-50m ‘buffer strip’ of open space along the southern boundary. This provides the opportunity to locate a strategic flood defence line further inland should a defence line along the Port frontage prove unfeasible.
4.3.8 A site specific flood emergency plan should be prepared, in consultation with Council emergency planners, emergency services, and with reference to Multi Agency Flood Plan to evacuate site users out of the floodplain in an ‘emergency’ flood event in the future.
4.3.9 Emergency services access should be made available for the site. 4.4 Option 1: Strategic Precautionary Approach
4.4.1 The site would benefit from strategic flood risk management infrastructure on the River Itchen and the River Test to a specified standard of protection informed by the FCERM Strategy (e.g. 1 in 200 year ARI tidal event in 2115 of 4.2m AOD + freeboard). Development should contribute to the overall delivery of strategic flood risk management.
4.4.2 Strategic flood risk management will need to consider the impact of flood defences on local groundwater levels.
Southampton City Council 4-3 August 2010
Southampton Level 2 SFRA: Volume 3
4.4.3 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD (development with a lifetime of 60 years can reduce this level to 4.1mAOD). Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of 1.4m.
Emergency services access:
4.4.4 A strategic flood risk management solution is unlikely to be designed to provide a standard of protection for a1 in 1000 year ARI tidal flood event. However, defences will provide some protection and it is likely that they would reduce the extent and depth of flooding that currently occurs without defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners. The priority for emergency access/evacuation should be given to users in the low –lying area in the centre of the site where flooding may last for extended periods. 4.5 Option 2: Site Specific Measures
4.5.1 Conceptual site specific flood risk management measures have been identified for each development type currently proposed:
Less Vulnerable: Commercial
4.5.2 The majority of the site is not at risk from the 1 in 200 year ARI tidal flood event for the lifetime of commercial development (assumed to be 60 years within this assessment). Wherever possible, finished floor levels for property should be 300mm above road and carparking levels to manage surface water flood risk. In the south of the site where shallow flooding is predicted from the 1 in 200 year ARI tidal flood event in 2070, raising floor levels 300mm above the surrounding ground can limit flood depths (into buildings) to approximately 200mm.
4.5.3 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
More Vulnerable: Residential
4.5.4 Three sub-options have been assessed for managing flood risk to the ‘more vulnerable’ infrastructure of the Major Development Quarter:
4.5.5 Option 1 (MDQ_SSM_R_O1): If residential development is required on upper floors of the commercial buildings, adopting a sequential approach to layout by locating it in areas where flood hazard is low will reduce the scale of mitigation required (Diagram 4.). Residential development on the upper floors that can access Central Station Bridge (via the intersection of Southern Road and Western Esplanade) would be considered safe.
4.5.6 Similarly, residential development on upper floors that could access Bargate Street at ground level would be unlikely to require further flood risk mitigation, as flood hazard in this area in generally ‘low’.
4.5.7 Diagram 4. identifies the areas which are considered the most appropriate for residential development based on the existing ground elevations, emergency and safe access/egress, and the susceptibility to surface water flooding.
Southampton City Council 4-4 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend: Preferred Residential Development Locations
Diagram 4.10: Preferred Residential Development Locations
4.5.8 Diagram 4.10 is indicative only and should be used as a basis for locating ‘more vulnerable’ development within the site. If appropriate flood risk management measures protect the site then wider areas should be utilised for this type of development. Appropriate evidence relating to safe access/egress and finished floor levels will need to be included within the proposed development(s) planning application.
4.5.9 Option 2 (MDQ_SSM_R_O2): if large scale residential development is required on upper floors, or if it is required in buildings that do not have direct access to the areas identified in Option 1, then safe access at ground level without mitigation will be difficult.
4.5.10 Raising West Quay Road through the site (Diagram 4.11) could provide ‘safe’ access to a broader range of residential areas where access is provided from upper storeys directly onto West Quay Road. To provide a ‘dry’ access route, 300mm above flood levels, would require the raising of the road by up to 1.0m from the roundabout at Mayflower Park through to the junction with the A33 (Mountbatten Way) – a distance of approximately 1.2km. This would also require road raising at the junction with Southern Road as well as the Mayflower Park roundabout.
4.5.11 This road could act as a defacto ‘defence’ to the MDQ land to the north, as well as potentially to Southampton Central Station and the mainline railway. It would also have a broader flood risk benefit to Southampton by making a strategic transport route more resilient to flooding.
Southampton City Council 4-5 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend: Road Raising
Diagram 4.11: Raising West Quay Road
4.5.12 Services (e.g. drainage, power, telecommunication) located in West Quay Road would either require protection or realignment. Overland flow routes would likely be affected by road raising, therefore additional surface water attenuation or drainage infrastructure may be required north of the raised road.
4.5.13 Partially raising West Quay Road so that it forms a ‘safe’ access route, with ‘low’ flood hazard has the benefit of requiring less road raising (filling depths of approximately 0.3-0.6m), however would be overtopped by a 1 in 200 year ARI tidal flood event in 2115, so would not protect development to the north of West Quay Road, and would be susceptible to ‘breaching’ should overtopping flood waters erode/compromise the integrity of road.
4.5.14 Option 3 (MDQ_SSM_R_O3): Creating a ‘defence’ line along the southern boundary of the MDQ offers the opportunity to protect the development, Central Station and wider community. This could be done as part of the realignment of West Quay Road or as public open space ‘buffer strip’ between the Port and MDQ.
4.5.15 This would likely need to be undertaken in conjunction with the redevelopment of Royal Pier, immediately to the south, where it could tie into similar flood defences.
Southampton City Council 4-6 August 2010
Southampton Level 2 SFRA: Volume 3
4.5.16 A flood defence bund constructed to 4.8mAOD (allowing 600mm freeboard above the predicted 2115 1 in 200 year ARI tidal flood tidal flood level) would require land raising by approximately 1.2-1.7m, and would require a width of approximately 20m-50m (refer to Diagram 4.12). This could offer the opportunity to provide a public access route or realignment route for the West Quay Road. The ‘flood defence bund’ could follow Southern Road and eventually connect into the higher ground where Southern Road approaches the Central Station. The ‘bund’ would also require land raising at the junction of West Quay Road and Southern Road in order for the raised areas to tie in with the existing road network.
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Legend:
Defence land raising 50m width for road Defence land raising Bund to tie into 20m width Royal Pier Defence
Diagram 4.12:Conceptual Raised Flood Defence Locations
More Vulnerable: Hotel
4.5.17 Similar to residential development, a hotel is a ‘more vulnerable’ use. If possible this use should be located on higher ground in the areas identified in Figure 4.10. The flood risk management measures necessary for hotel development may be linked to the expected lifetime of the development.
Southampton City Council 4-7 August 2010
Southampton Level 2 SFRA: Volume 3
4.5.18 Two sub options have been considered based on the potential development life of the hotel:
4.5.19 Option 1 (MDQ_SSM_H_O1): 60 year lifetime of development. On the basis of a lifetime of development for the hotel of approximately 60 years, there is no restriction on finished floor levels based on tidal flood risk (with the exception of the south eastern corner of the site). It is recommended that flood levels are a minimum of 300mm above road and carpark levels to manage the risk of surface water flooding.
4.5.20 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
4.5.21 Option 2 (MDQ_SSM_H_O2): 100 year lifetime of development. If the hotel is likely to have a lifetime of development of up to 100 years, it will need to be considered with a similar approach to residential development. Guest sleeping accommodation should be located above the ground floor (with a minimum finished floor level of 4.2mAOD plus freeboard).
4.5.22 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders, and raised internal services above 4.7mAOD.
4.5.23 Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of 0.9-1.2m.
Emergency Services Access:
4.5.24 Emergency services access during an ‘extreme’ flood event (4.4mAOD for the 1 in 1000 year tidal flood event for the year 2115) will be difficult to obtain for the lifetime of residential development without the residential ‘control and mitigate’ measures identified above. Without mitigation by 2115 the roads around the site are likely to be inundated in excess of 0.9m deep. Raising West Quay Road could provide a route that although flooded, may remain ‘safe’ for the lifetime of the development.
4.5.25 The ‘flood defence bund’ is unlikely to be designed to provide a standard of protection for a 1 in 1000 year ARI tidal flood event. However will provide some protection and it is likely that it would reduce the extent and depth of flooding that occurs without any defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners. 4.6 Option 3: Management Adaptive Approach
4.6.1 If a strategic flood risk management solution is brought forward before 2070, it will reduce the scale of flood risk management measures required for residential/hotel accommodation. Development should contribute to the overall delivery of strategic flood risk management.
4.6.2 Phasing of development could be used to fund either the contribution to strategic flood risk management, or contribution to site specific measures (e.g. the ‘flood defence bund’). For example, focussing on bringing forward ‘less vulnerable’ development (i.e. commercial only) in areas where residential development is not proposed could mean that a decision on how to manage flood risk to residential development could be delayed until the outcome, and funding, for the Southampton FCERM Strategy became clear.
Southampton City Council 4-8 August 2010
Southampton Level 2 SFRA: Volume 3
4.6.3 If the timeframes for development cannot be phased to coincide with the implementation of the strategic flood risk management measure and more vulnerable landuses are located in areas of flood risk, then the site should consider constructing the site specific measure to allow development to occur in a timely manner.
More Vulnerable: Residential/Hotel
4.6.4 Residential development on upper floors could be located across the site, however residential close to higher ground in the north and east should be favoured (sequential approach) to assist with managing the ‘residual’ risk of tidal flooding from an extreme flood event.
4.6.5 Ground floor residential levels should still be set 300mm above the surrounding road and carpark levels to reduce the risk of surface water flooding.
Less Vulnerable: Commercial
4.6.6 There is limited benefit to commercial development applying the managed adaptive approach, on the basis of a lifetime of development of 60 years, as there is limited flooding during the 1 in 200 year ARI tidal flood event before 2070.
4.6.7 Wherever possible, finished floor levels for property should be 300mm above road and carparking levels to manage surface water flood risk.
4.6.8 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
Emergency services access:
4.6.9 A strategic flood risk management solution is unlikely to be designed to provide a standard of protection for a 1 in 1000 year ARI tidal flood event. However, defences will provide some protection and it is likely that they would reduce the extent and depth of flooding that occurs without any defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners. 4.7 Environmental Impacts
4.7.1 Key potential environmental impacts associated with the proposed site specific mitigation proposals would primarily include the impact on the townscape of the immediate area. The raising of ground levels or potential installation of a defence bund approximately 1.2-1.7m above the existing ground levels may impact on views from the site across the River Test.
4.7.2 Listed buildings, SAMS & Old Town North and Old Town West Conservation areas located within and adjacent to the east of the site, however there may be potential for the disturbance of currently unidentified archaeological remains associated with the designation of the area as one of archaeological potential.
4.7.3 There may also be the potential to impact on terrestrial ecology in the area arising from the construction of the ‘buffer’ along the Southern boundary. This may need to be approximately 50m wide in places to accommodate access and may subsequently increase the development footprint of the existing area and potential impact on adjacent green sites.
Southampton City Council 4-9 August 2010
Southampton Level 2 SFRA: Volume 3
4.8 Site Summary
4.8.1 The site is currently located partially within Flood Zones 2 and 3. The Sequential (and likely Exception) Test will need to be demonstrated for any development proposed within this site
4.8.2 Tidal flooding (after 2070) and surface water flooding are the main risks to the site. Groundwater flooding may also be an issue and should be assessed through further site investigations.
4.8.3 A Strategic Flood Management solution for Southampton will benefit the site, in particular the ability to maximise the residential development options. If more vulnerable uses are proposed within the site then the construction of a flood bund around the southern perimeter of the site or raising the West Quay Road to 4.2mAOD (plus an allowance for freeboard) will be necessary in lieu of a strategic defence to provide safe development for the lifetime of the proposed land use (100 years for residential). The most logical staging for this to occur would be if development located near the area of road raising was also ‘regenerated’ so that new access roads and associated infrastructure can be constructed that would tie into the new West Quay Road levels. An alternative would be to raise West Quay Road and modify the entry/exit roads so that development north and south of the road would need to loop around and access this road via areas of high ground. No underpasses (for pedestrians or vehicles) should be constructed within the raised road as this could compromise the standard of protection.
4.8.4 No specific flood risk management measure for the tidal flood risk are required for commercial or less vulnerable land uses across the majority of the site due to the estimated lifetime of development (60 years) indicating that the tidal flood risk is low for the site.
4.8.5 Flood resilience and resistance techniques are likely to be required throughout much of the ground floor of new development to manage ‘residual’ flood risk, the scale and associated cost will be dependent on the development proposals.
4.8.6 It is recommended that SUDS are incorporated into the final layout to assist in reducing the volume of runoff entering the existing drainage infrastructure during the 1 in 100 year plus climate change event. As a minimum it is recommended that flood levels are raised 300mm above the existing ground level.
4.8.7 The Sequential Approach should be applied to the development of this site.
4.8.8 All site users are to receive an ‘information pack’ from developers identifying, as a minimum; the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
4.8.9 More detailed assessment of surface water flooding should be undertaken as part of either the Southampton SWMP, or detailed FRA to inform the site masterplanning, or a precautionary approach taken avoiding ‘more vulnerable’ or essential infrastructure in areas identified as susceptible to surface water flooding.
Southampton City Council 4-10 August 2010
Southampton Level 2 SFRA: Volume 3
5. Key Site 3: Royal Pier 5.1 Site Proposal
5.1.1 The proposed development envelope encompasses an area of approximately 14.9 ha and is located at National Grid Reference 441730 111060. The topography of the site varies from 2.8mAOD in the north-west, 3.2m AOD in the south-east and 3.3m AOD in the south west.
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Diagram 5.1: Site Location
5.1.2 Key spatial requirements for the site include:
Mixed used facilities including residential, commercial, retail and office buildings. There is also the potential for an arena, boat show and heritage area.
The existing Ferry Building is listed and will constrain any ground floor development in close proximity to the building. This may also constraint how flood risk is managed, in particular a strategic defence line
5.1.3 The proposed arena, retail and heritage areas are classified as a ‘less vulnerable’, the residential units as ‘more vulnerable’ and the Boat show area as ‘water compatible’ in accordance with Table D.2 of PPS 25. It is understood the likely proposed development will predominantly consist of less vulnerable land uses with any more vulnerable development being development located north of West Quay Road..
5.1.4 There are no historic records of flooding at the Royal Pier site.
Southampton City Council 5-1 August 2010
Southampton Level 2 SFRA: Volume 3
5.2 Technical Assessment: Flood Risk
Summary of Flood Risk
5.2.1 The site is currently at ‘low’ to ‘medium’ probability of tidal flooding, however by 2055 this is expected to be ‘high’ and by 2115 a ‘significant’ flood hazard can be expected over much of the site and surrounding area, however on the northside of Town Quay the ground levels quickly rise to a level higher than predicted flood levels.
5.2.2 Groundwater flooding may become an issue in the future and should be assessed and managed as part of a detailed site FRA. Future regional groundwater management should be considered by Hampshire County Council, the Environment Agency and SCC.
5.2.3 Further detail on the existing and future flood risk at the site are provided in the following sections.
Flood Zones
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Legend: Flood Zone 3
Flood Zone 2
Diagram 5.2: Environment Agency Flood Zones
5.2.4 The site is located within Flood Zones 1, 2 and 3 (Diagram 5.2). Current development proposals are compatible in this location, subject to the Sequential and potentially the Exception Test.
5.2.5 If the predicted effects of climate change are accurate then the majority of the site could be located within Flood Zone 3 in the future.
Southampton City Council 5-2 August 2010
Southampton Level 2 SFRA: Volume 3
Tidal Flood Risk and Hazard
5.2.6 Diagram 5.3 below indicates when the site may become susceptible to tidal flooding. The results of the detailed hydraulic model (discussed above and illustrated overleaf) should be used when assessing the risk to a site.
Flooding Regularity at Royal Pier
4.5
4.0 Astronomical Tide 3.5 1 in 20 year
3.0 1 in 50 year
1 in 100 year 2.5 1 in 200 year Water Level (mAOD)
2.0 Approximate flooding threshold level 1.5 2010 2030 2050 2070 2090 2110 Time (years)
Diagram 5.3: Susceptibility of the Royal Pier Site to Tidal Flooding
5.2.7 The site is currently not located in an area at ‘high’ probability of tidal flooding. Ground levels are generally 3.1-3.4mAOD. However, the proximity of the site to the River Test and little existing freeboard indicates that wave action may result in some water washing onto site during storm conditions.
5.2.8 By 2055 some flooding is predicted in Mayflower Park during the 1 in 200 year ARI tidal flood event (flood depths 0.3-0.5m). During this event the hazard rating for the site is predominantly ‘low’ with some areas of ‘moderate’ hazard.
5.2.9 By 2070 the majority of the site is predicted to flood and by 2115 the entire site will be inundated during a 1 in 200 year ARI tidal event. Flood depths are predicted to be approximately 0.6-1.0m.
5.2.10 Diagrams 5.4 to 5.6 identify the flood depths predicted within the site, whilst Diagrams 5.7 to 5.9 depict the flood hazard for identified 1 in 200 year ARI tidal flood event.
Residual Tidal Risk
5.2.11 Much of the site is potentially located within the ‘extreme’ 1 in 1000 year ARI tidal flood event (Flood Zone 2). By 2115 the entire site could be located within the ‘extreme’ 1 in 1000 year ARI tidal flood event flood event.
Fluvial Flood Risk
5.2.12 The site is located within an area identified at low probability of fluvial flooding.
Southampton City Council 5-3 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend (m)
Diagram 5.4 Flood Depth 2055 - 1 in 200 year ARI tidal flood event
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Legend (m)
Diagram 5.6 Flood Depth 2115 - 1 in 200 year ARI tidal flood event
Legend (m)
Diagram 5.5 Flood Depth 2070 - 1 in 200 year ARI tidal flood event
Southampton City Council 5-4 August 2010
Southampton Level 2 SFRA: Volume 3
N N
Legend: Low Moderate Significant
Extreme Legend: Low Diagram 5.7 Flood Hazard 2055 - 1 in 200 year ARI tidal flood event Moderate Significant Extreme
N Diagram 5.9: Flood Hazard 2115 - 1 in 200 year ARI tidal flood event
Legend: Low Moderate Significant Extreme
Diagram 5.8: Flood Hazard 2070 - 1 in 200 year ARI tidal flood event
Southampton City Council 5-5 August 2010
Southampton Level 2 SFRA: Volume 3
Surface Water Flood Risk
5.2.13 Surface water modelling indicates ponding is likely to occur in low ground in the centre of the site (Diagram 5.10). The ground levels are slightly higher than land immediately to the north in the MDQ, however a series of roads running north-south from higher ground towards Royal Pier are likely to act as overland flow routes during intense rainfall potentially resulting in some localised ponding on site. There are no historic records of surface water flooding on site.
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Diagram 5.10: Susceptibility to Surface Water Flooding
Groundwater Flood Risk
5.2.14 Existing ‘Low Risk’ due to permeability of the underlying geology. The Watermark West Quay Flood Risk Assessment (November 2008) identified that within the MDQ ‘groundwater was seen to be flowing under artesian conditions from standpipes up to 1994. In addition to this, artesian water was found to be issuing from a deep standpipe installed to the west of the Site in 2000. The Land Quality Assessment notes that groundwater was encountered at approximately 2m below ground level (bgl) indicating that there may be groundwater in the River Terrace Deposits which were found at depths of between 1.7m bgl and 4.3m bgl.’
Southampton City Council 5-6 August 2010
Southampton Level 2 SFRA: Volume 3
5.2.15 Sea level rise as a result of climate change is likely to also have an effect on the water table level in proximity to the river frontage. The available data indicates that if an associated 1m rise in the groundwater table occurs, groundwater levels at Royal Pier could approach the surface level during periods of prolonged rainfall. This could result in shallow, but long lasting flooding.
5.2.16 Development proposals will need to consider site ground conditions and groundwater levels over the lifetime of the development. In particular the design of any underground structures or services and foundations. Future regional groundwater management should be considered by Hampshire County Council, SCC and the EA.
Flood Risk from Artificial Sources
5.2.17 No artificial sources are located near the site. 5.3 General Flood Risk Management
5.3.1 There is the opportunity to adopt the sequential approach to layout on site. If residential development (‘more vulnerable’) is proposed, this should be located along the north away from the river frontage, wherever possible.
5.3.2 The following guidance applies independent of the overall approach to managing flood risk on site:
Wherever possible finished floor levels should be raised a minimum of 300mm above local road and ground floor carparking levels to manage the risk of surface water flooding.
All development in this location should be placed on the Environment Agency Flood Warning Register.
All site users are to receive an ‘information pack’ from developers identifying, as a minimum, the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Space constraints restrict the use significant SUDS features such as ponds and wetlands are unlikely to be appropriate. ‘At source’ control measures such as green roofs, rainwater harvesting and water butts should be included in the design. Permeable paving should be incorporated into carparking areas and hard landscape areas. Public gardens and open space should consider benefits of including swales and ‘rain gardens’ for both surface water management and biodiversity. The effects of ‘tide locking’ should be considered in the design of the drainage system, and where necessary additional storage included. Inclusion of SUDS measures will reduce the site runoff, and the scale of storage for ‘tide locking’ attenuation.
A site specific flood emergency plan should be prepared, in consultation with Council emergency planners, emergency services, and with reference to Multi Agency Flood Plan to evacuate site users out of the floodplain in an ‘emergency’ flood event.
Until the outcomes of the Southampton FCERM Strategy are known, it is recommended that the Royal Pier masterplanning set aside a 16m ‘buffer strip’ of open space along the site boundary with the River Test. This provides the opportunity to locate a strategic flood defence line along the river frontage.
Southampton City Council 5-7 August 2010
Southampton Level 2 SFRA: Volume 3
5.4 Option 1: Strategic Precautionary Approach
5.4.1 The site would benefit from strategic flood risk management infrastructure (e.g. flood defences) on the River Itchen and the River Test to a specified standard of protection (e.g. 1 in 200 year ARI tidal event in 2115 of 4.2m AOD + freeboard).
5.4.2 It is likely that a strategic flood defence line would be located within the site. This offers the opportunity for regeneration to directly contribute to strategic flood risk management in Southampton. Development also offers the opportunity to locate the defence line in an appropriate location, without the same constraint that existing development can present. Proposed ‘water compatible’ uses (e.g. boat show) would not necessarily need to contribute to strategic flood risk management if it did not benefit.
5.4.3 Flood resilient and resistance measures should be included in the ground floor building design to manage tidal residual risk over the lifetime of the development, such as external/internal wall renders and raised internal services above 4.7mAOD (if the lifetime of development is 100 years) or 4.1mAOD (if the lifetime of development is 60 years). Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths in excess of 1.4m.
More Vulnerable: Residential
5.4.4 Unless detailed breach/overtopping assessment demonstrates otherwise, ground floor residential development should be multi-storey, locating habitable rooms on the first floor (with a minimum finished floor level of 4.2mAOD plus freeboard) due to the potentially significant flood hazard from an extreme/emergency flood event (residual risk).
5.4.5 Safe refuge should be made available for all site users during the extreme flood event.
Emergency services access:
5.4.6 A strategic flood risk management solution is unlikely to be designed to provide a standard of protection for a 1 in 1000 year ARI tidal flood event. However, defences will provide some protection and it is likely that they would reduce the extent and depth of flooding that occurs without any defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners.
5.4.7 Strategic flood risk management will need to consider the impact of flood defences on local groundwater levels. 5.5 Option 2: Site Specific Measures
5.5.1 Conceptual site specific flood risk management measures have been identified for each development type currently proposed:
Southampton City Council 5-8 August 2010
Southampton Level 2 SFRA: Volume 3
Less Vulnerable: Commercial/Arena
5.5.2 The majority of the site is at risk from the 1 in 200 year ARI tidal flood event for the lifetime of commercial development.
5.5.3 Three sub-options have been assessed for managing flood risk to the ‘less vulnerable’ land uses such as the commercial development of the arena:
5.5.4 Option 1 (RP_SSM_C_O1): Constructing an on-site flood defence along the River Test frontage of the site to a level of approximately 3.9mAOD would provide protection to the site, as well as some benefit to Town Quay Road and the MDQ to the north (refer to Diagram 5.11). Some minor overtopping of the River Test to the west and east of Royal Pier is likely to still occur, however this is relatively minor and can be managed through on site measures (e.g. floor levels raised 300mm to manage surface water flooding). The exact line of the defence would be determined as part of the development of site proposals, however is likely to be in the order of 700m (assuming an existing defence line similar to the existing river frontage). The top of defence would be approximately 0.5-1.0m above the existing ground levels behind the defence. There are opportunities to maintain waterfront access through either setting back the defence line, or integrating the top of defence into a public access platform, or landscape feature within Mayflower Park. The multi-purpose use of a flood defence line offers the opportunity to reduce the associated ‘abnormal’ cost.
5.5.5 Raising the defence to 3.9mAOD would still result in overtopping during the 1 in 200 year ARI tidal flood event for the lifetime of residential development (e.g. in the MDQ). If protection is required for residential development on or off site this solution would not be appropriate in isolation, and would necessitate higher defences as part of the ‘strategic precautionary’ or ‘managed adaptive’ approach.
Bund to tie into MDQ Defence Line N
Legend:
20m wide flood defence
Diagram 5.11: Option 1 (RP_SSM_C_O1) 20m wide Flood Defence
Southampton City Council 5-9 August 2010
Southampton Level 2 SFRA: Volume 3
5.5.6 Option 2 (RP_SSM_C_O2): Raise finished floor levels to 3.9mAOD so that development remains dry and safe (0.5-1m above existing ground levels).
5.5.7 Minor ground raising (200-300mm) could be undertaken to provide strategic walkways that provide a link to the Town Quay roundabout and afford ‘safe’ access for site users.
5.5.8 Option 3 (RP_SSM_C_O3): Raise finished floor levels to a minimum of 3.4mAOD. This would assist in managing surface water flood risk, however the ground floor of development would be subject to ‘low’ tidal flood hazard. Provide ‘safe refuge’ for site users to upper floors and agree appropriate evacuation procedures based on duration of inundation lasting over 5 hours (during a 2115 1 in 200 year ARI tidal flood event).
5.5.9 In both Option 2 and Option 3, where a flood defence structure is not proposed, provision should be made for setting back development a minimum of 16m from the River Test frontage to allow for a future strategic flood risk management solution.
5.5.10 Flood resilient and resistance measures should be included in building design for all three options, such as external/internal wall renders and raised internal services above 4.1mAOD (assumed 60 year development lifetime).
Water Compatible: Boat Show
5.5.11 As a water compatible use it may not be necessary for the boat show to be protected (e.g. through flood defences) and a flood defence line (Option 1) could be located on the landward side of the boat show if necessary.
5.5.12 The boat show structure would not necessarily need to remain dry, however must be designed to withstand hydrostatic forces from an extreme flood event, which could result in water depths of 0.9m.
More Vulnerable: Residential
5.5.13 The extent and depth of flooding means that Residential development is likely to be constrained by the need for off-site safe access routes. Current proposals indicate that residential may be located north of West Quay Road.
5.5.14 Option 1 (RP_SSM_R_O1): Constructing flood defences within Royal Pier, in isolation, would not protect residential uses for the lifetime of development as flooding from the west and east would overtop adjacent lower ground. As identified in the MDQ assessment, there is the opportunity to undertake strategic works in a combined approach that provides protection to the MDQ and Royal Pier. This would be a combination of ‘Option 1’ for the commercial/arena (defences at 4.5mAOD), as well as Residential ‘Option 2’ or ‘Option 3’ for the MDQ site.
5.5.15 Additional off site works would be required to raise Town Quay in the vicinity of Bugle Street, French Street and High Street so that ingress of floodwater was from the east was limited. A raised road would act as a defacto flood defence, connecting a flood defence line at Royal Pier, with higher ground immediately on the north side of the road. Near the intersection of Bugle Street and Town Quay flood depths are lowest. Road levels would need to be raised by approximately 750mm in this area to provide a 300mm freeboard above flood levels, however a lower freeboard may be appropriate in this instance as the defence line is ‘in land’ and not subject to wave action that may reduce the overall standard of protection.
Southampton City Council 5-10 August 2010
Southampton Level 2 SFRA: Volume 3
5.5.16 This option offers the opportunity to provide a contiguous defence line from the A33 through to Town Quay, providing protection to existing development to the north, but also benefiting a number of the key development areas including the MDQ, Central Station, Lower High Street and Royal Pier.
5.5.17 The option will be constrained by the need to raise Town Quay (refer to Diagram 5.12). The extent of road raising will be dependent on the location of the flood defence ‘line’. For example, locating the ‘line’ near Bugle Street will mean works will required over a length of approximately 150-200m; however works further to the east, such as at the High Street intersection, would require road raising of approximately 900mm and potentially over a much longer length, as well as have an effect on a number of side road and vehicle entrances.
5.5.18 The road raising would be located in close proximity to Southampton Wall (heritage), and a number of existing buildings. In some instances footpaths may not be able to be raised, potentially creating a local health and safety issue with car movement located above footpath levels. Road raising is also likely to have an effect on services that either need to be reinstalled or protecting, and could alter overland flow paths during high intensity rainfall events, increasing surface water flood risk to others.
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Legend: Road Raising
Diagram 5.12: Option 1 (RP_SSM_R_O1) Raising Ground Levels at Town Quay
Southampton City Council 5-11 August 2010
Southampton Level 2 SFRA: Volume 3
5.5.19 Option 2 (RP_SSM_R_O2): ’More Vulnerable’ floor levels should be set at a minimum of 4.5mAOD (including 300mm freeboard) so that development remains ‘dry’ (Diagram 5.13). This is approximately 0.7-1.3m above local ground levels.
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Legend: Site Raising
Diagram 5.13: Option 2 (RP_SSM_R_O2) Raising Ground Levels.
5.5.20 If habitable rooms are to be located on the first floor and above, ground floor finished floor levels could be set at a minimum of 3.75mAOD, which would result in ‘low’ flood hazard within the ground floor of buildings (approximately 0.3-0.7m above surrounding ground levels).
5.5.21 On the basis that residential development is proposed only north of West Quay Road, access would be required directly onto the Harbour Parade roundabout in the northern corner of the site, which has only ‘low’ flood hazard for the lifetime of the development (Diagram 5.14). Hydraulic modelling indicates flood depths are less than 300mm along Harbour Parade, although there is an area of ‘significant’ flood hazard across the road. Limited raising of the footpath could be undertaken to provide a ‘dry’ pedestrian access route (80 m lifted by 100- 300mm) or ‘safe’ access route with low hazard (40m lifted by <100mm). Providing safe vehicular access is likely to require a significant further extent of raising along Harbour Parade (in a northerly direction), and is not considered practical at this stage.
Southampton City Council 5-12 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend:
Internal escape route External escape route
Diagram 5.14: Internal and External Escape Routes
5.5.22 The pedestrian route would need to be signposted and residents made aware of the safe access and egress arrangements in the event of a flood. Access would be required through the building above flood levels.
5.5.23 In the event that safe access and egress is not feasible for the site it is recommended that the incorporation of ‘safe refuge’ within the development layout are discussed with the Environment Agency and SCC.
Emergency Services Access:
5.5.24 Creation of a flood defence on site means there is the potential for a breach to occur and deep and fast flowing water to flood the Royal Pier area.
Southampton City Council 5-13 August 2010
Southampton Level 2 SFRA: Volume 3
5.5.25 Emergency services access to residential development in the north of the site is likely to be hindered by flooding on local access roads where flood depths are likely to exceed 0.9m during an ‘extreme’ flood event (4.4mAOD) for the lifetime of development. The existing public footpath from Harbour Parade to Western Esplanade is wide enough for emergency services vehicles; however vehicular access is blocked at present. Establishing this as a route for emergency services vehicles only (e.g. accessible bollards) would provide a route that emergency services could use to access the residential development in Royal Pier (and potentially areas of the MDQ if necessary). 5.6 Option 3: Management Adaptive Approach
5.6.1 A managed adaptive approach on the Royal Pier site could combine site specific flood risk management measures, with the overall strategic flood risk management solution. This could avoid the need to raise Town Quay, as the flood defence ‘line’ may continue to the east along the river frontage.
5.6.2 By undertaking the flood defence work identified in commercial/arena ‘Option 1’ above, the site will be largely protected until 2070.
5.6.3 If a strategic flood risk management solution is constructed by 2070, that ties the works undertaken on Royal Pier into a wider scheme, the site can be protected for the next 100 years.
5.6.4 This would require potential further raising the flood defence ‘line’ constructed within the Royal Pier site from 3.9mAOD to 4.5mAOD. Provision would need to be made within the flood defence design for future possible defence raising. Similarly, a contribution would need to be made by on site residential development for the future upgrading of the defence.
5.6.5 Phasing of development could be used to fund either the contribution to strategic flood risk management. For example, focussing on bringing forward ‘less vulnerable’ development (i.e. commercial only) first would mean a decision on how to manage flood risk to residential development (e.g. off-site footpath works or raising of Town Quay) could be delayed until the outcome, and funding, for the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane), was clear.
5.6.6 If the timeframes for development cannot be phased to coincide with the implementation of the strategic flood risk management measure and more vulnerable landuses are located in areas of flood risk, then the site should consider adopting site specific measures to allow development to occur in a timely manner.
More Vulnerable: Residential
5.6.7 Residential development on upper floors could be located across the site, however residential close to higher ground in the north should still be favoured (sequential approach) to assist with managing the ‘residual’ risk of tidal flooding from an extreme flood event.
5.6.8 Ground floor residential levels should still be set 300mm above the surrounding road and carpark levels to minimise the risk of surface water flooding.
Less Vulnerable: Commercial
5.6.9 Wherever possible, finished floor levels for property should be 300mm above road and carparking levels to manage surface water flood risk.
Southampton City Council 5-14 August 2010
Southampton Level 2 SFRA: Volume 3
5.6.10 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
Emergency services access:
5.6.11 A strategic flood risk management solution is unlikely to be designed to provide a standard of protection for a 1 in 1000 year ARI tidal flood event. However, defences will provide some protection and it is likely that they would reduce the extent and depth of flooding that occurs without any defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners. In the event that emergency access is not available for the site then ‘safe refuge’ options are to be discussed with the Environment Agency and SCC. 5.7 Environmental Impacts
5.7.1 The proposed development site would benefit from the city wide strategic mitigation strategy described in Section 5.5.
5.7.2 Site specific mitigation proposals may impact on the townscape of the site and surrounding area due to raised ground levels or construction of a flood defence along River Test frontage. However, as this area is already heavily industrialised no significant impact is anticipated.
5.7.3 The Royal Pier is a listed structure and located within the boundary of the site. The installation of the proposed flood mitigation may result in a temporary impact on the views or setting of these structures during the construction process, however this will only culminate in a short term impact and any risks can be minimised via the incorporation of specific mitigation/management plans.
5.7.4 The site is located close to the boundary of the Old Town North, Old Town South and Old Town West Conservation Areas. Views of the conservation area may be impacted by the construction process and the final raised elevation levels of the site. However, this is not anticipated to create a significant impact.
5.7.5 The proposed development site is located within an area designated by SCC as having archaeological potential .There may be the potential to disturb currently unknown archaeological remains as part of the construction process. This is not thought to culminate in a significant impact due to the heavily urbanised nature of the area. Appropriate mitigation will be specified to minimise impacts. 5.8 Site Summary
5.8.1 The site is currently located within Flood Zones 1, 2 and 3. The Sequential and Exception Test will need to be demonstrated for any development proposed within this site. The Sequential Approach should be applied to the development of this site, locating residential in the north first.
5.8.2 Tidal flooding (after 2070) and surface water flooding are the main risks to the site. Groundwater flooding may also be an issue and should be assessed through further Site Investigations.
5.8.3 A Strategic Flood Management solution for Southampton will benefit the site and maximise the residential development potential of the site.
Southampton City Council 5-15 August 2010
Southampton Level 2 SFRA: Volume 3
5.8.4 If no strategic measures are selected for the area then site specific measures will be required for any development type proposed within the site. The construction of a defence line (only) along the southern property boundary will reduce the risk of immediate flooding to the site to allow site users sufficient time to escape but the risk of flooding form western and eastern overland flow routs will still exist unless measures are incorporated to design. This could involve a site specific defence line, ground raising and strategic road raising (Town Quay and High Street intersection) to limit the ingress of water into the site. These measures will need to be undertaken in parallel with the site specific measures for the MDQ site (refer to Section 4.2) to offer sufficient protection to development in this area. The raising of roads in these areas may be constrained by health and safety concerns (depending on finished footpath and road levels) the extent, phasing and duration of works and existing access points along with the relocation of existing services in the area. This would require careful logistical planning to minimise disturbance sit property owners and users in the area along with a key transport route.
5.8.5 It is recommended that development is located in the north of the site with safe access being made available for residential land uses.
5.8.6 Flood resilience and resistance techniques are likely to be required throughout much of the ground floor of new development to manage ‘residual’ flood risk, the scale and associated cost will be dependent on the development proposals.
5.8.7 It is recommended that SUDS are incorporated into the final layout to assist in reducing the volume of runoff entering the existing drainage infrastructure during the 1 in 100 year plus climate change event. As a minimum it is recommended that flood levels are raised 300mm above the existing ground level.
5.8.8 All site users are to receive an ‘information pack’ from developers identifying, as a minimum; the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Southampton City Council 5-16 August 2010
Southampton Level 2 SFRA: Volume 3
6. Key Site 4: Fruit and Vegetable Market 6.1 Site Proposal
6.1.1 The proposed development envelope encompasses an area of approximately 1.25 ha and is located at National Grid Reference 442170 111210. It is possible that this development allocation may include the existing parcels of land located between Brunswick Square and Orchard Place. The topography of the site varies from 5.8mAOD on the north-west of the site to 3.8mAOD in the south-east.
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Diagram 6.1: Site Location
6.1.2 Key spatial requirements for the site include:
Proposals for Queensway and Orchard Place are to include a variety of land uses including housing, cafes and bars, restaurants, hotel, leisure, some retail, offices and health (e.g. doctor’s surgery or dentists).
Retail or commercial units as part of a terrace area are proposed to replace the existing buildings at 50-42 High Street. These units will be ground floor properties with housing above.
On Bernard Street it is suggested that a small office complex could be sited, with small scale occupiers with housing above, which would complement the existing offices on the opposite side of Bernard Street. Another option for this area is leisure or health facilities.
The junction of Bernard Street and Queensway plays a key role in identifying the area and it has been suggested that a foodstore could be located here, which could utilise the existing basement as parking.
Southampton City Council 6-1 August 2010
Southampton Level 2 SFRA: Volume 3
The Queensway is proposed to be lined with apartments and townhouses, with on-street parking and trees to provide traffic-calming.
The vacant two-storey building in the south east of the development site is proposed to be replaced with a taller hotel development which would integrate into the larger scale blocks in the area.
6.1.3 The new buildings within the development area will consider, the inclusion of green roofs and site wide sustainable drainage solutions.
6.1.4 The proposed Residential and hotel units are classified as a ‘more vulnerable’ and the commercial units as ‘less vulnerable’ in accordance with Table D.2 of PPS 25 this report has assumed the minimum lifetime a residential development is 100 years and 60 years for a non- residential land uses. 6.2 Technical Assessment: Flood Risk
Summary of Flood Risk
6.2.1 The site is located at ‘low’ to ‘medium’ risk of flooding over the next 100 years. Limited flood risk management measures will be required to demonstrate the site remains ‘safe’ in accordance with PPS 25, for the lifetime of the development.
6.2.2 This can be considered a compatible location for the type of development proposed, however it is recommended the Sequential test is undertaken for residential uses considering Flood Zones over the lifetime of the development.
6.2.3 Further detail on the existing and future flood risk at the site are provided in the following sections.
Flood Zones
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Legend: Flood Zone 3
Flood Zone 2
Diagram 6.2: Environment Agency Flood Zones (Current Day)
Southampton City Council 6-2 August 2010
Southampton Level 2 SFRA: Volume 3
6.2.4 The site is located within Flood Zones 1, all land uses are appropriate within this zone (refer to Diagram 6.2).
6.2.5 If the predicted effects of climate change are accurate there is the potential for the site to be partially located in Flood Zone 2 and 3 in the future ( refer to Figure 2.4 in Volume 2).
Tidal Flood Risk and Hazard
6.2.6 The site is not currently located in an area at tidal risk of flooding (0.7-2.5m above 1 in 200 year ARI tidal flood level). By 2115 tidal flooding is predicted to extend to Briton St, Brunswick Square and Orchard Place, adjacent to the site; however modelling indicates the site is outside the flood extent (with some flooding expected in the wider development area)- refer to Diagram 6.3. This may be a result of the modelling techniques adopted, therefore adopting a precautionary approach, it is assumed that the south east corner of the site will be at risk of flooding beyond 2070.
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Legend (m)
Diagram 6.3: Flood Depth 2115 - 1 in 200 year ARI tidal flood event
Southampton City Council 6-3 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend: Low Moderate Significant Extreme
Diagram 6.4: Flood Hazard 2115 1 in 200 year ARI tidal flood event
Residual Risk
6.2.7 The site is currently located outside the ‘extreme’ 1 in 1000 year ARI tidal flood event. By 2115 the south east corner of the site may be partially located within the ‘extreme’ 1 in 1000 year ARI tidal flood event.
Fluvial Flood Risk
6.2.8 The site is located within an area identified at low probability of fluvial flooding.
Surface Water Flood Risk
6.2.9 The site is located on high ground within Southampton. Surface water modelling indicates some areas of lower lying ground within the site can be classified as having a ‘less’ to ‘intermediate’ susceptibility to surface water flooding (refer to Diagram 6.5). There are no records of surface water flooding on site. Bernard Street and Queensway through the site may act to convey overland flow during high intensity rainfall events.
Southampton City Council 6-4 August 2010
Southampton Level 2 SFRA: Volume 3
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Diagram 6.5: Susceptibility to Surface water flooding
Groundwater Flood Risk
6.2.10 Existing ‘Moderate Risk’ due to the permeability of the underlying geology. Development proposals should investigate ground conditions and groundwater levels if underground design features are proposed as part of the final development.
Flood Risk from Artificial Sources
6.2.11 No artificial sources located near the site. 6.3 General Flood Risk Management
6.3.1 The following guidance applies independent of the overall approach to managing flood risk on site:
6.3.2 Where possible, residential and hotel ‘habitable rooms’ should be located away from the ground floor in the south east portion of the site to manage any uncertainty in flood modelling.
6.3.3 Wherever possible property levels should be raised a minimum of 300mm above local road and carparking levels to manage the risk of surface water flooding.
6.3.4 All site users are to receive an ‘information pack’ from developers identifying, as a minimum, the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Southampton City Council 6-5 August 2010
Southampton Level 2 SFRA: Volume 3
6.3.5 Land availability will restrict the feasibility of SUDS features such as ponds and wetlands within the development (although they should still be investigated). ‘At source’ control measures such as green roofs, rainwater harvesting and water butts should be included in the design. Permeable pavement should be considered in carparking areas and any hard landscape areas. Public gardens and open space should consider benefits of including swales and ‘rain gardens’ for both surface water management and biodiversity. Development should seek to reduce the rate and volume of water discharging from the site. Inclusion of SUDS measures such as these will significantly assist with this.
6.3.6 Emergency services access should be available for the site via Bernard St and Queensway for the lifetime of the development. 6.4 Option 1: Strategic Precautionary Approach
6.4.1 The site is likely to have limited benefit from strategic flood risk management in Southampton, however this should be reviewed following completion of the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane).
6.4.2 There may be some minor benefit for building levels in the south east corner of the site (refer ‘Managed Adaptive’ mitigation) and possible wider area between Brunswick Street and Orchid Place. 6.5 Option 2: Site Specific Measures
6.5.1 Conceptual site specific flood risk management measures have been identified for each development type currently proposed:
More Vulnerable: Residential
6.5.2 Residential development on upper floors will remain ‘safe’ for the lifetime of the development.
6.5.3 Where ground floor residential development is proposed, including property entrances, these should be set at a minimum of 4.1mAOD (approximately 300mm above adjacent flood levels).
More Vulnerable: Hotel
6.5.4 Hotel: 60 year lifetime of development - flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
6.5.5 Hotel: 100 year lifetime of development - If the hotel is likely to have a lifetime of development of up to 100 years, the ground floor should be located at a minimum of 4.4mAOD (approximately 0.5m above the lowest ground levels on site).
6.5.6 If the hotel is fronting Briton St and Brunswick Square it should be provided with a safe access route onto Queensway, in the direction of higher ground.
6.5.7 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.7mAOD for a 100 year lifetime of development and 4.1mAOD for a 60 year lifetime of development.
Southampton City Council 6-6 August 2010
Southampton Level 2 SFRA: Volume 3
Less Vulnerable: Commercial
6.5.8 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
Basements:
6.5.9 Basement carpark access is indicated off Queenway and Briton Street. Accesses in these locations are predicted to remain above the 1 in 200 year tidal flood event up to the year 2115. If residential carpark basement access is proposed off Briton St or Brunswick Place the entrance levels should seek to be located above 4.3mAOD to prevent water ingress in a flood and significant hazard to site users. Where this is not achievable, appropriate warning and signage should be incorporated into basement design.
6.5.10 Further detailed investigation of groundwater levels, as well as the impact of climate change, is recommended to inform the design of underground structures and understand any local impact on groundwater levels. 6.6 Option 3: Management Adaptive Approach
6.6.1 The site is likely to have limited benefit from strategic flood risk management in Southampton, however this should be reviewed following completion of the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane).
6.6.2 If a strategic flood risk management scheme is in place by 2070 it would then not be necessary to have the hotel development entrance levels raised to a minimum of 4.3mAOD (assuming a 100 year lifetime of development).
6.6.3 Phasing of development could be used to fund either the contribution to the strategic flood risk management. For example, focussing on bringing forward development in the north-eastern portion of the site first would mean a decision on how to manage flood risk to more vulnerable landuses (e.g. hotel and residential development in the south east of the site) could be delayed until the outcome, and funding, for the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane), was clear. If the timeframes for development cannot be phased to coincide with the implementation of the strategic flood risk management measure and more vulnerable landuses are located in areas of flood risk (in particular the area between Brunswick Street and Orchid Place), then the site should consider adopting site specific measures to allow development to occur in a timely manner.
6.6.4 If more vulnerable land uses in the south east corner were required as part of the initial phasing of development then dependent upon the access into the development it may be necessary to raise ground levels to 4.3mAOD, refer to Diagram 6.6)
Southampton City Council 6-7 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend: Site raising to 4.3mAOD Site raising to 4.3mAOD (wider development area)
Diagram 6.6: Potential land raising to accommodate ‘more vulnerable’ landuses before 2070 6.7 Environmental Impacts
6.7.1 Key potential environmental impacts associated with the proposed site specific mitigation of flood risk at this location may include the visual impact on the townscape of the area arising from the raised ground levels. The slight change in elevation may impact on the views of the area and those to the conservation area from the east.
6.8 Site Summary 6.8.1 The site is currently located entirely within Flood Zone 1. Tidal flooding (after 2070) may pose a risk in the south east corner of the site. The Sequential Approach should be applied to the development of this site. 6.8.2 A Strategic Flood Management option for Southampton will be of limited benefit the site. It is recommended that site levels are raised to 4.2mAOD plus freeboard to minimise any risk of flooding to the site.
6.8.3 Flood resilience and resistance techniques may be required in the South East corner of the site to manage ‘residual’ flood risk, the scale and associated cost will be dependent on the development proposals. These would also need to be considered for the wider development is the area between Brunswick Street and Orchard Place were to be brought forward. It is recommended that a cost benefit analysis is undertaken on the cost of raising ground levels above the design flood event and the costs for incorporating flood resilience design into the construction of the exiting or any future development.
Southampton City Council 6-8 August 2010
Southampton Level 2 SFRA: Volume 3
6.8.4 It is recommended that SUDS are incorporated into the final layout to assist in reducing the volume of runoff entering the existing drainage infrastructure during the 1 in 100 year plus climate change event. As a minimum it is recommended that flood levels are raised 300mm above the existing ground level. 6.8.5 All site users are to receive an ‘information pack’ from developers identifying, as a minimum; the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details. 6.8.6 No specific flood risk management measure for the tidal flood risk are required for commercial or less vulnerable landuses due to the estimated lifetime of development (60 years) indicating that the tidal flood risk is low for the site. If residential uses are proposed in this area than it may be necessary to raise portions of the south eastern corner to accommodate habitable ground floors (if proposed). It is recommended that finished ground floors are located at a minimum 300mm above local road and footpath levels to reduce the risk of surface water flooding.
Southampton City Council 6-9 August 2010
Southampton Level 2 SFRA: Volume 3
7. Key Site 5: Town Depot 7.1 Site Proposal
7.1.1 The development site currently coves 8.3ha. The Town Depot occupies approximately 3.6ha with the balance of the site being a mixture of Council freehold properties and land in third party ownership. the site is located at National Grid Reference 443000 111400. The topography of the site varies between 2-3mAOD.
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Diagram 7.1: Site Location
7.1.2 Key spatial requirements for the site include:
Into an indoor snow ski facility, which has the potential to be a major leisure draw in the city.
The site is designated within the local plan as a city centre location and therefore should consider leisure, employment and residential development as part of mixed-use proposals.
The site is classified as ‘out of centre’ in retail terms and so a general retail development option would not be appropriate, with the scale and nature of any retail land use within the redline boundary requiring careful management.
7.1.3 The proposed leisure facility is classified as a ‘less vulnerable’ in accordance with Table D.2 of PPS25. Any proposed commercial would also be classified as ‘less vulnerable’ with residential development being a’ more vulnerable’ land use.
7.1.4 It is understood that along with the flood risk to the site, contamination and potential archaeological constraints will need to be assessed which would require significant intrusive investigations.
Southampton City Council 7-1 August 2010
Southampton Level 2 SFRA: Volume 3
7.2 Technical Assessment: Flood Risk
Summary of Flood Risk
7.2.1 The site is currently at ‘high’ probability of tidal flooding, and this is predicted to increase over the lifetime of the development. Flooding is expected to affect much of the local area over the next 100 years.
7.2.2 ‘Tide locking’ of the local drainage system during periods of heavy rainfall and high tide has historically led to flooding within site. This form of flooding is likely is increase in frequency if the predicted effects of climate change are accurate.
7.2.3 Groundwater flooding may become an issue in the future and should be assessed and managed as part of a detailed site FRA. Future regional groundwater management should be considered by the Environment Agency and SCC.
7.2.4 Further detail on the existing and future flood risk at the site are provided in the following sections.
Historic Flooding
7.2.5 There are two recorded instances of flooding in 2008 (SCC records). On the 27 May heavy rain lead to flooding of Endle Street and Crosshouse Road the source was determined to be an exceedance of the river channel.
7.2.6 Whilst on the 13 August (SCC record) heavy rain lead to the flooding of Albert Road North, the source was determined to be overland flow and drainage exceedance.
Flood Zones
7.2.7 The site is located almost entirely within Flood Zone 3. Current development proposals are compatible in this location, subject to the Sequential (potentially) and Exception Test.
Legend: Flood Zone 3 N Flood Zone 2
Diagram 7.2: Environment Agency Flood Zones
Southampton City Council 7-2 August 2010
Southampton Level 2 SFRA: Volume 3
Tidal Flood Risk and Hazard
7.2.8 The River Itchen frontage is generally low lying and is identified as being at a at high probability of tidal flooding. Overland flow paths along the low lying roads may also develop during times of extreme tidal flooding. This is supported by historic tidal flood records (2008). The land immediately fronting the River Itchen may have shallow tidal flooding in events more frequent than a 1 in 20 year return period, and by 2055 the river frontage may flood on an ‘average’ tide. Ground levels vary from 2.3mAOD on the River Itchen frontage to 3.0mAOD.
Flooding Regularity at Town Depot
4.5
4.0 Astronomical Tide 3.5 1 in 20 year
3.0 1 in 50 year
1 in 100 year 2.5 1 in 200 year Water Level (mAOD)
2.0 Approximate flooding threshold level 1.5 2010 2030 2050 2070 2090 2110 Time (years)
Diagram 7.3: Susceptibility to Tidal Flooding at Town Depot
7.2.9 During a 1 in 200 year ARI tidal flood event in 2055 tidal flooding is expected to inundate the majority of the site and some of the nearby streets, with on-site depths of approximately 0.4- 1.0m.
7.2.10 By 2070 the entire site is predicted to be flooded during the 1 in 200 year ARI tidal flood event flood event, and significant flood hazard will cover much of the site. By 2115 areas of extreme flood hazard (flooding depth >1.5m) may begin to develop within the site.
7.2.11 Predicted flood depths are shown in Diagrams7.4 – 7.7 whilst Diagrams 7.8 – 7.11 identify the predicted flood hazard for the site.
Southampton City Council 7-3 August 2010
Southampton Level 2 SFRA: Volume 3
Legend (m) Legend (m)
N N
Diagram 7.4: Flood Depth - 2010 1 in 200 year Tidal Flood Event Diagram 7.5: Flood Depth - 2055 1 in 200 year Tidal Flood Event
Legend (m) Legend (m)
N N
Diagram 7.6: Flood Depth - 2070 1 in 200 year Tidal Flood Event Diagram 7.7: Flood Depth - 2115 1 in 200 year Tidal Flood Event
Southampton City Council 7-4 August 2010
Southampton Level 2 SFRA: Volume 3
Legend: Legend: Low Low Moderate Moderate N Significant N Significant Extreme Extreme
Diagram 7.8: Flood Hazard - 2010 1 in 200 year Tidal Flood Event Diagram 7.9: Flood Hazard 2055 - 1 in 200 year Tidal Flood Event
Legend: Legend: Low Low N Moderate Moderate Significant N Significant Extreme Extreme
Diagram 7.10: Flood Hazard 2070 - 1 in 200 year Tidal Flood Event Diagram 7.11: Flood Hazard 2115 - 1 in 200 year Tidal Flood Event
Southampton City Council 7-5 August 2010
Southampton Level 2 SFRA: Volume 3
Tidal Residual Risk
7.2.12 The site is potentially partially at risk from the ‘extreme’ 1 in 1000 year ARI tidal flood event (Flood Zone 2). By 2115 the entire site is likely to located within the ‘extreme’ 1 in 1000 year ARI tidal flood event.
Fluvial Flood Risk
7.2.13 The site is located within an area identified at low probability of fluvial flooding.
Surface Water Flood Risk
7.2.14 Surface water modelling indicates the site is not particularly susceptible to surface water flooding (refer to Diagram 7.12). However, flooding that has occurred on site (2008) through a combination of heavy rainfall combining with a high tides resulted in ‘tide locking’ of the local drainage system. The local road network within the site is likely to be particularly susceptible to this form of flooding.
N
Diagram 7.12: Susceptibility to Surface Water Flooding
Southampton City Council 7-6 August 2010
Southampton Level 2 SFRA: Volume 3
Groundwater Flood Risk
7.2.15 Existing ‘Moderate Risk’ due to permeability of the underlying geology (refer to Figure 7.1 and 7.2 in Volume 2).
7.2.16 Sea level rise as a result of climate change is likely to also have an effect on the water table level in proximity to the river frontage. The available data indicates that if an associated 1m rise in the groundwater table occurs, groundwater levels at the Town Depot site could approach the surface level during periods of prolonged rainfall. This could result in shallow, but long lasting flooding.
7.2.17 Development proposals will need to consider site ground conditions and groundwater levels over the lifetime of the development. In particular the design of any underground structures or services and foundations. Future regional groundwater management should be considered by Hampshire County Council, the Environment Agency and SCC.
Flood Risk from Artificial Sources
7.2.18 No artificial sources located near the site. 7.3 General Flood Risk Management
7.3.1 The following guidance applies independent of the overall approach to managing flood risk on site:
Locating ‘less vulnerable’ or ‘water compatible’ uses closer to the river, and residential development set further back, may assist in managing ‘residual’ risk, however there is limited opportunity to adopt a sequential approach to layout, as over the next 100 years flood hazard is predicted to be significant across the majority of the site. Wherever possible property levels should be raised a minimum of 300mm above local road and ground floor carparking levels to manage the risk of surface water flooding. All development in this location should be placed on the Environment Agency Flood Warning Register. All site users are to receive an ‘information pack’ from developers identifying, as a minimum, the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details. ‘At source’ control measures such as green roofs, rainwater harvesting and water butts should be included in the design. Permeable paving should be incorporated within carparking and hard landscape areas. Public gardens and open space should consider benefits of including swales, wetlands, ponds and ‘rain gardens’ for both surface water management and biodiversity. The effects of ‘tide locking’ should be considered in the design of the drainage system, and where necessary additional storage included. Inclusion of SUDS measures will reduce the site runoff, and the scale of storage for ‘tide locking’ attenuation. The raising of ground levels on site will assist in providing a gravity drainage system that could be able to freely discharge into the river minimising the regularity of flooding on site.
Southampton City Council 7-7 August 2010
Southampton Level 2 SFRA: Volume 3
Existing surface water storage tanks and associated pumping (Albert Road North Pump Station) are located in the north west of the site. Southern Water are planning to review the works required to make this infrastructure more resilient to flooding during the next plan period (to 2015). Site specific or strategic flood risk management on the Town Depot site offers the opportunity for Southern Water and developers to work together to make this infrastructure more resilient to flooding (e.g. raising ground around the tanks or pump station may provide protection from flooding) for mutual benefit. A site specific flood emergency plan should be prepared, in consultation with Council emergency planners, emergency services, and with reference to Multi Agency Flood Plan to evacuate site users out of the floodplain in an ‘emergency’ flood event. Until the outcomes of the Southampton FCERM Strategy are known, it is recommended that the Town Depot masterplanning set aside a minimum 16m ‘buffer strip’ of open space along the site boundary with the River Itchen. This provides the opportunity to locate a strategic flood defence line along the river frontage. The current site frontage with the River Itchen predominantly consists of concrete and sheet pile hard edge. It is likely that redevelopment of the site will need to replace the river frontage. This does offer the opportunity to create a ‘softer’ edge that encourages both habitat creation and public access to the water whilst also acting to prevent erosion and provide flood protection to the area.
7.4 Option 1: Strategic Precautionary Approach
7.4.1 The site would benefit from strategic flood risk management infrastructure on the River Itchen and the River Test to a specified standard of protection (e.g. 1 in 200 year return period tidal event in 2115 of 4.2m AOD plus freeboard).
7.4.2 It is likely that a strategic flood defence line would be located within the site. This offers the opportunity for regeneration to directly contribute to strategic flood risk management in Southampton. Development also offers the opportunity to locate the defence line in an appropriate location, without the same constraint that existing development can present (e.g. raising ground could act as a flood defence). This is discussed further in ‘site specific’ mitigation. Redevelopment also offers the opportunity to ‘set back’ defences for habitat creation.
7.4.3 Development should contribute to the overall delivery of strategic flood risk management. ‘Water compatible’ uses would not necessarily need to contribute to strategic flood risk management if they did not benefit (i.e. if it was located on the riverside of the defence line).
7.4.4 Strategic flood risk management will need to consider the impact of flood defences on local groundwater levels.
7.4.5 Two sub-options have been assessed for managing flood risk to all development of the Town Depot:
Southampton City Council 7-8 August 2010
Southampton Level 2 SFRA: Volume 3
7.4.6 Option 1 (TD_SM_A_O1): Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD for development with a lifetime of 100 years and 4.1mAOD for development with a 60 year lifetime. Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of over 2m (dependent on existing site elevations and residential land use).
7.4.7 Option 2 (TD_SM_A_O1): If resistance to 2m hydrostatic forces is not achievable; ground levels will need to be raised to a level where buildings can withstand forces. For example, raising residential site levels to 3.3mAOD (up to 1.0m above existing levels) would reduce the residual risk, hydrostatic forces and potentially enable emergency services access during the extreme flood event for the lifetime of development.
More Vulnerable: Residential 7.4.8 Unless detailed breach/overtopping assessment proves otherwise, ground floor residential development should be multi-storey, locating habitable rooms on the first floor (finished floor level of 4.2mAOD plus freeboard) due to the potentially high ‘residual’ risk of tidal flooding. Emergency services access: 7.4.9 A strategic flood risk management solution is unlikely to be designed to provide a standard of protection for a 1 in 1000 year ARI tidal flood event. However, defences will provide some protection and it is likely that they would reduce the extent and depth of flooding that occurs without any defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners. 7.4.10 Should overtopping occur at the Town Depot site, emergency services may be able to reach the site boundary via the Itchen Bridge, however access through the site may be limited after 2055. 7.4.11 By 2115 emergency services are unlikely to be able to access through the site for approximately 4-5 hours during the peak of the extreme tidal flood event (i.e. the period when water will remain greater than 900mm deep - at or above 3.3mAOD). It is recommended SCC emergency planners are consulted on the acceptability of this risk. Raising site levels as indicated in sub Option 2 would manage ‘residual’ tidal flooding to currently acceptable levels. 7.5 Option 2: Site Specific Measures
7.5.1 Conceptual site specific flood risk management measures have been identified for each development type currently proposed:
7.5.2 The significant flood risk over the next 50-100 year on the site means that a site specific solution to manage flood risk is likely to include the need for raising site levels.
Southampton City Council 7-9 August 2010
Southampton Level 2 SFRA: Volume 3
Safe Access:
7.5.3 ‘Safe’ access (maximum flood depth of 250mm) through the site is not currently available for the lifetime of commercial and residential development, however providing a route through the site to the Itchen Bridge would enable site users to reach land out of the floodplain to the east. Strategic land raising within the site would enable ‘safe’ access to be available for all site users.
7.5.4 The level of roads or access routes for ‘safe’ access within the site would be dictated by the flood risk for the lifetime of development. For example, commercial development could be afforded ‘safe’ access if a route was at or above 3.3mAOD (2070 tidal flood levels of 3.6mAOD), whereas residential development would require access at or above 3.9mAOD (2115 tidal flood levels 2115 of 4.2mAOD).
Water Compatible: Marine
7.5.5 Marine uses are expected to require access to the River Itchen frontage as part of their business use. The existing ground levels in this area are approximately 2.3-3.5mAOD. The level in which this ground need to be raised to will be dependent upon the acceptability of ‘regular’ flooding to the proposed development. The current astronomical peak tidal level is approximately 2mAOD. This is expected to rise to approximately 2.5mAOD over the next 60 years. The lowest parts of the site are likely to flood during a 1 in 20 year return period at present. By 2070, the 1 in 20 year flood depth will result in approximately 0.8m flood depth (Diagram 7.3).
7.5.6 Raising the site to 3.3mAOD (raising ground levels by up to 1m) Option (TD_SSM_Ma) would protect the area during a 1 in 200 year ARI tidal flood event, however this would decrease to a 1 in 20 year standard of protection over the next 60 years (estimated lifetime of development). During a 1 in 200 year ARI event in 2070 flood depths would be approximately 300mm (likely ‘low’ to ‘moderate’ flood hazard). It may be possible to rationalise the amount of land raising depending upon what level of disruption is acceptable to marine site users. Alternatively, floor levels only could be raised 0.8m above the current ground levels, if higher flood depths around the marine use were acceptable, and building access was not a constraint.
7.5.7 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services. The buildings must be designed to withstand hydrostatic forces from an extreme flood event, which could result in water depths exceeding 1m.
7.5.8 Flood warning and evacuation procedures as part of a site specific emergency plan would be critical for the safety of site users.
Less Vulnerable: Commercial/Leisure
7.5.9 Raising ground levels to 3.3mAOD (up to 1m above existing ground levels) would result in flooding of carparking and internal road areas to a depth of approximately 300mm (for the life time of the development), however flood hazard is likely to remain ‘low’. Raising finished floor levels to 3.9mAOD (this includes a 300mm freeboard) would ensure that buildings remain dry and safe for the lifetime of development. Finished floor levels could be set lower (e.g. 3.6mAOD), providing protection from surface water flooding is investigated. This could potentially lead to built development being located in an areas with a ‘low’ flood risk hazard.
Southampton City Council 7-10 August 2010
Southampton Level 2 SFRA: Volume 3
Less Vulnerable: Carparking Associated with the Leisure Facility
7.5.10 The leisure centre and its associated carparking are assumed to have a lifetime of development of 60 years. If ground floor carparking is retained at the existing ground level then there could potentially be a significant flood hazard to vehicles. Cars located in water greater than 0.3m can potentially float. Ensuring the carpark is enclosed will minimise the potential for cars to float and cause additional hazard to people or structures however this will require access ramps entering the structure to above 3.9mAOD.
More Vulnerable: Residential
7.5.11 Site levels for residential development will be dictated by the need to provide a ‘safe’ access route for residents through to Albert Road North and over the Itchen Bridge.
7.5.12 Three sub-options have been assessed for managing flood risk to the ‘more vulnerable’ land uses in the Town Depot:
7.5.13 Option 1 (TD_SSM_R_O1): raising ground levels to 3.95mAOD, and finished floor levels at 4.5mAOD would mean properties were protected.
7.5.14 Option 2 (TD_SSM_R_O2): raising ground levels to 3.95mAOD, and ground floor levels at 4.0mAOD, with habitable rooms on upper floors only (i.e. undercroft carparking and property access only).
7.5.15 Option 3 (TD_SSM_R_O3): raising ground levels to 4.2mAOD, and finished floor levels at or above 4.5mAOD would provide dry access for development and allow for ground floor habitable rooms.
7.5.16 Proposed levels of 3.95mAOD would require land raising by over 1.5m above the existing ground levels. It is likely that the existing road levels will need to be graded up through the site to the proposed platform level. This would mean that during a tidal flood event some roads may flood, however provision of a route through the site to Albert Road North near the Itchen Bridge is likely to deliver the requirements for safe access through the site.
Emergency Services Access:
7.5.17 Though site raising and providing an access off Albert Road North near the Itchen Bridge, emergency services should be able to access through the site during an ‘extreme’ flood event (flood level of 4.4mAOD for the 2115 1 in 1000 year tidal flood event) for the lifetime of residential development. 7.6 Option 3: Management Adaptive Approach 7.6.1 A managed adaptive approach on the Town Depot site could combine site specific flood risk management measures, with the overall strategic flood risk management solution. This could reduce the need for extensive earthworks within the boundaries (and any tie in works adjacent to the site). 7.6.2 Raising site levels as indicated in the ‘site specific’ mitigation would enable a strategic flood risk management solution to tie into the north and south (defences located upstream and downstream of the site). Provision would need to be made within the site layout for future raising of the coastal frontage (i.e. flood defences) to provide protection to the wider community, as the site raising may only provide protection until 2030, depending on the final site layout.
Southampton City Council 7-11 August 2010
Southampton Level 2 SFRA: Volume 3
7.6.3 Phasing of development could be used to fund either the contribution to strategic flood risk management or further site specific works. For example, focussing on bringing forward residential development with access directly to the Itchen Bridge first and adopting local site specific management measures (i.e. land raising) would mean a decision on how to manage flood risk to commercial or leisure development (e.g. further land raising or strategic flood risk management solution) could be delayed until the outcome, and funding, for the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane), was clear.
7.6.4 If the timeframes for development cannot be phased to coincide with the implementation of the strategic flood risk management measure and in particular if more vulnerable landuses are located in areas of flood risk, then the site should consider adopting site specific measures to allow development to occur in a timely manner.
7.6.5 Strategic flood risk management will need to consider the impact of flood defences on local groundwater levels.
7.7 Strategic Flood Risk Management Solution by 2055
7.7.1 The 1 in 200 year tidal flood level prior to construction of a strategic flood risk management solution would be 3.4mAOD. The development would need to set aside an appropriate corridor for future flood defences (minimum 16m from the top of the river bank with work proposed in these areas requiring a Flood Defence Consent from the Environment Agency). This could be on the landward side of the marine uses, or directly along the river frontage – dependent upon layout designs.
7.7.2 Development in this area should contribute to the overall delivery of strategic flood risk management and in addition would require the following interim flood risk management measures:
Site Levels:
7.7.3 A consistent development platform level of 3.15mAOD Option (TD_MAA_SL_2055) would mean the ground floor of residential and commercial development was subject to ‘low’ flood hazard. This would require land raising of 0.1-0.7m over much of the site.
More Vulnerable: Residential
7.7.4 Habitable rooms would need to be located above ground floor level. Finished floor levels should be at 3.4mAOD.
7.7.5 If habitable rooms are located on the ground floor, finished floor levels at 4.2mAOD are likely to be necessary.
7.7.6 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD. Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths greater than 1.25m.
Southampton City Council 7-12 August 2010
Southampton Level 2 SFRA: Volume 3
Less Vulnerable: Commercial/Leisure
7.7.7 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.1mAOD. Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths approximately 1m – dependent upon final ground levels.
Marine:
7.7.8 If Marine uses are protected by a strategic flood defence scheme, ground levels could remain at existing site levels if a ‘significant’ flood hazard is acceptable to these uses. ‘Flood gates’ would need to be included in any proposed flood defences to provide these businesses with access to the river.
7.7.9 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD (or 4.7mAOD if the lifetime of development is estimated to be 100 years). The buildings must be designed to withstand hydrostatic forces from an extreme flood event, which could result in water depths of up to 1.5m.
7.7.10 Flood warning and evacuation procedures as part of a site specific emergency plan would be critical for the safety of site users. 7.8 Strategic Flood Risk Management Solution by 2070
7.8.1 The design flood level prior to construction of a strategic flood risk management solution would be 3.6mAOD for the 1 in 200 year tidal flood event. The development would need to set aside an appropriate corridor for future flood defences (minimum 16m from the top of the river bank with work proposed in these areas requiring a Flood Defence Consent from the Environment Agency). This could be on the landward side of the marine industrial uses, or directly along the river frontage.
7.8.2 Development should contribute to the overall delivery of strategic flood risk management and in addition would require the following interim flood risk management measures:
Site Levels:
7.8.3 A consistent development platform level of 3.3mAOD would mean the ground floor of residential and commercial development was subject to ‘low’ flood hazard. This would require land raising of around 0.3–0.8m over much of the site.
More Vulnerable: Residential
7.8.4 If habitable rooms are located above ground floor level, then finished floor levels could be approximately 3.6mAOD.
7.8.5 If habitable rooms are located on the ground floor the finished floor levels would need to be 4.5mAOD (includes an allowance for freeboard above the 1 in 200 year tidal level).
7.8.6 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD (includes a freeboard above the 1 in 1000 year ARI tidal level). Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths greater than 1m.
Southampton City Council 7-13 August 2010
Southampton Level 2 SFRA: Volume 3
Less Vulnerable: Commercial
7.8.7 Commercial development would not significantly benefit from a strategic flood risk management solution from 2070, on the basis of the assumed lifetime of development being 60 years.
7.8.8 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.1mAOD (for a development lifetime of 60 years) or 4.7mAOD (for a development lifetime of 100 years). Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of over 1m.
Marine Industrial:
7.8.9 If Marine uses are protected by a strategic flood defence scheme, ground levels could remain at existing site levels if a ‘significant’ flood hazard is acceptable to these uses. ‘Flood gates’ would need to be included in any proposed flood defences to provide these businesses with access to the river.
7.8.10 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD (for a development lifetime of 60 years). The buildings must be designed to withstand hydrostatic forces from an extreme flood event, which could result in water depths that are greater than 1m.
7.8.11 Flood warning and evacuation procedures as part of a site specific emergency plan would be critical for the safety of site users.
Emergency services access:
7.8.12 A strategic flood risk management solution is unlikely to be designed to provide a standard of protection for a 1 in 1000 year ARI tidal flood event. However, defences and site raising will provide some protection and it is likely that they would reduce the extent and depth of flooding that occurs without any defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners.
7.8.13 Should overtopping or breach occur at the Town Depot site, emergency services may be able to reach the site boundary via the Itchen Bridge. However by 2115 there may be a 4-5 hour period (possibly longer due to the tide locking of the drainage network) during the peak of the extreme tidal surge during in which flood depths are greater than 0.9m and emergency services may not be able to gain access through the site (if site levels are unchanged). It is recommended SCC emergency planners are consulted on the acceptability of this risk. 7.9 Environmental Impacts
7.9.1 Key potential environmental aspects associated with the site specific mitigation strategy are anticipated to be a temporary townscape impact resulting from the construction process. There may be a slight impact on the townscape from the raised ground levels, however this is not thought to culminate in a significantly adverse impact.
7.9.2 There are several listed buildings near the site but none are located within the development boundary.
Southampton City Council 7-14 August 2010
Southampton Level 2 SFRA: Volume 3
7.9.3 The proposed development site is located close to the Canute Road and Oxford Street conservation areas. Views of the area from the waters edge may be slightly impacted by the proposed flood mitigation strategy in terms of the elevated ground levels.
7.10 Site Summary
7.10.1 The site is currently located within all of the Flood Zones. The Sequential and Exception Test will need to be demonstrated for any development proposed within this site. Although flood risk is fairly uniform across the site in the future, there is the opportunity to use the sequential approach in phasing development (e.g. developing close to the Itchen Bridge as a safe access route first) prior to a strategic flood risk management solution being brought forward.
7.10.2 Tidal flooding and surface water flooding are the main risks to the site. Groundwater flooding may also be an issue and should be assessed through further Site Investigations.
7.10.3 A Strategic Flood Management option for Southampton will benefit the site and maximise the residential development potential of the site.
7.10.4 If no strategic measures are selected for the area then site specific measures will be required for any development type proposed within the site. The most feasible option for developing the site will involve raising the ground to various levels depending upon the acceptable level of risk to the proposed development (e.g. higher finished ground levels for ‘more vulnerable’ development). Phasing of the necessary ground raising could be undertaken so that development located near safe access (e.g. residential with direct access to Itchen Bridge) is constructed first along with any commercial uses which require reduced levels of filling. Any land raising will need to consider the convergence of proposed and existing site levels and how these limit the ingress of floodwater onto the site. Any final design should consider how the flood management technique can combine with any proposed strategic regional flood defence for Southampton.
7.10.5 Flood resilience and resistance techniques are likely to be required throughout much of the ground floor of new development to manage ‘residual’ flood risk, the scale and associated cost will be dependent on the development proposals.
7.10.6 It is recommended that SUDS are incorporated into the final layout to assist in reducing the volume of runoff entering the existing drainage infrastructure during the 1 in 100 year plus climate change event. As a minimum it is recommended that flood levels are raised 300mm above the existing ground level.
7.10.7 All site users are to receive an ‘information pack’ from developers identifying, as a minimum; the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Southampton City Council 7-15 August 2010
Southampton Level 2 SFRA: Volume 3
8. Key Sites 6 and 7: Meridian and Drivers Wharf 8.1 Site Proposal 8.1.1 The proposed development envelope encompasses an area of approximately 7.8 ha (5. ha for the Drivers Wharf site, eastern portion, and 2.8ha for the Meridian site, western portion) and is located at National Grid Reference 443320 112760. Ground levels vary from 2.4mAOD to 4.0mAOD.
N
Diagram 8.1: Site Location
8.1.2 Key spatial requirements for the site include:
The proposed development for the Meridian/Drivers Wharf site is likely to be mixed use commercial/residential with waterside space to provide pedestrian and cycle routes along the waterfront. Current proposals for Drivers Wharf site involve spliting the development into two zones - a commercial area in the east, which will act as a continuation of the Prices Street Business Centre; and residential zone in the west. The mixed-use proposal for the site includes marine and industrial units, restaurants and/or bars, offices, a food store, residential units and an area for the rowing club and a community facility.
Southampton City Council 8-1 August 2010
Southampton Level 2 SFRA: Volume 3
8.1.3 The proposed marine units are classified as a ‘water compatible’ (subject to confirming uses), the Industrial units and commercial sectors are ‘less vulnerable’ and the residential properties are ‘more vulnerable’ in accordance with Table D.2 of PPS 25. 8.2 Technical Assessment: Flood Risk Summary of Flood Risk
8.2.1 Although the majority of the site is currently outside an area classified at ‘high’ probability of ‘actual’ tidal flooding, this is predicted to increase over the lifetime of the development. Almost the entire site will be at ‘high’ probability of flooding by 2070, with significant flood hazard.
8.2.2 Groundwater flooding may become an issue in the future and should be assessed and managed as part of a detailed site FRA. Future regional groundwater management should be considered by the Environment Agency and SCC.
8.2.3 Further detail on the existing and future flood risk at the site are provided in the following sections.
Flood Zones
N
Legend: Flood Zone 3
Flood Zone 2
Diagram 8.2: Environment Agency Flood Zones
8.2.4 The development site is located predominantly within Flood Zone 3 (refer to Diagram 8.2). Current development proposals are compatible in this location, subject to the Sequential Test and potentially the Exception Test.
8.2.5 If the predicted effects of climate change are accurate then the majority of the site could be located within Flood Zone 3 in the future.
Southampton City Council 8-2 August 2010
Southampton Level 2 SFRA: Volume 3
Tidal Flood Risk and Hazard
8.2.6 The River Itchen frontage is generally low lying and is identified as being at a high probability of tidal flooding. The land immediately fronting the River Itchen may have shallow tidal flooding in events more frequent than a 1 in 20 year return period, and by 2055 the river frontage may flood on an ‘average’ tide (refer to Diagram 8.3). Ground levels vary from 2.3mAOD on the River Itchen frontage to 3.8mAOD.
Flooding Regularity at Meridian and Drivers Wharf
4.5
4.0 Astronomical Tide 3.5 1 in 20 year
3.0 1 in 50 year
1 in 100 year 2.5 1 in 200 year Water Level (mAOD)
2.0 Approximate flooding threshold level 1.5 2010 2030 2050 2070 2090 2110 Time (years)
Diagram 8.3: Susceptibility to tidal flooding at Meridian and Drivers Wharf
8.2.7 Diagrams 8.4 to 8.7 identify the predicted flood depths within the site whilst Diagrams 8.8 to 8.11 identify the associated flood hazard to the site.
During a 1 in 200 year ARI flood event in 2055 tidal flooding is expected to include some of the surrounding streets, and depths on parts of the site of 0.4-1.0m. By 2070 the majority of the site is predicted to be flooded during the 1 in 200 year ARI flood event, and significant flood hazard will cover much of the site. By 2115 the entire site and surrounding road network is expected to be inundated, with ‘significant’ flood hazard predicted across the majority of the development area.
Tidal Residual Risk
8.2.8 The site is potentially partially at risk from the ‘extreme’ 1 in 1000 year ARI tidal flood event (Flood Zone 2). By 2115 the entire site could be located within the ‘extreme’ 1 in 1000 year ARI tidal flood event.
Fluvial Flood Risk
8.2.9 The site is located within an area identified at low probability of fluvial flooding.
Southampton City Council 8-3 August 2010
Southampton Level 2 SFRA: Volume 3
Legend (m) Legend (m)
N N
Diagram 8.4: Flood Depth 2010 – 1 in 200 year ARI tidal flood event Diagram 8.5: Flood Depth 2055 – 1 in 200 year ARI tidal flood event
Legend (m) Legend (m)
N
Diagram 8.6: Flood Depth 2070 – 1 in 200 year ARI tidal flood event Diagram 8.7: Flood Depth 2115 – 1 in 200 year ARI tidal flood event
Southampton City Council 8-4 August 2010
Southampton Level 2 SFRA: Volume 3
Legend: Legend: Low Low Moderate Moderate N Significant N Significant Extreme Extreme
Diagram 8.8: Flood Hazard 2010 – 1 in 200 year ARI tidal flood event Diagram 8.9: Flood Hazard 2055 – 1 in 200 year ARI tidal flood event
Legend: Legend: Low Low N Moderate N Moderate Significant Significant Extreme Extreme
Diagram 8.10: Flood Hazard 2070 – 1 in 200 year ARI tidal flood event Diagram 8.11: Flood Hazard 2115 – 1 in 200 year ARI tidal flood event
Southampton City Council 8-5 August 2010
Southampton Level 2 SFRA: Volume 3
Surface Water Flood Risk
8.2.10 Broad scale surface water modelling indicates the site is not particularly susceptible to surface water flooding (refer to Diagram 8.12). There are no records of surface water flooding on site, however ‘tide locking’ of the local drainage system during periods of heavy rainfall coinciding with high tides have the potential to cause flooding in low lying areas (particularly the local road network). Areas to the south of the site can be classified as having a ‘less’ to ‘intermediate’ susceptibility to surface water flooding.
N
Diagram 8.12: Susceptibility to Surface Water Flooding
Groundwater Flood Risk
8.2.11 Existing ‘Moderate Risk’ due to permeability of the underlying geology (Refer to Figure 7.1 and 7.2 in Volume 2). Sea level rise, as a result of climate change, is likely to also have an effect on the water table level in close proximity to the river frontage. The available data indicates that if an associated 1m rise in the groundwater table occurs, groundwater levels at Meridian and Drivers Wharf could approach the surface level during periods of prolonged rainfall. This could result in shallow, but long lasting flooding.
8.2.12 Development proposals will need to consider site ground conditions and groundwater levels over the lifetime of the development. In particular the design of any underground structures or services and foundations. Future regional groundwater management should be considered by Hampshire County Council, the Environment Agency and SCC.
Flood Risk from Artificial Sources
8.2.13 No artificial sources located near the site.
Southampton City Council 8-6 August 2010
Southampton Level 2 SFRA: Volume 3
8.3 General Flood Risk Management
8.3.1 The following guidance applies independent of the overall approach to managing flood risk on site:
The sequential approach is recommended with lower vulnerability land uses located within areas identified at higher flood event probabilities (in particular any residential proposals in Flood Zone 1).
Locating ‘less vulnerable’ or ‘water compatible’ uses closer to the river, and residential development set further back on raised ground, may assist in managing ‘residual’ risk, however there is limited opportunity to adopt a sequential approach to layout, as over the next 100 years flood risk is predicted to be significant across the majority of the site.
Wherever possible property levels should be raised a minimum of 300mm above local road and carparking levels to manage the risk of surface water flooding.
All development in this location should be placed on the Environment Agency Flood Warning Register.
All site users are to receive an ‘information pack’ from developers identifying, as a minimum, the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
‘At source’ control measures such as green roofs, rainwater harvesting and water butts should be included in the design. Permeable paving should be incorporated into carpark and hard landscaping areas. Public gardens and open space should consider benefits of including swales, wetlands, ponds and ‘rain gardens’ for both surface water management and biodiversity. The effects of ‘tide locking’ should be considered in the design of the drainage system, and where necessary additional storage included. Inclusion of SUDS measures will reduce the site runoff, and the scale of storage for ‘tide locking’ attenuation. The raising of ground levels on site will assist in providing a gravity drainage system that could be able to freely discharge into the river and minimising the regularity of flooding on site.
A site specific flood emergency plan should be prepared, in consultation with Council emergency planners, emergency services, and with reference to Multi Agency Flood Plan to evacuate site users out of the floodplain in an ‘emergency’ flood event.
Until the outcomes of the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane) are known, it is recommended that the Meridian and Drivers Wharf masterplanning set aside a minimum of 16m ‘buffer strip’ of open space along the site boundary with the River Itchen. This provides the opportunity to locate a strategic flood defence line along the river frontage and will be required as part of a Flood Defence Consent.
The current site frontage with the River Itchen predominantly consists of concrete hard edge that does not appear to be maintained regularly. It is likely that redevelopment of the site will need to replace the existing river frontage. This does offer the opportunity to create a ‘softer’ edge that encourages both habitat creation and public access to the water whilst acting to prevent erosion and provide flood protection.
Southampton City Council 8-7 August 2010
Southampton Level 2 SFRA: Volume 3
8.4 Option 1: Strategic Precautionary Approach
8.4.1 The site would benefit from strategic flood risk management infrastructure on the River Itchen and the River Test to a specified standard of protection (e.g. 1 in 200 year ARI tidal event in 2115 of 4.2m AOD + freeboard).
8.4.2 It is likely that a strategic flood defence line would be located within the boundaries of the site. This offers the opportunity for regeneration to directly contribute to strategic flood risk management in Southampton. Development also offers the opportunity to locate the defence line in an appropriate location, without the same constraint that existing development can present (e.g. raising ground on site could act as a flood defence). This is discussed further in ‘site specific’ mitigation. Redevelopment of the site also offers the opportunity to ‘set back’ defences to generate new areas of habitat.
8.4.3 Development should contribute to the overall delivery of a strategic flood risk management measure for the city. The marine uses would not necessarily need to contribute to strategic flood risk management measure if they do not directly benefit from the scheme (i.e. if the proposed land use was located on the riverside of the defence).
8.4.4 Strategic flood risk management will need to consider the impact of flood defences on local groundwater levels.
8.4.5 Flood resilient and resistance measures should be included within the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD (4.4mAOD 2115 1 in 1000 year ARI tidal flood level including freeboard) for residential development and 4.1m AOD of commercial and industrial development (with an estimated lifetime of 60 years). Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of up to 1.8m.
More Vulnerable: Residential
8.4.6 Unless detailed breach/overtopping assessment proves otherwise, ground floor residential development should be multi-storey, locating habitable rooms on the first floor due to the potentially high ‘residual’ risk of flooding.
Emergency services access:
8.4.7 A strategic flood risk management solution is unlikely to be designed to provide a standard of protection for a 1 in 1000 year ARI tidal flood event. However, defences will provide some protection and they may reduce the extent and depth of flooding that occurs without any defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners.
Southampton City Council 8-8 August 2010
Southampton Level 2 SFRA: Volume 3
8.4.8 Should overtopping occur at the Meridian/Drivers Wharf site, emergency services may not be able to access the site for approximately 1.5-2 hours during the peak of the extreme tidal flood event (i.e. water will remain greater than 900mm deep - at or above 4-4.1mAOD). 8.5 Option 2: Site Specific Measures
8.5.1 Conceptual site specific flood risk management measures have been identified for each development type currently proposed:
8.5.2 Due to the significant flood risk over the next 50-100 years a site specific solution to manage flood risk is likely to include the need for raising site levels.
Safe Access:
8.5.3 ‘Safe’ access through the site and out of the floodplain is not currently available for the lifetime of commercial or residential development. Strategic land raising within and outside the site would enable ‘safe’ access to be available for all site users.
8.5.4 The level of roads or access routes for ‘safe’ access within the site would be dictated by the flood risk for the lifetime of development. For example, commercial development could be afforded ‘safe’ access if a route was at or above 3.3mAOD, whereas residential development would require access at or above 3.9mAOD.
8.5.5 Safe access from the Drivers Wharf site (to the east) is constrained by the surrounding extent and depth of flooding predicted in the surrounding road network and would require a link through to the Meridian.
8.5.6 Meridian Wharf does benefit from off-site ‘safe’ access to the west until 2070 via Mount Pleasant Road. By 2115 this route is likely to experience a significant flood hazard with flood depths reaching 0.9m.
8.5.7 Providing access between Drivers Wharf and Meridian Wharf is considered key to providing ‘safe’ access for the entire site.
8.5.8 Two sub-options have been assessed for managing flood risk to the safe access of Meridian and Drivers Wharf:
Option 1 (MDW_SSM_SA_O1): A raised promenade beneath Northam Bridge (along the River Itchen frontage) set at 4.2mAOD would provide a dry link between the sites for the lifetime of residential development. This could form part of a 16m ‘buffer strip’ along the river frontage for public access and habitat creation but would be subject to a Flood Defence consent approval from the Environment Agency.
Option 2 (MDW_SSM_SA_O2): A route ‘over’ Northam Road Bridge between Meridian and Drivers Wharf would provide a route for residents and site users further away from the river. This would require a designated crossing location due to the potential for high traffic volumes.
Southampton City Council 8-9 August 2010
Southampton Level 2 SFRA: Volume 3
N
Legend:
Raised Public Access Raised Footpath
Diagram 8.13: Raised defence and footpath
8.5.9 This link would need to continue through the Meridian Wharf site to Mount Pleasant Road (refer to Diagram 8.13).
8.5.10 Raising the existing pedestrian footpath along Mount Pleasant Road by up to 0.6m from the site boundary through to the railway level crossing (100m), and a further 200mm raising for 30m west of the railway line, would provide a ‘safe’ access route out of the floodplain for residential development on the Meridian and Drivers Wharf sites.
8.5.11 The pedestrian route would need to be signposted and residents made aware of the safe access and egress arrangements in the event of a flood.
Marine Industrial:
8.5.12 Marine industrial uses are expected to require access to the River Itchen frontage as part of their business use. The existing ground levels in this area are approximately 2.8mAOD. The elevation that the development platform might need to be raised to will be dependent upon the acceptability of regular flooding to the site. The current astronomical peak tidal level is approximately 2mAOD. This is expected to rise to approximately 2.5mAOD over the next 60 years. The lowest waterfront margins of the site are likely to flood during a 1 in 20 year return period at present. By 2070 the 1 in 20 year flood depth will result in approximately 0.5m flood depth.
8.5.13 Raising the site to 3.3mAOD (raising by 0.5m) would protect the area during a 1 in 200 year return period event now, however this would decrease to a 1 in 20 year standard of protection over the next 60 years. During a 1 in 200 year return period event in 2070 flood depths would be approximately 300mm (likely ‘low’ to ‘moderate’ flood hazard). It may be possible to rationalise the amount of land raising depending upon what level of disruption is acceptable to site users. Alternatively, floor levels only could be raised 0.5m, if higher flood depths around the marine use were acceptable, and building access was not a constraint.
Southampton City Council 8-10 August 2010
Southampton Level 2 SFRA: Volume 3
8.5.14 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD (1 in 1000 year tidal level for a 60 year lifetime of development including freeboard).. The buildings must be designed to withstand hydrostatic forces from an extreme flood event, which could result in water depths in excess of 1m.
8.5.15 Flood warning and evacuation procedures as part of a site specific emergency plan would be critical for the safety of site users.
Commercial:
8.5.16 The majority of the site is at risk from the 1 in 200 year ARI tidal event for the lifetime of commercial development (assumed to be 60 years within this report). Hydraulic modelling indicates the flood level on site is approximately 3.6mAOD during a 1 in 200 year ARI tidal event in 2070.
8.5.17 Raising ground levels to 3.3mAOD (0.4-0.8m above existing ground levels) would mean flooding of carparking and internal roads to a depth of 300mm, however flood hazard is likely to remain ‘low’. Raising finished floor levels to 3.9mAOD would mean buildings remain dry and safe, including a 300mm freeboard. Finished floor levels could be set lower (e.g. 3.6mAOD) if ‘low’ hazard within buildings was acceptable.
More Vulnerable: Residential
8.5.18 Site levels for residential development will be dictated by the need to provide a ‘safe’ access route for residents through the Mount Pleasant Road.
8.5.19 Three sub-options have been assessed for managing flood risk to the ‘more vulnerable’ land uses in the area:
Option 1 (MDW_SSM_R_O1): raising ground levels to 3.95mAOD, and finished floor levels at 4.5mAOD would provide ‘safe’ access and protect properties against the 1 in 200 year ARI tidal flood event.
Option 2 (MDW_SSM_R_O2): raising ground levels to 3.95mAOD, and ground floor levels at 4.2mAOD, with habitable rooms on upper floors only (i.e. undercroft carparking, storage and property access only). This option would provide safe access within the development.
Option 3 (MDW_SSM_R_O3): raising ground levels to 4.2mAOD, and finished floor levels at or above 4.5mAOD would provide safe and dry access for development and allow for ground floor habitable uses.
8.5.20 Proposed levels of 3.95mAOD would require land raising by up to 1.2m above existing ground levels whilst raising ground levels to 4.2mAOD would increase this to nearly 1.5m.
Southampton City Council 8-11 August 2010
Southampton Level 2 SFRA: Volume 3
Emergency Services Access:
8.5.21 Emergency services access during an ‘extreme’ flood event (4.4mAOD for the 2115 1 in 1000 year ARI tidal flood event) will be difficult to obtain for the lifetime of residential development. By 2115 the roads around the site are likely to be significantly inundated. The shallowest flooding is likely to be along Mount Pleasant Road (road levels 3.2mAOD), however in areas this may exceed 0.9m. Emergency services may not be able to access the site for approximately 1-2 hours during the peak of the extreme tidal flood event (i.e. water will remain greater than 900mm deep - at or above 4.1mAOD), however detailed hydraulic modelling is required to understand the local flood mechanisms and timings and should be investigated during the masterplanning process.
8.6 Option 3: Management Adaptive Approach
8.6.1 A managed adaptive approach on the Meridian and Drivers Wharf site could combine site specific flood risk management measures, with the overall strategic flood risk management solution. This could reduce the need for extensive site raising, and off site works to provide safe access.
8.6.2 Raising site levels as indicated in the ‘site specific’ mitigation could enable a strategic flood risk management measures to tie into the east and west. Provision would need to be made within the site layout for future raising of the coastal frontage (i.e. flood defences) to provide protection to the wider community, as the site raising may only provide protection until 2030, depending on the final site layout and levels.
8.6.3 Careful phasing of the sites development could be used to fund contribution to strategic flood risk management. For example, focussing on bringing forward ‘less vulnerable’ development (i.e. commercial only) first would mean a decision on how to manage flood risk to residential development (e.g. off-site footpath works) could be delayed until the outcome, and funding, for the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane), was clear.
8.6.4 If the timeframes for development cannot be phased to coincide with the implementation of the strategic flood risk management measure and in particular more vulnerable landuses are located in areas of flood risk, then the site should consider adopting site specific measures to allow development to occur in a timely manner. 8.7 Strategic Flood Risk Management Solution by 2055
8.7.1 The design flood level prior to construction of a strategic flood risk management solution would be 3.4mAOD. The development would need to set aside an appropriate corridor for future flood defences. This could be on the landward side of the marine industrial uses, or directly along the river frontage.
8.7.2 Development should contribute to the overall delivery of strategic flood risk management.
Site Levels:
8.7.3 A consistent development platform level of 3.15mAOD would mean the ground floor of residential and commercial development was subject to ‘low’ flood hazard. Detailed investigation would be required to determine whether fill would need to be imported, however it is likely to be relatively limited Option (MDW_MAA_SL_2055).
Southampton City Council 8-12 August 2010
Southampton Level 2 SFRA: Volume 3
More Vulnerable: Residential
8.7.4 Habitable rooms are located above ground floor level (at a minimum level of 4.2mAOD). Ground floor levels would be approximately 3.4mAOD.
8.7.5 If habitable rooms are located on the ground floor ground raising to 4.2mAOD is likely to be necessary to manage potentially ‘significant’ flood hazard from a breach/overtopping of defences in the future.
8.7.6 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD. Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths exceeding 1m.
Less Vulnerable: Commercial
8.7.7 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.1mAOD. Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths exceeding 1m.
Water Compatible: Marine Industrial
8.7.8 If Marine uses are protected by a strategic flood defence scheme, ground levels could remain at existing site levels if a ‘significant’ flood hazard is acceptable. ‘Flood gates’ would need to be included in flood defences to provide the business with access to the river.
8.7.9 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD. All buildings must be designed to withstand hydrostatic forces from an extreme flood event, which could result in water depths exceeding 1m.
8.7.10 Flood warning and evacuation procedures as part of a site specific emergency plan would be critical for the safety of site users. 8.8 Strategic Flood Risk Management Solution by 2070
8.8.1 The design flood level prior to construction of a strategic flood risk management solution would be 3.6mAOD. The development would need to set aside an appropriate corridor for future flood defences. This could be on the landward side of the marine industrial uses, or directly along the river frontage.
8.8.2 Residential development should contribute to the overall delivery of strategic flood risk management, however commercial development is unlikely to significantly benefit based on a development lifetime of 60 years.
Site Levels:
8.8.3 A consistent development platform level of 3.3mAOD would mean the ground floor of residential and commercial development was subject to ‘low’ flood hazard. This would require land raising of 0.2-0.5m over much of the site. Option (MDW_MAA_SL_2070).
Southampton City Council 8-13 August 2010
Southampton Level 2 SFRA: Volume 3
More Vulnerable: Residential
8.8.4 Habitable rooms are located above ground floor level. Ground floor levels would be approximately 3.6mAOD.
8.8.5 If habitable rooms are located on the ground floor ground raising to 4.2mAOD is likely to be necessary to manage potentially significant hazard from a breach/overtopping of defences in the future.
8.8.6 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD. Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of up to 0.8m.
Less Vulnerable: Commercial
8.8.7 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.1mAOD. Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of up to 0.8m.
Water Compatible: Marine Industrial
8.8.8 If Marine uses are protected by a strategic flood defence scheme, ground levels could remain at existing site levels if a ‘significant’ flood hazard is acceptable. ‘Flood gates’ would need to be included in flood defences to provide the business with access to the river.
8.8.9 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD. The buildings must be designed to withstand hydrostatic forces from an extreme flood event, which could result in water depths exceeding 0.8m.
8.8.10 Flood warning and evacuation procedures as part of a site specific emergency plan would be critical for the safety of site users.
Emergency services access:
8.8.11 Up until 2070 emergency services should be able to access the Meridian site via Mount Pleasant Road, as flood depths are lower than 0.9m. A strategic flood risk management solution is unlikely to be designed to provide a standard of protection for a 1 in 1000 year ARI tidal flood event. However, defences will provide some protection and this may reduce the extent and depth of flooding that occurs without any defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners. 8.9 Environmental Impacts
8.9.1 Site specific mitigation strategies may have the potential to impact on the townscape of the site and surrounding area due to the elevation of ground levels to approximately 0.5m above existing ground levels. However, this is not anticipated to constitute a significant impact due to the urbanised nature of the area.
Southampton City Council 8-14 August 2010
Southampton Level 2 SFRA: Volume 3
8.9.2 There are 2 listed building designations within close proximity to the proposed development site. The Marsh Hotel and Royal Albert Hotel are both located on Albert Road South. There may be the potential for a temporary impact on the setting and general character of these buildings associated with the elevation of ground levels for flood mitigation. It is not anticipated that there will be a permanent impact on the setting or character of these buildings associated with the proposed flood mitigation strategy.
8.9.3 There are no conservation areas within close proximity to the proposed development site.
8.10 Site Summary
8.10.1 The site is currently located predominantly within Flood Zone 3. The Sequential and potentially the Exception Test will need to be demonstrated for any development proposed within this site. although flood risk is reasonably uniform over the site for the lifetime of development, there is the potential to use the sequential approach in phasing development (e.g. developing the western portion of the site first close to Mount Pleasant Road, the safe access route).
8.10.2 Tidal flooding is the main risk to the site. Groundwater flooding may also be an issue and should be assessed through further site investigations.
8.10.3 A Strategic Flood Management option for Southampton will benefit the site and maximise the residential development potential of the site.
8.10.4 If no strategic measures are selected for the area then site specific measures will be required for any development type proposed within the site. This could involve a site specific defence line, ground raising and strategic road raising to limit the ingress of water into the site. These measures will need to factor in safe access for site users and emergency access for emergency services.
8.10.5 Regardless of the approach taken to manage tidal flood risk it is likely some raising of site levels will be required to manage tidal residual risk from breach or overtopping of defences. A site specific approach will require significantly more land raising than reliance on defences as well as require off site works on Mount Pleasant Road to create a safe access route, however it is unlikely development would need to also contribute separately to a strategic flood defence solution. A managed adaptive approach where defences are brought forward by 2055 is likely to be most economical for the site, as it would minimise site raising, whilst still managing tidal residual risk behind defences to an acceptable level.
8.10.6 The practicality of these options would require a detailed cost benefit analysis based on the anticipated densities of residential and less vulnerable landuses against the cost of raising the site or incorporating measures to minimise risk until a strategic flood risk measure is constructed – or even if the option selected to manage the site can be incorporated into the final strategic design therefore reducing the contribution of the site to this structure. Landraising will need to consider impacts of raised access to the site and how it can merge with existing infrastructure whilst minimising disruptions to local residents/site users and the road network.
8.10.7 Flood resilience and resistance techniques are likely to be required throughout much of the ground floor of new development to manage ‘residual’ flood risk, the scale and associated cost will be dependent on the development proposals.
Southampton City Council 8-15 August 2010
Southampton Level 2 SFRA: Volume 3
8.10.8 It is recommended that SUDS are incorporated into the final layout to assist in reducing the volume of runoff entering the existing drainage infrastructure during the 1 in 100 year plus climate change event. As a minimum it is recommended that flood levels are raised 300mm above the existing ground level.
8.10.9 All site users are to receive an ‘information pack’ from developers identifying, as a minimum; the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Southampton City Council 8-16 August 2010
Southampton Level 2 SFRA: Volume 3
9. Key Site 8: Lower High Street
9.1 SITE PROPOSAL
9.1.1 The proposed development envelope encompasses an area of approximately 0.7ha and is located at National Grid Reference 441940 111040. The proposed development is residential that retains the archaeology context and heritage at ground level with some commercial units on High Street. The topography of the site varies between 3 – 5.6m AOD
N
Diagram 9.1: Site Location
9.1.2 Key spatial requirements for the site include:
The development of this site must include the preservation of the historic vaults within the site. Due to the current condition and the archaeological importance of the site, the vaults which are likely to be made accessible to the public. The vaults at 86-88 High Street are proposed to continue as a repository for Southampton Archaeology Unit, containing artefacts and archaeology, with the vaults at 95-98 High Street having the potential to be a ‘museum piece’ for the public to view.
Quilters vault requires management and maintenance as part of this proposal if it is to be used as a multi-purpose events and exhibitions space, as suggested. This venue would cater for such activities as music performances, public meetings and art exhibitions.
Southampton City Council 9-1 August 2010
Southampton Level 2 SFRA: Volume 3
Additional development proposals include commercial or leisure uses and potentially some form of residential development.
9.1.3 The proposed leisure, tourism and commercial uses are classified as a ‘less vulnerable’ in accordance with Table D.2 of PPS25. Any proposed residential development can be classifies as a ‘more vulnerable’ landuse.
9.1.4 The development proposal encourages construction of residential above ‘the vaults’ with ground floor car parking. 9.2 Technical Assessment: Flood Risk
Summary of Flood Risk
9.2.1 The site is generally located at ‘low’ to ‘medium’ risk of flooding over the next 100 years, however the southern boundary of the site on Porters Lane is likely to be at a ‘high’ risk after 2070.
9.2.2 The existing heritage value in the southern portion of the site is potentially at risk from flooding in the future – if measures are not included to manage flood risk (either strategically or on site).
9.2.3 Dependant on the proposed development layout, flood risk management measures will be required in the southern portion of the site to demonstrate the site remains ‘safe’ in accordance with PPS 25, for the lifetime of the development.
9.2.4 Further detail on the existing and future flood risk at the site are provided in the following sections.
Flood Zones
N
Legend: Flood Zone 3
Flood Zone 2
Diagram 9.2: Environment Agency Flood Zones
Southampton City Council 9-2 August 2010
Southampton Level 2 SFRA: Volume 3
9.2.5 At present the site is located within Flood Zone 1 (refer to Diagram 9.2), all landuses are appropriate within this site (areas shown as shown as Flood Zone 3 appear to be an anomaly in the Flood Zone maps).
9.2.6 If the predicted effects of climate change are accurate then there is the potential for the site to be partially located within Flood Zone 2 and 3 in the future.
Tidal Flood Risk and Hazard
9.2.7 The site is not currently located in an area at tidal risk of flooding due to ground levels between the site and the River Test. The majority of the site is located above 4mAOD, however there is a small area of land immediately fronting Porters lane that is at 2.8-3mAOD (the historic ruins of Canute’s Palace).
9.2.8 The site remains outside the 1 in 200 year ARI tidal flood event until 2070. By 2115 flooding in the low lying land adjacent to Porters Lane is predicted (0.5-1.0m deep) – refer to Diagram 9.3. Flood hazard is expected to be ‘significant’ (Diagram 9.4). Significant flood hazard is also predicted on Porter Lane, the bottom of High Street and Town Quay.
Legend (m)
N
Diagram 9.3: Flood Depth 2115 - 1 in 200 year Tidal Flood Event
Tidal Residual Risk
9.2.9 The site is currently located outside the ‘extreme’ 1 in 1000 year ARI tidal flood event. By 2115 the southern half of the site may be partially located within the ‘extreme’ 1 in 1000 year ARI tidal flood event.
Southampton City Council 9-3 August 2010
Southampton Level 2 SFRA: Volume 3
Legend: Low Moderate N Significant Extreme
Diagram 9.4: Flood Hazard - 2115 1 in 200 year Tidal Flood Event
Fluvial Flood Risk
9.2.10 The site is located within an area identified at low probability of fluvial flooding.
Surface Water Flood Risk
9.2.11 The site is generally located on higher ground. Surface water modelling indicates some areas of lower lying ground within the site (e.g. the historic ruins on Porter Lane) can be classified as having a ‘less’ to ‘intermediate’ susceptibility to surface water flooding. There are no records of surface water flooding on site. High Street, immediately to the east, and French Street, immediately to the west, may act to convey overland flow during high intensity rainfall events.
N
Diagram 9.5: Susceptibility to Surface Water Flooding
Southampton City Council 9-4 August 2010
Southampton Level 2 SFRA: Volume 3
Groundwater Flood Risk
9.2.12 Existing ‘Moderate Risk’ due to permeability of the underlying geology. Sea level rise as a result of climate change is likely to also have an effect on the water table level in low lying ground in Southampton.
9.2.13 Development proposals will need to consider site ground conditions and groundwater levels over the lifetime of the development. In particular the design of any underground structures or services and foundations. Future regional groundwater management should be considered by the Environment Agency and SCC.
Flood Risk from Artificial Sources
9.2.14 No artificial sources located near the site. 9.3 General Flood Risk Management
9.3.1 The following guidance applies independent of the overall approach to managing flood risk on site:
Seeking to retain and enhance the site heritage offers opportunity to adopt a sequential approach to layout and avoid locating residential development fronting Porter Lane in the area with the highest risk.
Wherever possible property levels should be raised a minimum of 300mm above local road and carparking levels to manage the risk of surface water flooding.
All site users are to receive an ‘information pack’ from developers identifying, as a minimum, the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Space constraints mean significant SUDS features such as ponds and wetlands are unlikely to be appropriate. ‘At source’ control measures such as green roofs, rainwater harvesting and water butts should be included in the design. Permeable pavement should be considered in surface carparking and hard landscaping areas. Public gardens and open space should consider benefits of including swales and ‘rain gardens’ for both surface water management and biodiversity. Development should seek to reduce the rate and volume of water discharging from the site. Inclusion of SUDS measures such as these will significantly assist with this.
Emergency services access should be available for the site via High Street and French Street for the lifetime of the development.
A site specific flood emergency plan should be prepared, in consultation with Council emergency planners, emergency services, and with reference to Multi Agency Flood Plan to evacuate site users out of the floodplain in an ‘emergency’ flood event. 9.4 Option 1: Strategic Precautionary Approach
9.4.1 The site, in particular residential development, would benefit from strategic flood risk management infrastructure on the River Itchen and the River Test to a specified standard of protection (e.g. 1 in 200 year ARI tidal event = 4.2m AOD + freeboard).
9.4.2 Development should contribute to the overall delivery of strategic flood risk management.
Southampton City Council 9-5 August 2010
Southampton Level 2 SFRA: Volume 3
9.4.3 This has the benefit of protecting the existing heritage value of the site, as well as future development on site.
9.4.4 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD for residential development and 4.1mAOD for commercial development. Although they do not necessarily need to remain dry, buildings fronting Porter Lane must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of up to 0.9m.
9.4.5 Strategic flood risk management will need to consider the impact of flood defences on local groundwater levels
Basements:
9.4.6 If necessary, basement carpark access off Porter Lane would be acceptable, if accompanied by appropriate flood warning and signage to advise on risk. 9.5 Option 2: Site Specific Mitigation
9.5.1 Conceptual site specific flood risk management measures have been identified for each development type currently proposed:
More Vulnerable: Residential
9.5.2 Residential development on upper floors will remain ‘safe’ for the lifetime of the development. Where ground floor residential development is proposed, including property entrances, these should be set at 4.4mAOD (approximately 300mm above adjacent flood levels). This should be achievable assuming residential land uses are not located along the southern boundary of the site.
9.5.3 If residential development is proposed on upper storeys above Canute’s Palace, these should have access available out of the building either in a northerly direction or onto French Street, not onto Porter Lane which could be at potentially significant flood hazard. This access can be a ‘secondary’ or ‘emergency’ exit (above 4.1mAOD), with the main entrance on Porter Lane if necessary.
9.5.4 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.7mAOD.
Less Vulnerable: Commercial
9.5.5 Wherever possible, finished floor levels for property on High Street and French Street should be 300mm above road levels to reduce the risk of surface water flooding.
9.5.6 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
Southampton City Council 9-6 August 2010
Southampton Level 2 SFRA: Volume 3
Canute’s Palace and Quilter’s Vault area:
9.5.7 English Heritage should be consulted on whether they consider specific flood risk management measures necessary for maintaining the value of Canute’s Palace and Quilter’s Vault. Development does offer the opportunity to reduce flood risk to these structures in the future. Specific flood risk management for the structures is unlikely to be necessary to meet the requirements of PPS 25.
9.5.8 Option 1 (LH_SSM_CPQV_O1): If development, including commercial/retail offices are proposed around these structures, finished floor levels/a wall could be raised to 4.2mAOD (plus freeboard), providing a ‘secondary defence’ protection from tidal flooding for the next 100 years (approximately 700mm above Porters Lane and approximately 1.3m above existing levels on Canute’s Palace) – refer to Diagram, 9.6.
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Legend: Wall (to 4.2m AOD) Area of restricted development
Diagram 9.6: Secondary defence protection
9.5.9 Option 2 (LH_SSM_CPQV_O2): In accordance with guidance for ‘less vulnerable’ development, finished floor levels around the structures should be raised to a minimum of 3.9mAOD. This would mean some shallow flooding would occur within the development, and 1-1.2m deep water within Canute’s Palace.
9.5.10 Option 3 (LH_SSM_CPQV_03): If all built development is to be located on upper floors, flood resilience measures could be incorporate in the proposed structure, this could include raising external/internal wall renders and elevated internal services above 4.7mAOD. This would allow the structure(s) to flood, but recovery works may be able to be undertaken more rapidly following a flood. The structures may flood as frequently as 1 in 20 years by 2115 in this scenario.
Southampton City Council 9-7 August 2010
Southampton Level 2 SFRA: Volume 3
9.5.11 Option 4 (LH_SSM_CPQV_O4): allow the historic structures to flood in the future.
Basements:
9.5.12 Basement carparking has not currently been indicated as part of development proposals. If proposed, it is recommended basement carpark entrances are located off French Street and High Street, rather than Porter Lane, to avoid potentially deep water flowing into the carpark. Entrance levels for basement carparking access off Porter Lane will be difficult to deliver due to the predicted flood depth and road levels.
9.5.13 Further detailed investigation of groundwater levels, as well as the impact of climate change, is recommended to inform the design of underground structures and understand any local impact on groundwater levels. 9.6 Option 3: Management Adaptive Approach
9.6.1 If a strategic flood risk management solution is brought forward before 2070, the scale of site specific flood risk management measures on site can be reduced.
9.6.2 Residential development should contribute to the overall delivery of strategic flood risk management. Commercial development is unlikely to significantly benefit from strategic flood risk management on the basis of a 60 year development lifetime.
Residential:
9.6.3 ‘Secondary’ or ‘emergency’ exit to the north for residents living above Canute’s Palace is not required as ‘safe’ access during the design event would be available via the French Street or High Streets.
9.6.4 Ground floor residential levels should be set 300mm above the surrounding road and carpark levels to manage the risk of surface water flooding.
Canute’s Palace and Quilter’s Vault area:
9.6.5 Raised floor levels or a ‘wall’ in this area would not be necessary. The area is expected to flood less frequently than a 1 in 200 year return period event and any further flood risk management measures would be based on specific English Heritage requirements, rather than PPS 25.
9.6.6 Flood resilient and resistance measures should be included in the ground floor design, such as external/internal wall renders and raised internal services above 4.7mAOD. Although they do not necessarily need to remain dry, buildings fronting Porter Lane must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of 0.9m.
Basements:
9.6.7 If necessary, basement carpark access off Porter Lane would be acceptable, if accompanied by appropriate flood warning and signage to advise on risk.
Southampton City Council 9-8 August 2010
Southampton Level 2 SFRA: Volume 3
9.7 Environmental Impacts
9.7.1 The site is located within the Old Town South Conservation area. The key potential environmental aspects associated with the incorporation of flood mitigation strategies into this proposed development site focus upon the impact on the various listed buildings and SAMS located to the West of the site and the possible impacts to Canutes Palace and Quilters Vault. The setting of these structure may be impacted on a temporary basis from the construction process. There may be a slight impact on the setting of designation arising from the elevation of ground levels to mitigate the impact of flood waters, however this is not thought to be significant.
9.7.2 Due to the fact that the proposed development site is located within an area of archaeological potential as designated by SCC, there may be the potential for the disturbance of undiscovered archaeological features associated with the designation. This should be investigated during the necessary site investigation works for the site. 9.8 Site Summary
9.8.1 The site is currently located within Flood Zone 1. All land uses are appropriate for this site.
9.8.2 Tidal flooding will be a consideration for site planning, however does not have to pose a significant constraint or require significant mitigation with careful layout of development proposals.
9.8.3 The Sequential Approach should be applied to the development of this site based on the potential future flood risk of the site.
9.8.4 A Strategic Flood Management option for Southampton will benefit residential development on site however does not need to rely on strategic flood risk management.
9.8.5 Site specific measures such as a water tight wall, raising floor levels or adopting a sequential approach could all reduce tidal flood risk to acceptable levels. These measures will need to factor in safe access for site users and emergency access for emergency services. These options are very practical for the redevelopment of the site, and with careful management should generate minimal impacts to nearby properties and transport routes.
9.8.6 Flood resilience and resistance techniques may be required in the south of the site to manage ‘residual’ flood risk, the scale and associated cost will be dependent on the development proposals.
9.8.7 It is recommended that SUDS are incorporated into the final layout to assist in reducing the volume of runoff entering the existing drainage infrastructure during the 1 in 100 year ARI plus climate change event. As a minimum it is recommended that flood levels are raised 300mm above the existing ground level.
9.8.8 All site users are to receive an ‘information pack’ from developers identifying, as a minimum; the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Southampton City Council 9-9 August 2010
Southampton Level 2 SFRA: Volume 3
10. Key Site 9: College Street
10.1 Site Proposal
10.1.1 The College Street development site, located at National Grid Reference 442490 111250, occupies 0.4ha of land and is currently a public car park. The topography of the site varies between 2 – 3.5mAOD.
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Diagram 10.1: Site Location
10.1.2 Key spatial requirements for the site include:
A mixed-use development for the site/area is proposed, with a possible combination of uses such as café, bar and restaurant uses, hotels, offices, housing and small-scale retail.
Leisure and retail facilities should be located in the south east corner due to the close proximity to Oxford Street and the Mercury Tower, with its existing retail unit.
Alternative options include the redevelopment of the site into a multi storey carpark.
10.1.3 The proposed commercial units and carpark are classified as a ‘less vulnerable’ and the residential properties are ‘more vulnerable’ in accordance with Table D.2 of PPS 25. 10.2 Technical Assessment: Flood Risk
Summary of Flood Risk
10.2.1 The site is generally located at ‘low’ to ‘medium’ probability of tidal flooding over the next 60 year, however much of the site is at high probability after 2070.
10.2.2 Further detail on the existing and future flood risk at the site are provided in the following sections.
Southampton City Council 10-1 August 2010
Southampton Level 2 SFRA: Volume 3
Flood Zones
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Legend: Flood Zone 3
Flood Zone 2
Diagram 10.2: Environment Agency Flood Zones
10.2.3 The site is predominantly located within Flood Zones 2 and 3 (refer to Diagram 10.2). Current development proposals compatible in this location, subject to the Sequential and potentially the Exception Test.
10.2.4 If the predicted effects of climate change are accurate then the majority of the site and surrounding area could be located within Flood Zone 3 in the future.
Tidal Flood Risk and Hazard
10.2.5 The site is not currently located in an area at tidal risk of flooding due to higher ground levels between the site and the River Itchen. Ground levels vary between 2m AOD (on the eastern boundary) to 3.5m AOD (on the western boundary).
The site remains outside the 1 in 200 year ARI tidal flood event until about 2070. During a 1 in 200 year ARI tidal flood event in 2055 tidal flooding is expected to reach the site (from the north), however flood hazard remains ‘low’ with a small portion of the site being classified as having moderate flood risk (refer to Diagrams10.3 and 10.6) .
By 2070 over two thirds of the site is predicted to be flooding during the 1 in 200 year ARI tidal flood event, with flood depths reaching approximately 600-800mm along the eastern boundary (refer to Diagram 10.4 and 10.7)
By 2115 flood water are expected to cover the entire site during a 1 in 200 year ARI tidal flood event. Flood hazard is expected to be ‘significant’. Extreme flood hazard is predicted along Terminus Terrace, however Central Bridge leading onto the Itchen Bridge is predicted to remain ‘dry’ (refer to Diagrams 10.5 and to 10.8).
Southampton City Council 10-2 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend (m)
Diagram 10.3: Flood Depth 2055 - 1 in 200 year Tidal Flood Event
Legend (m)
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Diagram 10.5: Flood Depth 2115 - 1 in 200 year Tidal Flood Event
Legend (m)
Diagram 10.4: Flood Depth 2070 - 1 in 200 year Tidal Flood Event
Southampton City Council 10-1 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend: Low Moderate Significant Extreme
Diagram 10.6: Flood Hazard 2055 - 1 in 200 year Tidal Flood Event
N Legend: Low Moderate Significant Extreme
gure Diagram 10.8: Flood Hazard 2055 - 1 in 200 year Tidal Flood Event
Legend: Low Moderate Significant Extreme
Diagram 10.7: Flood Hazard 2070 - 1 in 200 year Tidal Flood Event
Southampton City Council 10-2 August 2010
Southampton Level 2 SFRA: Volume 3
Residual Risk
10.2.6 The site is potentially partially at risk from the ‘extreme’ 1 in 1000 year ARI tidal flood event (Flood Zone 2). By 2115 the entire site could be located within the ‘extreme’ 1 in 1000 year ARI tidal flood event.
Fluvial Flood Risk
10.2.7 The site is located within an area identified at low probability of fluvial flooding.
Surface Water Flood Risk
10.2.8 The south-eastern half of the site is classified as having a ‘less’ to ‘intermediate’ susceptibility to surface water flooding (refer to Diagram 10.9). This area is part of a local ‘basin’ which may collect surface water runoff from the surrounding local catchment. College Street and Bernard Street are likely to act as overland flow routes during high intensity rainfall events, with water flowing in an easterly direction. There are no historic surface water/sewer flooding records on site.
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Diagram 10.9: Susceptibility to Surface Water Flooding
Groundwater Flood Risk
10.2.9 Existing ‘Moderate Risk’ due to permeability of the underlying geology. Sea level rise as a result of climate change is likely to also have an effect on the water table level in low lying ground in Southampton.
Southampton City Council 10-1 August 2010
Southampton Level 2 SFRA: Volume 3
10.2.10 Development proposals will need to consider site ground conditions and groundwater levels over the lifetime of the development. In particular the design of any underground structures or services and foundations. Future regional groundwater management should be considered by the Environment Agency and SCC.
Flood Risk from Artificial Sources
10.2.11 No artificial sources located near the site. 10.3 General Flood Risk Management
10.3.1 The following guidance applies independent of the overall approach to managing flood risk on site:
The Sequential approach is recommended with lower vulnerability land uses located within areas identified at higher flood event probabilities.
There is the opportunity to adopt the sequential approach to development layout by locating higher vulnerability development in the west of the site (e.g. no ground floor residential across the site or car park only in the eastern portion of the site).
Wherever possible property levels should be raised a minimum of 300mm above local road and carparking levels to manage the risk of surface water flooding.
All site users are to receive an ‘information pack’ from developers identifying, as a minimum, the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Space constraints mean significant SUDS features such as ponds and wetlands are unlikely to be appropriate. ‘At source’ control measures such as green roofs, rainwater harvesting and water butts should be included in the design. Permeable pavement should be considered in surface carparking and hard landscaping areas. Public gardens and open space should consider benefits of including swales and ‘rain gardens’ for both surface water management and biodiversity. Development should seek to reduce the rate and volume of water discharging from the site. Inclusion of SUDS measures such as these will significantly assist with this.
A site specific flood emergency plan should be prepared, in consultation with Council emergency planners, emergency services, and with reference to Multi Agency Flood Plan to evacuate site users out of the floodplain in an ‘emergency’ flood event. 10.4 Option 1: Strategic Precautionary Approach
10.4.1 The site would benefit from strategic flood risk management infrastructure on the River Itchen and the River Test to a specified standard of protection (e.g. 1 in 200 year ARI tidal event in 2115 of 4.2m AOD + freeboard).
10.4.2 Development should contribute to the overall delivery of strategic flood risk management.
10.4.3 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD for residential development. Based on the lifetime of development for commercial development this level can be reduced to 4.1mAOD.
Southampton City Council 10-2 August 2010
Southampton Level 2 SFRA: Volume 3
10.4.4 Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths exceeding 2m.
More Vulnerable: Residential
10.4.5 Unless detailed breach/overtopping assessment proves otherwise, ground floor residential development should be multi-storey, locating habitable rooms on the first floor due to the potentially high residual flood hazard.
10.4.6 A strategic flood risk management solution may provide protected safe access to the site, however will need to be reviewed once the outcomes of the Flood and Coastal Erosion Risk Management Strategy for Southampton (Redbridge to Woodmill Lane) are known.
Emergency services access:
10.4.7 A strategic flood risk management solution is unlikely to be designed to provide a standard of protection for a 1 in 1000 year ARI tidal flood event. However, defences will provide some protection and it is likely that they would reduce the extent and depth of flooding that occurs without any defences. It is recommended that detailed ‘overtopping’ and/or ‘breach’ modelling is undertaken to inform the assessment of emergency services provision and consultation undertaken with the SCC emergency planners. 10.5 Option 2: Site Specific Mitigation
10.5.1 Conceptual site specific flood risk management measures have been identified for each development type currently proposed:
Multi-storey carpark:
10.5.2 Carparking is assumed to have a lifetime of development of 60 years. Ground floor carparking could be retained at existing ground levels on the basis that this is the existing use and therefore development does not result in an increase in the ‘consequences’ of this type of landuse. There could potentially be significant flood hazard in the eastern part of the carpark (exceeding 0.8m) over the lifetime of the development. Cars located in water greater than 0.3m can potentially float. Ensuring the carpark is enclosed will minimise the potential for cars to float and cause additional hazard to people or structures reducing flood risk overall.
10.5.3 If the ground floor carpark can be completely enclosed with a wall 0.3-0.9m high (Diagram 10.10), and carpark entrances off College Street located in the western half of the site, the ground floor carpark could remain ‘dry’ for the assumed lifetime of development.
10.5.4 Reserving upper floors for residents carparking could reduce the risk of ground floor carparks being occupied during a 1 in 200 year ARI tidal flood event.
Southampton City Council 10-3 August 2010
Southampton Level 2 SFRA: Volume 3
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Key: Wall (to 3.0m AOD)
Diagram 10.10: Carpark enclosure wall
Less Vulnerable: Commercial
10.5.5 Commercial development will require raised finished floor levels in the east of the site.
10.5.6 Option 1 (Co_SSM_C_O1): Finished floor levels are raised to 3mAOD – 300mm above local flood levels. This would mean floor levels would gradually rise above street level from west to east. At the lowest point on site, commercial ground floor levels may be 1m above the road level (south east corner).
10.5.7 Option 2 (Co_SSM_C_O2): Finished floor levels are raised to 2.45mAOD – there would be approximately 250mm of floodwater in commercial properties during the 1 in 200 year ARI tidal flood event. This would mean the maximum difference between floor levels and Bernard Street are approximately 450mm.
10.5.8 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
10.5.9 Safe access for the commercial development would be available to the west via Threefield Lane, however this would require access through the development (e.g. through the carpark).
More Vulnerable: Residential
10.5.10 Option 1 (Co_MAA_R_O1): Raise ground floor levels to 4.2mAOD (plus freeboard) provide ground floor residential development. this will create a safe development which will offer ‘safe refuge’ to users and allow direct access to any offsite access proposals.
10.5.11 Option 2: Locating all residential development on upper floors (eg. above commercial development or carparking) will mean people are ‘safe’ within the property. Safe access through the site will be a key requirement for the future development of the site.
Southampton City Council 10-4 August 2010
Southampton Level 2 SFRA: Volume 3
10.5.12 Residents could be afforded direct access to the upper floors of the carpark directly from their property, enabling them to move to the edge of the development freely. Stairs and vehicle ramp access to the ground floor level (preferably in the north west corner) could provide a route for all site users. A ‘safe’ access route could then be available as detailed below.
Off Site Safe Access:
10.5.13 Option 1 (Co_SSM_SA_O1): The central bridge, leading onto the Itchen Bridge is located on land above the 1 in 200 year ARI flood event. A ‘high level’ walkway connected to the Central Bridge at a level above 4.1mAOD would provide a safe ‘dry’ access route from the site, however this is unlikely to comply with local design guidance. Using the area of land at the end of Terminus Terrace (current parking and open space) more effectively may enable an access route to be created to the Central Bridge that doesn’t appear as a ‘high level’ walkway. This could include ground raising, and creating a feature at the end of the development linked to the desire for ‘public art’ in Southampton.
10.5.14 Option 2 (Co_SSM_SA_O2): ‘safe’ access in a westerly direction could be achieved through the raising of land off site from the intersection of Threefield Lane and College Street, along an existing public footway, to King Street (150m). Raising the road intersection and footpath by 300-450mm will provide ‘dry’ access for the lifetime of the development. The route would require appropriate signage so residents were aware of evacuation routes. There would be a need to raised parts of College Street and Threefield Lane to accommodate a 450mm rise in level at the intersection. This option could also provide a safe access route for residents to the north east of College Street, reducing flood risk to off-site areas.
10.5.15 Option 3 (Co_SSM_SA_O3): Providing ‘safe’ access with limited flood depth (250mm) may be acceptable. Approximately 130m of footpath and intersection would need to be raised, however only by 150-200mm (Diagram 10.11). This could be accommodated at the intersection through the incorporation of a raised ‘traffic calming’ measure, which would limit impacts to College Street and Threefield Lane. This option could also provide a safe access route for residents to the north east of College Street, reducing flood risk to off-site areas.
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Legend:
Internal Escape Route
External Escape Route
Internal Meeting Point
External Meeting Point
Raised Fill Platform
Diagram 10.11: Internal and External Escape routes and Meeting Points
Southampton City Council 10-5 August 2010
Southampton Level 2 SFRA: Volume 3
10.5.16 If safe access is not feasible then the option for incorporating safe refuge into the building design should be discussed with the Environment Agency and SCC.
Emergency Services Access:
10.5.17 Emergency services access during an ‘extreme’ flood event (4.4mAOD) will be difficult to obtain for the lifetime of residential development. By 2115 the roads around the site are likely to be significantly inundated. The shallowest flooding is likely to be at the intersection of College St and Threefields Lane (800mm). Flooding at the intersection of Threefields Lane and Bernard Street is estimated at 1.2m deep. Further north on Threefields Lane water remains less than 900mm until close to Russell St, after which it is expected to exceed a depth of 1m. Emergency services may not be able to access the site for approximately 1-2 hours during the peak of the extreme tidal flood event (i.e. water will remain greater than 900mm deep - at or above 4mAOD), however detailed hydraulic modelling is required to understand the local flood mechanisms and timings and should be investigate during the masterplanning phase of the development. 10.6 Option 3: Management Adaptive Approach
10.6.1 There is a benefit to the site of adopting a managed adaptive approach to flood risk management on the site.
10.6.2 Development should contribute to the overall delivery of strategic flood risk management. 10.7 Strategic Flood Risk Management by 2055
10.7.1 If a strategic flood risk management solution is brought forward before 2055, it is unlikely that site levels will need to be raised. Safe access would be available for residential development for the lifetime of the development.
10.7.2 If the timeframes for development cannot be phased to coincide with the implementation of the strategic flood risk management measure and more vulnerable landuses are located in areas of flood risk, then the site should consider adopting site specific measuress to allow development to occur in a timely manner.
More Vulnerable: Residential
10.7.3 Ground floor residential development should be multi-storey (unless a strategic flood management measure is adopted and a breach and or overtopping analysis indicates the site remains dry), locating habitable rooms on the first floor due to the potentially high ‘residual’ risk of flooding.
10.7.4 Off site works to provide ‘safe’ access would not be required.
Less Vulnerable: Commercial
10.7.5 Wherever possible, finished floor levels for property should be 300mm above road and carparking levels to manage surface water flood risk.
10.7.6 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
10.7.7 Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of up to 1.2m.
Southampton City Council 10-6 August 2010
Southampton Level 2 SFRA: Volume 3
10.8 Strategic Flood Risk Management by 2070
10.8.1 If a strategic flood risk management solution is brought forward before 2070, commercial development will not gain significant benefit.
Less Vulnerable: Commercial
10.8.2 Similar to the ‘site specific’ measures, commercial development will require raised finished floor levels in the east of the site.
10.8.3 Option 1 (Co_MAA_C_O1): Finished floor levels are raised to 3mAOD – 300mm above local flood levels. This would mean floor levels would gradually rise above street level moving west to east down Bernard St. At the lowest point, commercial ground floor levels may be 1m above the road level (south east corner).
10.8.4 Option 2 (Co_MAA_C_O2): Finished floor levels are raised to 2.45mAOD – there would be approximately 250mm of floodwater in commercial properties during the 1 in 200 year ARI tidal flood event. This would create a 450mm (maximum) difference in elevation between floor levels and Bernard Street.
10.8.5 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
10.8.6 Safe access for the commercial development would be available to the west via Threefield Lane, however this would require access through the development (e.g. through the carpark).
Residential
10.8.7 Locating all residential development on upper floors will mean people are ‘safe’ within the property. Safe access through the site will be a key requirement.
10.8.8 Residents could be afforded direct access to the upper floors of the carpark directly from their property, enabling them to move to the edge of the development freely. Stairs and vehicle ramp access to the ground floor level (preferably in the western half of the site) could provide a route for all site users. 10.9 Environmental Impacts
10.9.1 The site is located near Central Bridge (listed structure), adjacent to the Oxford Street Conservation area and two listed building (opposite side of Bernard Street). In terms of commercial development on the site, it is not thought that the environmental impact would be significant. There may be a temporary impact on the townscape of the site and surrounding area during the construction process to raise the ground levels, however, it is not thought that that will culminate in a significant permanent impact as the proposed elevation is not considerable.
10.9.2 The raising of pedestrian walkways may have slightly larger impact in terms of townscape due to higher walkway levels and associated lighting etc. The lighting has the potential to impact on a wider area due to its relative elevation.
10.9.3 The proposed development site is also located within an area designated as having archaeological potential. For this reason, there is the potential to disturb undiscovered archaeological remains as part of associated earthworks.
Southampton City Council 10-7 August 2010
Southampton Level 2 SFRA: Volume 3
10.10 Site Summary
10.10.1 The site is currently located within Flood Zones 2 and 3. The Sequential and Exception Test will need to be demonstrated for any development proposed within this site.
10.10.2 Tidal flooding and surface water flooding are the main risks to the site. Groundwater flooding may also be an issue and should be assessed through further Site Investigations.
10.10.3 A Strategic Flood Management option for Southampton will benefit the site and maximise the ‘more vulnerable;’ land use development potential of the site.
10.10.4 If no strategic measures are available for the area then site specific measures will be required for any development type proposed within the site. This could involve the creation of a water tight wall, ground raising and strategic road raising to limit the ingress of water into the site or excluding particular development from specific area of the site that are at risk of flooding. These measures will need to factor in safe access for site users and emergency access for emergency services.
10.10.5 If a carpark was located on the ground floor this would not increase the consequence of flooding and could through appropriate design could reduce flood risk to the site thought the incorporation of additional measures (water tight walls, locating residential above the capark area, etc).
10.10.6 If required, the raising of the land to create the evacuation route from the site is likely to be a significant constraint. A detailed assessment of the impacts on infrastructure, transport routes and access to existing building will need to be undertaken for the development.
10.10.7 It is recommended that SUDS are incorporated into the final layout to assist in reducing the volume of runoff entering the existing drainage infrastructure during the 1 in 100 year plus climate change event. As a minimum it is recommended that flood levels are raised 300mm above the existing ground level.
10.10.8 Flood resilience and resistance techniques are likely to be required throughout much of the ground floor of new development to manage ‘residual’ flood risk, the scale and associated cost will be dependent on the development proposals.
10.10.9 The Sequential Approach should be applied to the development of this site due to the potential future flood risk of the site.
10.10.10 All site users are to receive an ‘information pack’ from developers identifying, as a minimum; the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Southampton City Council 10-8 August 2010
Southampton Level 2 SFRA: Volume 3
11. Key Site 10: Solent Sky Area
11.1 Site Proposal
11.1.1 The proposed development envelope encompasses an area of approximately 1.32ha and is located at National Grid Reference 442840 111120. The proposed development is for a mixed development with residential and commercial development on the ground floor. The average topography of the site is between 2.5 – 3.2m AOD
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Diagram 11.1: Site Location
11.1.2 Ground levels vary around the site:
Along Canute Road and Royal Crescent Road - 2.9 – 3.2mAOD
Along Saltmarsh Road – 3.3 rising to 7mAOD at the Itchen Bridge roundabout
Within the site – 2.5 – 3.5mAOD
11.1.3 Key spatial requirements for the site include:
There are four possible development options for the site that are being investigated.
The proposals include variations on mixed use redevelopment on site with the current museum either enlarged in its original location or relocated on to Canute Road with the possibility of residential and commercial units incorporated into the new building.
These conceptual options can be seen in Diagram 11.2 with blue areas denoting the proposed museum and yellow areas identified as commercial and residential areas.
Southampton City Council 11-1 August 2010
Southampton Level 2 SFRA: Volume 3
Option 1 Option 2
Option 3 Option 4
Diagram 11.2: Indicative 3D Massing Model of the proposed Development Option(s) (Source: Albert Road South Masterplan Options Report 2009)
11.1.4 The proposed options include ground floor commercial uses with residential uses on the upper floors along Canute Road. The options that involve the relocation of the museum propose a housing development on the existing location of the museum on Saltmarsh Road, whereas the options that retain the museum in its current position propose an additional housing development on Canute Road. 11.2 Technical Assessment: Flood Risk
Summary of Flood Risk
11.2.1 The site is currently at a ‘low’ probability of tidal flooding, however after 2070 this is expected to ‘high’ and by 2115 ‘significant’ flood hazard is predicted within the majority of the site.
11.2.2 Groundwater flooding may become an issue in the future and should be assessed and managed as part of a detailed site FRA. Future regional groundwater management should be considered by Hampshire County Council, the Environment Agency and SCC.
11.2.3 It is recommended that Southampton City Council undertakes the Sequential Test to inform future development on the site.
Southampton City Council 11-2 August 2010
Southampton Level 2 SFRA: Volume 3
11.2.4 Further detail on the existing and future flood risk at the site are provided in the following sections.
Flood Zones
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Legend: Flood Zone 3
Flood Zone 2
Diagram 11.3: Environment Agency Flood Zones
11.2.5 The site is predominantly located within Flood Zones 2 and 3 (refer to Diagram 11.3). Sequential approach recommended with higher vulnerability land uses located within areas identified at lower probability of flooding. Current development proposals compatible in this location, subject to the Sequential and potentially the Exception Test.
11.2.6 If the predicted effects of climate change are accurate then the majority of the site could be located within Flood Zone 3 in the future.
Historic Flooding
11.2.7 No historic flood records were recorded within the site. Combined tidal/surface water flooding has been recorded at the Town Depot site to the north east.
Tidal Flood Risk and Hazard
11.2.8 The site is not currently located in an area at tidal risk of flooding due to higher ground levels between the site and the River Itchen.
11.2.9 The site remains outside the 1 in 200 year ARI tidal flood event until about 2070. During a 1 in 200 year ARI tidal flood event in 2055 overland flow routes are predicted to develop along Royal Crescent Road (from the north), but are not predicted to reach the site. By 2070 flooding is predicted to reach the site via overland flow along Royal Crescent Road and Canute Road (to the east) with a ‘low’ flood hazard. Flooding on the site and surrounding roads is predicted to have a ‘significant’ flood hazard in 2115. Diagrams 11.4 – 11.7 identify the flood depth and hazard predicted for the site.
Southampton City Council 11-3 August 2010
Southampton Level 2 SFRA: Volume 3
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Legend (m) Legend: Low Moderate Significant Extreme
Diagram 11.4: Flood Depth 2070 - 1 in 200 year Tidal Flood Event Diagram 11.5: Flood Hazard 2070 - 1 in 200 year Tidal Flood Event
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Legend: Legend (m) Low Moderate Significant Extreme
Diagram 11.6: Flood Depth 2115 - 1 in 200 year Tidal Flood Event Diagram 11.7: Flood Hazard 2115 - 1 in 200 year Tidal Flood Event
Southampton City Council 11-4 August 2010
Southampton Level 2 SFRA: Volume 3
Tidal Residual Risk
11.2.10 The site is potentially partially at risk from the ‘extreme’ 1 in 1000 year ARI tidal flood event (Flood Zone 2). By 2115 the entire site could be located within the ‘extreme’ 1 in 1000 year ARI tidal flood event.
Fluvial Flood Risk
11.2.11 The site is located within an area identified at low probability of fluvial flooding.
Surface Water Flood Risk
11.2.12 Surface water modelling indicates some areas of lower lying ground within the site (e.g. near the intersection of Royal Crescent Road and Canute Road) that can be classified as being susceptible to surface water flooding (refer to Diagram 11.8). There are no records of surface water flooding on site. Saltmarsh Road on the northern boundary of the site may act to convey overland flow from the Itchen Bridge roundabout during high intensity rainfall events.
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Diagram 11.8: Susceptibility to Surface Water Flooding
Groundwater Flood Risk
11.2.13 Existing ‘Moderate Risk’ due to permeability of the underlying geology. Sea level rise as a result of climate change is likely to also have an effect on the water table level in low lying ground in Southampton.
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11.2.14 Development proposals will need to consider site ground conditions and groundwater levels over the lifetime of the development. In particular the design of any underground structures or services and foundations. Future regional groundwater management should be considered by the Environment Agency and SCC.
Flood Risk from Artificial Sources
11.2.15 No artificial sources located near the site. 11.3 General Flood Risk Management
11.3.1 The following guidance applies independent of the overall approach to managing flood risk on site:
There is limited opportunity to adopt a sequential approach to layout on site, as ground levels are relatively consistent within the site boundary. Locating residential development in the north of the site would be preferable, as this provides a more direct route to a ‘dry’ safe access route over the Itchen Bridge.
Wherever possible property levels should be raised a minimum of 300mm above local road and carparking levels to manage the risk of surface water flooding. In particular access off Saltmarsh Road should be designed so overland flow from higher ground is not channelled through the site, but is retained in the carriageway.
The basement carpark design will need to consider the existing and future groundwater levels on site, in particular whether it alters off site groundwater flow (increasing flood risk to others), how this will change over time with climate change, managing groundwater ingress into the basement, and ensuring both the basement and services are not susceptible to ‘floating’.
All site users are to receive an ‘information pack’ from developers identifying, as a minimum, the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Space constraints mean significant SUDS features such as ponds and wetlands are unlikely to be appropriate. ‘At source’ control measures such as green roofs, rainwater harvesting and water butts should be included in the design. Permeable pavement should be considered in surface carparking and hard landscaping areas. Public gardens and open space should consider benefits of including swales and ‘rain gardens’ for both surface water management and biodiversity. Development should seek to reduce the rate and volume of water discharging from the site. Inclusion of SUDS measures such as these will significantly assist with this.
Emergency services access should be available for the site via Itchen Bridge and Saltmarsh Road for the lifetime of the development.
A site specific flood emergency plan should be prepared, in consultation with Council emergency planners, emergency services, and with reference to Multi Agency Flood Plan to evacuate site users out of the floodplain in an ‘emergency’ flood event.
Southampton City Council 11-6 August 2010
Southampton Level 2 SFRA: Volume 3
11.4 Option 1: Strategic Precautionary Approach
11.4.1 The site would benefit from strategic flood risk management infrastructure on the River Itchen and the River Test to a specified standard of protection (e.g. 1 in 200 year ARI tidal event = 4.2m AOD + freeboard).
11.4.2 Development should contribute to the overall delivery of strategic flood risk management.
11.4.3 Flood resilient and resistance measures should be included in the ground floor building design, such as external/internal wall renders and raised internal services above 4.7mAOD. The Museum should consider the location of valuable artefacts, and wherever possible locate items that would not be resilient to flooding on upper floors, or above a level of 4.7mAOD.
11.4.4 Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths of 0.7-1.7m.
More Vulnerable: Residential
11.4.5 Unless detailed breach/overtopping assessment demonstrates the site remains dry during a residual risk event , ground floor residential development should be multi-storey, locating habitable rooms on the first floor due to the potentially high ‘residual’ risk of flooding.
Basements:
11.4.6 Basement carpark access off Royal Crescent Road would be acceptable, if accompanied by appropriate flood warning and signage to advise on risk. 11.5 Option 2: Site Specific Measures
11.5.1 Conceptual site specific flood risk management measures have been identified for each development type currently proposed:
Less Vulnerable: Commercial
11.5.2 Commercial development along Canute Road is at limited risk during the design flood event for the lifetime of the development. Finished floor levels for commercial property along Canute Road should be 300mm above road levels to manage surface water flood risk and the ‘low’ tidal flood hazard.
11.5.3 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.7mAOD for a 100 year lifetime of development and 4.1mAOD for a 60 year lifetime of development.
Basement Carpark:
11.5.4 Basement carpark access is proposed off Royal Crescent Road. Due to the ‘significant’ flood hazard on this road , there is the potential for water to flow into the basement carpark resulting in extensive flooding. The ground level on Royal Crescent Road near the carpark access is approximately 3.2mAOD. Creating a slope from the road up to either 3.95mAOD or 4.2mAOD, and then down into the carpark would prevent floodwater from entering the basement carpark. The basement carpark would need to be ‘tanked’ with ventilation at or above 4.2mAOD. The up-and-down basement access may reduce the development footprint by approximately 400- 600m2 to create the access ramp.
Southampton City Council 11-7 August 2010
Southampton Level 2 SFRA: Volume 3
11.5.5 If the basement carpark access can be located off Saltmarsh Road it will require a limited up- and-down access ramp (potentially 300mm raised to prevent surface water runoff ingress), however the ramp down is likely to be longer and may reduce the available carpark spaces.
More Vulnerable: Residential
11.5.6 The site is expected to have a ‘significant’ flood hazard during the lifetime of any proposed residential development.
11.5.7 Safe access through the site will be a key requirement and is likely to require some land raising if access through upper floors of buildings or ‘high level’ accessways to Saltmarsh Road are not appropriate.
11.5.8 Two sub-options have been assessed for managing flood risk to the ‘more vulnerable’ land uses in the area.
11.5.9 Option 1 (SW_SSM_R_O1): Creating a site that slopes or ‘steps up’ from existing levels on Canute Road (approximately 3mAOD) where ground floor commercial is proposed, up to the existing levels on Saltmarsh Road (6-7mAOD) can provide a basis for ‘safe’ access for residential development. This site raising would assist in developing a ‘positive edge’ for the development onto Saltmarsh Road.
11.5.10 Raising ground levels to approximately 4.2mAOD where residential development is located would provide a safe ‘dry’ access route to Saltmarsh Road.
11.5.11 Residential development above the commercial frontage on Canute Road would require access onto the higher ground in the north, not onto Canute Road which could potentially be at significant flood hazard. This access can be a ‘secondary’ or ‘emergency’ exit, with the main entrance on Canute Road if necessary.
11.5.12 Finished floor levels for future residential development should be located 300mm above the 4.2mAOD ground level to manage both uncertainty in hydraulic modelling, and surface water flood risk.
11.5.13 This scenario is likely to mean ground floor residential development (including habitable rooms) would be acceptable as ‘residual’ flood risk from breach or overtopping of defences is likely to result in only ‘low’ flood hazard across this area, with safe access, albeit with limited flood depth, available to Saltmarsh Road.
11.5.14 Option 2 (SW_SSM_R_O2): Site levels are raised to create a slope, or ‘step up’, from Canute Road (3mAOD), to the existing levels on Saltmarsh Road (6-7mAOD) providing the basis for ‘safe’ residential development. This site raising would assist in developing a ‘positive edge’ for the development onto Saltmarsh Road.
11.5.15 Raising ground levels to approximately 3.95mAOD or higher where residential development is located would provide a safe access route to Saltmarsh Road, however it would flood to limited depth (250mm).
11.5.16 Residential development above the commercial frontage on Canute Road would require access onto the 3.95mAOD ground in the north. This access can be a ‘secondary’ or ‘emergency’ exit, with the main entrance on Canute Road if necessary.
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11.5.17 Finished floor levels for future residential development should be located at 4.5mAOD (i.e. 300mm above flood levels), which would make them approximately 600mm above the surrounding site levels. This would be an additional access consideration in the scheme design.
11.5.18 If ground floor levels are raised to only 4.2mAOD (250mm above site levels), they may be subject to flooding during the design event. In this instance ground floor residential should not locate habitable rooms on the first floor and above to manage ‘residual’ risk of flooding.
11.5.19 Raising ground levels across the site is likely to reduce the amount of excavated material required to create the basement carpark, albeit by not as much as Option 1. Material that is excavated on site may be reused (if determined to be suitable fill material) to raise ground levels and create slopes with the land immediately to the east, and Royal Crescent to the west. This would contribute to the overall sustainability of development. Retaining walls (with root barriers) may be required where existing trees require protection on Royal Crescent Road.
11.5.20 Option 3 (SW_SSM_R_03): providing commercial land uses on the ground floor throughout the development would mean large scale site raising may not be necessary, as commercial development would be safe for the lifetime of the development, however ‘safe’ access would still be required for residential properties on upper floors.
11.5.21 This would mean key pedestrian routes at or above a minimum of 3.95mAOD, with commercial development located slightly lower than this level. This is likely to create design and access challenges.
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Legend: Wall (to 4.2m AOD) Raised carpark entrance
Diagram 11.9: Conceptual location of raised carpark entrance and flood wall.
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Southampton Level 2 SFRA: Volume 3
11.5.22 Option 4 – creating a wall along Canute Road (or immediately behind the commercial frontage) and along Royal Crescent Road would prevent water ingress into the site, and remove the need for ground raising, however access over the wall would be required for the basement carpark, and a wall is likely to create design and access challenges.
Museum:
11.5.23 A museum is most appropriately classified as a ‘less vulnerable’ use, however the value of its contents and SCC view on the design life will be important considerations in how flood risk is managed.
11.5.24 Option 1(SW_SSM_M_O1): Museum: 60 year lifetime of development On the basis of a lifetime of development for the museum of approximately 60 years, there is no restriction on finished floor levels based on tidal flood risk. It is recommended that flood levels are a minimum of 300mm above road and carpark levels to manage the risk of surface water flooding.
11.5.25 The Museum should consider flood resilience and resistance measures including the location of valuable artefacts, and wherever possible locate items that would not be resilient to flooding on upper floors, or above a level of 4.1mAOD.
11.5.26 Option 2 - Museum: 100 year lifetime of development - If the museum is likely to have a lifetime of development of up to 100 years, the ground floor should be raised to a minimum of 3.95mAOD (approximately 1m above Canute Road levels). In this instance, some flooding is still likely to occur, albeit at depths of approximately 250mm. If a higher standard of protection is required, ground floor levels should be further raised.
11.5.27 The Museum should also consider flood resilience and resistance measures including the location of valuable artefacts, and wherever possible locate items that would not be resilient to flooding on upper floors, or above a level of 4.7mAOD.
11.5.28 The buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depth exceeding 1.5m (depending on the final land elevations). 11.6 Option 3: Management Adaptive Approach
11.6.1 If a strategic flood risk management solution is brought forward before 2070, it will be unlikely to be necessary to raise site levels. Safe access would be available for residential development for the lifetime of the development.
11.6.2 If the timeframes for development cannot be phased to coincide with the implementation of the strategic flood risk management measure and more vulnerable landuses are located in areas of flood risk, then the site should consider adopting site specific measures to allow development to occur in a timely manner.
11.6.3 Development should contribute to the overall delivery of strategic flood risk management.
Residential:
11.6.4 Ground floor residential levels should still be set 300mm above the surrounding road and carpark levels to manage the risk of surface water flooding.
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11.6.5 Unless detailed breach/overtopping assessment demonstrates the site remains dry during a residual risk event, ground floor residential development should be multi-storey, locating habitable rooms on the first floor due to the potentially high ‘residual’ risk of flooding. If this is not achievable it is likely that ground raising, similar to that described in Option 2 above, would be necessary.
Commercial/Museum:
11.6.6 There is limited benefit to commercial development applying the managed adaptive approach, on the basis of a lifetime of development of 60 years, as the site is at limited flood risk from the 1 in 200 year ARI tidal flood event before 2070.
11.6.7 Wherever possible, finished floor levels for property should be 300mm above road and carparking levels to minimise the surface water flood risk.
11.6.8 Flood resilient and resistance measures should be included in building design, such as external/internal wall renders and raised internal services above 4.1mAOD.
11.6.9 The Museum should consider the location of valuable artefacts, and wherever possible locate items that would not be resilient to flooding on upper floors, or above a level of 4.1mAOD or 4.7mAOD, depending upon the lifetime of the development.
11.6.10 Although they do not necessarily need to remain dry, buildings must be designed to withstand hydrostatic forces from a breach in defences or extreme flood event, which could result in water depths exceeding 1.5m.
Basements:
11.6.11 Basement carpark access off Royal Crescent Road would be acceptable, if accompanied by appropriate flood warning and signage to advise on risk. 11.7 Environmental Impacts
11.7.1 The site is located next to the Canute Road Conservation area and near several listed buildings (not located within the boundaries of the development). Site specific flood mitigation proposals are anticipated to potentially have a permanent impact on the townscape of the area due to elevated ground levels. There may also be a temporary impact during the construction process, although this is not anticipated to be significant.
11.7.2 As the proposed site is also located in an area of archaeological potential, there is the potential for the disturbance of undiscovered archaeological remains as part of the earth works to create a sloping or stepped site to reduce the impact from flood events. 11.8 Site Summary
11.8.1 The site is currently located within Flood Zones 2 and 3. The Sequential and potentially the Exception Test will need to be demonstrated for any development proposed within this site.
11.8.2 Tidal flooding and surface water flooding are the main risks to the site. Groundwater flooding may also be an issue and should be assessed through further Site Investigations.
11.8.3 A Strategic Flood Management option for Southampton will not benefit less vulnerable development options with a design life of 60 years but will maximise the development potential of the site for landuses with a design life of 100 years.
Southampton City Council 11-11 August 2010
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11.8.4 If no strategic measures are selected for the area then site specific measures will be required. This could involve the creation of a water tight wall, locating habitable residential rooms above ground floor less vulnerable development and site specific ground raising – however this could be offset with below ground carparking which could create raised ground levels but would require a specific raised access to prevent the ingress of floodwaters during an extreme event. These measures will need to factor in safe access for site users and emergency access for emergency services. Providing access to the basement carpark that will restrict the ingress of flood water but still have sufficient grades to allow vehicles to enter the proposed basement structure may prove a constraint to development. Any ventilation into the basement will need to be located above the design flood levels to restrict the ingress of floodwaters into the basement.
11.8.5 It is recommended that SUDS are incorporated into the final layout to assist in reducing the volume of runoff entering the existing drainage infrastructure during the 1 in 100 year plus climate change event. As a minimum it is recommended that flood levels are raised 300mm above the existing ground level.
11.8.6 Flood resilience and resistance techniques are likely to be required throughout much of the ground floor of new development to manage ‘residual’ flood risk, the scale and associated cost will be dependent on the development proposals.
11.8.7 The Sequential Approach should be applied to the development of this site due to the potential future flood risk of the site.
11.8.8 All site users are to receive an ‘information pack’ from developers identifying, as a minimum; the risk of flooding, how this is being managed on site, actions site users should take in the event of a flood, appropriate emergency contact details.
Southampton City Council 11-12 August 2010
Southampton Level 2 SFRA: Volume 3
Appendix A: Environmental Appraisal
Southampton City Council A1 August 2010
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1. Introduction
1.1.1 Capita Symonds has been appointed by Southampton City Council (SCC) to undertake an appraisal of an overall strategic flood mitigation plan for Southampton. In addition, there is a requirement to produce an appraisal of more site specific flood risk assessments for a number of areas around the city that have been selected as potential re-development sites. As part of these tasks, in order to identify potential environmental impacts related to the proposed flood mitigation proposals, an initial desk-top environmental sieve analysis has been undertaken. 1.1.2 This desk-top sieve analysis includes an initial review of readily available published information enabling the identification of a series of potential constraints and opportunities that may influence the approach to strategic and site specific flood mitigation, as well as recommendations for further work as appropriate. The initial review has covered from the River Itchen and frontage around the Docklands area to the town of Redbridge to the west. See Figure 1.1 in Volume 2 of the SFRA. 1.1.3 Information was primarily obtained from the following sources:
Multi-Agency Geographic Information for the Countryside (MAGIC) interactive mapping (www.magic.gov.uk); Southampton City Council website (www.southampton.gov.uk); Natural England’s ‘Nature on the Map’ interactive mapping (http://www.natureonthemap.org.uk/map.aspx); and, Environment Agency ‘What’s in your backyard’ interactive mapping (http://maps.environmentagency.gov.uk/wiyby/wiybyController?ep=maptopics&lang=_ e). 1.1.4 The environmental features considered in this analysis correspond with those environmental aspects identified in Schedule 2 of the SEA Regulations 2004 (The Environmental Assessment of Plans and Programmes Regulations 2004 (Statutory Instrument 2004 No 1633). The suitability of each Option to adapt to climate change is assessed elsewhere in this document.
1.2 THE OPTIONS
The purpose of this environmental sieve analysis is to assess the environmental impact associated with the installation of a strategic flood mitigation strategy and a number of site specific strategies.
Strategic Flood Mitigation Scheme A strategic flood mitigation scheme has been proposed for Southampton as a whole. This involves the installation of sheet piling or re-enforcement of existing banks that have been hard engineered to cope with a 1 in 200 year flood event. The proposed defences would begin at a point just south of Woodmill Lane on the River Itchen and follow the western bank along Southampton Water to where the River Test discharges into Southampton Water at the town of Redbridge.
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Site Specific Flood mitigation Option A number of site specific proposals have also been considered , focussed on a selection of development sites around the City that are particularly at risk from current or future flood events. The development proposals and associated flood mitigation proposals are outlined in Appendix 1, Table 1, with the environmental impact of the proposed flood mitigation described in further detail in Section 3 of the main report.
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2. Scene Setting
2.1.1 The proposed flood mitigation schemes are primarily associated with the implications of tidal flooding associated with the River Itchen and the River Test.
RIVER ITCHEN 2.1.2 The source of the River Itchen is located just south of the village of Cheriton, Mid- Hampshire rising from the chalk aquifer of the Hampshire Downs. It is approximately 28 miles long and is noted as a world premier site for fly fishing. The River is designated as a Site of Special Scientific Interest (SSSI) and Special Area of Conservation (SAC) along certain points of its length due to the valuable habitat it supports, namely including, Otter, White Clawed Crayfish, Water Vole and Brook Lamprey. 2.1.3 It enters the jurisdiction of SCC, passing under the M27 to the north of Mansbridge Road. The river then passes through a medium sized weir where it is culverted and forms the Mansfield Reservoir 2.1.4 The River then flows in an easterly direction, running relatively parallel with the A27, (Mansbridge Road) where it is joined by a small tributary. The surrounding landscape is relatively flat in profile and is dissected into small fields by mature trees and hedgelines. 2.1.5 The river then turns in a southerly direction and passes under the A27 (Mansbridge Road) and is lined on the northern side by a bank of mature trees. The southern bank of the river is bounded by a man-made pathway. 2.1.6 Before passing under Woodmill Lane, the River widens considerably and breaks into two small, linked ponds. The river then passes under Woodmill Lane, widening in the vicinity of Woodmill Canoeing and Activity Centre. Along the northern bank, there are a number of houses with boat mooring or small jetty facilities backing directly onto the river, whilst the southern shore is flanked by a band of mature trees. 2.1.7 The River then meanders naturally south through relatively open countryside, before passing a sewage works where the surrounding landscape becomes significantly more urbanised. At this point both banks are the River are culverted owing to the housing estates located on the western bank and a narrow road on the eastern bank. It passes under the A3035 (St Denys Road) and enters a highly urbanised residential area with managed banks and boat mooring/jetty facilities. 2.1.8 The River then passes under a railway line before crossing the A3024 and flowing into an area of mixed residential, commercial and industrial use with port and docking facilities for large ships, before passing under the Itchen Toll Bridge before discharging into Southampton Water where its meets the River Test.
THE RIVER TEST 2.1.9 The river rises from a chalk aquifer 10km west of Basingstoke and continues for approximately 40 miles before meeting the River Itchen and flowing into the sea as Southampton Water.
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2.1.10 The River Test is classified as a SSSI along the entirety of its length due to its diverse species assemblage and fen and flood pasture. It has several abstraction points along its length, both for public and agricultural use and receives a number of discharges from a sewage works and paper factory. 2.1.11 For the purposes of this assessment, the affected part of the River Test will be confined to the heavily industrialised frontage of Western Docks to a point just north of the A35.
2.2 ENVIRONMENTAL BASELINE
STATUTORY AND NON STATUTORY NATURE CONSERVATION DESIGNATIONS 2.2.1 The River Itchen is widely regarded as one of the finest examples of chalk streams in the world. As such it supports a wide variety of botanical and ecological species. The site has a European nature conservation designation as a Special Area of Conservation (SAC) from its source to a point at Woodmill within the city of Southampton. It is also designated as a SSSI at certain points along its length owing to its classification as a classic chalk stream and river, fen meadow, flood pasture and swamp habitat. The site is also designated for its nationally rare populations of southern Damselfly (Coenagrion mercurial) and assemblages of White Clawed Crayfish (Austropotamobius pallipes). The site is also notified for Otter (Lutra lutra), Water Vole (Arvicola terrestris), fresh water fishes including Bullhead (Cottius gobo), Brook Lamprey (Lampetra planeri) and Atlantic Salmon (Salmo salar). The River is also home to a number of breeding bird species. 2.2.2 As highlighted above, the River Itchen supports a wide variety of botanical and ecological species. There are an additional two smaller SSSI and Ramsar designations further downstream named Lee-on-the-Solent to Itchen Estuary. The first site is located on a bend of the River Itchen and the second within the estuary at the mouth of the River. Both sites have been designated due to the varied intertidal sediment resource that provides a valuable habitat for barnacles and molluscs. 2.2.3 On the far side of the Solent, there are two further SSSI designations. Hythe to Calshot Marshes SSSI and Ramsar site is so designated due to its extensive salt marshes and mud flats that support thousands of over-wintering and migrating waders and wildfowl, however, it is thought that the quality of this designation is declining due to ‘coastal squeeze’ i.e. the effect of implementing flood defences. The second designation, Dibden Bay lies further inland than the Hythe to Calshot designation and was formed by the deposition of marine dredging material in the late 1970’s and has since re-colonised naturally and now supports a variety of wet and dry grassland, together with salt marsh vegetation, saltpans, swamp, reedbeds, scrub and open water. It also supports a wide variety of invertebrates, wildfowl and nesting Lapwing. 2.2.4 The River Test is designated as a SSSI along the entirety of its length before reaching Southampton Water. Owing to its diverse species assemblage, the River Test is widely regarded as one of the most species rich lowland rivers in England. 2.2.5 The Solent and Southampton Water Special Protection Area (SPA), SSSI and Ramsar site is a site of international importance and located opposite the Western Docks area of Southampton and designated due to the breeding and overwintering birds it attracts. 2.2.6 A Site of Importance for Nature Conservation (SINC) has been designated at Peartree Green, with an additional area just after the River enters the jurisdiction of Southampton City Council. These areas are so designated by the SCC Local Plan (2002), under policy NE3
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and are usually based around naturally occurring stream ways. Development on these sites will be restricted unless it can be demonstrated that the development is in the overriding public interest and outweighs the nature conservation value of the site. 2.2.7 Much of the initial route of the River Itchen as it enters Southampton is bounded by mature trees and fields with the potential for protected flora and fauna or the presence of designated Biodiversity Action Plan (BAP) habitats and species.
AIR 2.2.8 Since 1997 Local Authorities (LAs) in the UK have been carrying out a review and assessment of air quality in their areas. The aim of the review is to assist LAs in carrying out their statutory duty to work towards meeting national air quality objectives. If a LA finds any places where the objectives are not likely to be achieved, it must declare an Air Quality Management Area (AQMA) there. SCC has declared an AQMA for eight areas within its jurisdiction:
Bevois Valley Road – an area encompassing Bevois Valley Road, Onslow Road and Charlotte Place. Bitterne Road West – an area encompassing Bitterne Road West from Hawkswood Road to Little Lance’s Hill. Red Bridge Road (A33) – encompassing the immediate area around the Redbridge Flyover. Romsey Road – encompassing the intersection between Romsey and Winchester Road. Town Key – encompassing Platform Road, Terminus Terrace and Town Quay. Winchester Road – encompassing the roundabout off Winchester Road Millbrook Road – encompassing Millbrook Road East and West. Commercial Road – encompassing Commercial Road where it joins the A348. 2.2.9 The main source of pollution in the above AQMA’s is nitrogen oxide derived from road traffic. Nitrogen oxide pollution arises from emissions of nitrogen oxide and nitric oxide from combustion processes such as vehicle engines. The AQMA’s were put in place with the aim to reduce Nitrogen Oxide emission levels to achieve the national air quality objective levels. The national pollution targets are health based standards and therefore the Southampton AQMA’s have only been located in areas where human exposure would be high, for example, in areas where buildings act as receptor units.
ARCHAEOLOGY 2.2.10 A review of Southampton City Council’s proposal maps indicates that much of the water front area has been designated as having archaeological potential. A number of wards located along the River Itchen and River Test are listed on the website of Southampton City Council Archaeology department as containing a number of sites of archaeological potential and include:
Redbridge Ward; Freemantle Ward; Bargate Ward; Bevois Ward, and; Portswood Ward
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BUILT HERITAGE 2.2.11 The Southampton Interactive Plan for Conservation Areas, Scheduled Monument and Listed Buildings indicates there are a number of Conservation Areas, Listed Buildings and Scheduled Monuments within Southampton. Conservation Areas designated as such by SCC within the Local Plan are defined by the Planning (Listed Buildings and Conservation Areas) Act 1990 as local areas of special architectural or historic interest, the character or appearance of which it is desirable to preserve or enhance. Features can include both built heritage and archaeological potential. The following Conservation Areas are of particular note to this review, due to their location on or adjacent to the River Itchen:
Oxford Street Conservation Area – Bernard Street, Canute Road, John Street, Latimer Street, Lower Canal Walk, Orchard Place, Oxford Street, Platform Road, Queen’s Park, Queens Terrace and Terminus terrace. Old Town North Conservation Area – includes Bargate Road, Castle Way, East Bargate, East Street, Hamtun Street, High Street, Holyrood Street, Simnel Street, St Michael’s Street, West Bargate and West Street. Old Town South Conservation Area – encompasses Briton Street, Castle Way, Frech Street, High Street, Lower Canal Walk, Porters Lane, Town Quay, Winkle Street. Old Town West Conservation Area – Masonic Hall in Albion Way, Blue Anchor Lane, Bugle Lane, Castle Lane ,Castle Way, Cement Terrace, Church Lane, Cuckoo Lane, Forest View, French Street, Landsdowne Hill, Madison Street, Merchants Walk, Sinmel Street, St Michaels Square, Town Quay, Upper Bugle Street, Western Esplanade and Westgate Street. Canute Road Conservation Area – includes both the south and north side, Platform Road and the west side of Royal Crescent Road. Itchen Valley Conservation Area – includes Claude Ashby Close, Cutbush Lane, Mansbridge Road, Monks Way, St Marys Church Close, Wessex Lane and Woodmill Lane. 2.2.12 There are numerous listed buildings within these Conservation Areas in addition to a number of Churches along the River and Southampton docklands including old warehouses, the King George V Dry Dock, Town Pier and a number of buildings adjacent to the A35. 2.2.13 There is one Scheduled Monument (SM) within the locality of the River, located on the eastern bank of the River Itchen – Bitterne (Clauentum) Roman Station, Bitterene Manor. SMs are designated by the Secretary of State under the Ancient Monuments and Archaeological Areas Act 1979, to give legal protection to nationally important sites and monuments and their setting.
LANDSCAPE / TOWNSCAPE 2.2.14 At its entry point into the jurisdiction of Southampton City Council the River Itchen flows through a reasonably rural area, with open views and some stretches of the bank being bounded by mature trees. As it enters the City of Southampton, the landscape becomes significantly more urban, flowing through dense residential areas, where the river is used for recreational purposes (sailing/canoeing). 2.2.15 A significant area of land from the M27 to the A3035 bridge crossing has been designated as ’open space’ on the SCC proposals maps. Land designated as such by the Council will be protected from development that could result in the loss of this public or private space. 2.2.16 There are a number of parks located near the docklands areas: Queens Park, Mayflower
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Park, Central Park, Town Quay, Cuckoo Lane and Riverside Park located further upstream. There are no designated Green Corridors along the River. 2.2.17 Large trees are also present along the length of the River. Where these fall within Conservation Areas they will be subject to Tree Preservation Orders (TPOs). A number of additional trees along the river are also likely to be subject to TPOs. 2.2.18 Much of the frontage along the affected section of the River Test is comprised of dockland areas with concrete or manmade banks. The opposite shore, due to the nature conservation designations, has natural banks with open views of Southampton Water and the city itself.
POPULATION 2.2.19 Southampton is an urban area with a population of approximately 217,445 people. According to 2001 Census data 68.3% of its population are in good health compared to a national statistic of 68.7%. Southampton has an economically active age group of approximately 39.6% which is slightly lower than the 43.2% average for the South East. The percentage of the population that have no qualifications in Southampton is considerably higher than the regional average at 26.4% compared to 23.9% for the South East as a whole. 2.2.20 The Solent Way and Itchen Way public footpaths are both designated as long distance footpaths on the SCC Local Plan. The Itchen Way follows the River Itchen from its source near Hinton to its mouth at Woolston with the 60 mile Solent Way running West to East from Milford-on-Sea to Emsworth Harbour passing close to the mouth of the River Itchen. There are numerous smaller footpaths that follow the course of the River as it enters Southampton designated as public footpaths on the local planning maps.
SOIL: CONTAMINATED LAND 2.2.21 Limited information is available on contaminated land in Southampton from readily accessible sources. The current land use of the River Itchen and the surrounding landscape suggest there may be significant potential for contaminated land arising, especially, from the dockland and harbour areas of the city. Environment Agency data indicates that there have been pollution incidents to water in these areas. There may be pollution incidents to land, although records of which have been inaccessible at this stage of the assessment. 2.2.22 Construction works involving earth movement and excavations and the use of plant adjacent to the river has the potential to generate contamination pathways if contamination is present in these areas. 2.2.23 A geomorphology baseline for the study area will need to be derived to inform future stages of the project.
WATER: QUALITY 2.2.24 A recent study to assess the sustainability of the River Itchen has concluded that the current level of water abstraction and discharge activities on the River are having a detrimental effect on the biodiversity; notably; Damslefly, Salmon and Invertebrates. There was also found to be high levels of soluble phosphorous and ammonia in the River, potentially due to the discharges from the sewage works. 2.2.25 The Environment Agency website ‘What’s in my backyard’ feature identifies 4 significant pollution incidents that have occurred along the River Itchen. This is in addition to 2 industrial pollution outfalls that were classified as Water and Radioactive that are actively monitored by the Environment Agency.
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2.2.26 There are no recorded pollution incidents that have occurred along the frontage of the River Test. 2.2.27 The geology of the catchment is predominantly upper and middle Chalk and baseflows are highly dependent on the local groundwater levels.
NOISE 2.2.28 Although no readily available information is available for this topic area, it is assumed that the background noise levels will be similar in Southampton to any urban area. As such potentially noise sensitive receptors include the residential areas, schools, and users of open space.
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3. Analysis
3.1 STRATEGIC FLOOD MITIGATION OPTION
The strategic flood mitigation strategy for Southampton is illustrated in Figure 11.1 in Volume 2 of the SFRA. It involves the installation of sheet piling or the reinforcement of existing hard banks. The proposed defences begin on the Western bank of the River Itchen just after Woodmill Lane and continue down into Southampton Water in an easterly direction along the Docklands area to where the River Test meets Southampton Water at the town of Redbridge.
Key environmental impacts associated with this strategic defence include the indirect impact to a number of key SSSIs and one Site of Important Nature Conservation Interest (SINC) within close proximity to the proposed defences. The Lee-on-Solent to Itchen Estuary SSSI has previously been subject to a decline in the value of the site due to the implementation of coastal flood defences in other areas. There is a potential risk that this decline may be exacerbated by the installation of the new Southampton flood defences. However, due to the industrialised nature of the area in the vicinity of these designations and the fact that the banks that will be reinforced as part of this mitigation strategy are already culverted or managed in some way, it is not anticipated that there will be a significant impact on the condition of the designations resulting from the mitigation strategy.
There is the potential for impact on both aquatic and terrestrial ecology as part of the implementation and operation of this mitigation strategy. Accounting for the limited ecological information regarding the sites that may be affected by the mitigation scheme it is difficult to ascertain the level of impact. There may be the potential to impact on aquatic ecology as part of the installation of the defences arising from sediment loading or disturbance of habitat. Terrestrial ecology located close to the waters edge may be impacted by the installation of the defences through loss of habitat and disturbance.
Certain sections of the River Itchen are bounded by relatively mature trees that may have the potential to provide habitat for bats or other protected species that may be disturbed as part of construction works.
In terms of archaeological impacts, much of the dockland area is designated as having archaeological potential on SCC Local Plan. However, it is not anticipated that the proposed works would culminate in a significant impact in this area due to the industrialised nature of the area, and the fact that much of the ground has been disturbed previously.
There may be a limited potential for temporary impact on the setting of a number of listed buildings within close proximity to the proposed mitigation site associated with the construction activities.
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The townscape of the affected area during the construction phase of the works may be impacted due to the presence of heavy machinery and earth works, however this is not anticipated to be significant in nature.
Due to the urbanised nature of the dockland area there may be the potential for disturbance to previously unrecorded areas of contaminated land arising from the ongoing industrial use of the area.
3.2 SITE SPECIFIC FLOOD MITIGATION
The following section outlines the potential environmental impact of each proposed flood mitigation option for the 9 proposed development sites. There are a number of impacts that will be common across each strategy associated with the construction period. It is anticipated that each of the below site specific solutions will have a slight temporary impact on air quality and noise levels throughout the construction period. The majority of this disruption is likely to be mitigated by the use of Best Practice Techniques and responsible working practices to minimise disruption to nearby sensitive receptors. In each case there may be the potential to encounter contaminated land due to the industrialised nature of the area.
3.3 CENTRAL STATION
3.3.1 It is anticipated that this site would benefit from the strategic flood mitigation option described in Section 3.1. 3.3.2 Key potential environmental aspects associated with the site specific mitigation may include an impact on the townscape of the area due to the raised ground levels. This may be especially prominent where the residential elements of the development are proposed. 3.3.3 Views of the site may also be impacted by the installation of a small wall along the site boundary with Western Esplanade which would be 320m long and approximately 4.1m AOD. Views may also be adversely impacted by the proposed option to raise Western Esplanade Road. 3.3.4 There may be the potential for disturbance to unknown archaeological remains as part of associated earthworks. However, significant impact is not anticipated due to the urbanised nature of the area and that much of the ground is likely to have been disturbed previously. 3.3.5 The Listed Wyndam Court building is in close proximity to the site and is located on Commercial Road. There may be a slight, temporary disruption to the setting of this building whilst construction works are carried out; however, this is not anticipated to be a significant impact.
3.4 MAJOR DEVELOPMENT QUARTER
3.4.1 The proposed development site located on the bank of the River Itchen would likely benefit from the strategic flood mitigation proposal. 3.4.2 Key potential environmental aspects associated with the proposed site specific mitigation proposals would primarily include the impact on the townscape of the immediate area. The raising of ground levels or potential installation of a defence buffer approximately 1.2-1.7m
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above the existing ground levels may impact on views from the site across the River Itchen. 3.4.3 There are no listed buildings or conservation areas within close proximity to the site, however there may be potential for the disturbance of currently unidentified archaeological remains associated with the designation of the area as one of archaeological potential. 3.4.4 There may also be the potential to impact on terrestrial ecology in the area arising from the construction of the ‘buffer’ along the Southern boundary. This may need to be approximately 50m wide in places to accommodate access and may subsequently increase the development footprint of the existing area and potential impact on adjacent green sites. 3.4.5 The mitigation strategy also suggests the possibility of creating some form of surface water attenuation. Consideration will need to be given to the aforementioned constraints in addition to how this surface water will be managed.
3.5 ROYAL PIER
3.5.1 The proposed development site would benefit from the city wide strategic mitigation strategy described in Section 3.1. 3.5.2 Site specific mitigation proposals may impact on the townscape of the site and surrounding area due to raised ground levels or construction of a flood defence along River Test frontage. However, as this area is already heavily industrialised no significant impact is anticipated. 3.5.3 There are a number of listed buildings within close proximity to the site. The installation of the proposed flood mitigation may result in a temporary impact on the views or setting of these buildings during the construction process, however this will only culminate in a short term impact. 3.5.4 The site is located close to the boundary of the Old Town North Conservation Area. Views of the conservation area may be impacted by the construction process and the final raised elevation levels of the site. However, this is not anticipated to create a significant impact. 3.5.5 The proposed development site is located within an area designated by SCC as having archaeological potential .There may be the potential to disturb currently unknown archaeological remains as part of the construction process. This is not thought to culminate in a significant impact due to the heavily urbanised nature of the area. Appropriate mitigation will be specified to minimise impacts.
3.6 FRUIT AND VEGETABLE MARKET
3.6.1 The Queensway and Orchard Place proposed development site is currently located in an area of ‘low’ to ‘medium’ risk of flooding over the next 100 years. The site is located just outside of the Old Town and Canute Conservation Areas. 3.6.2 Strategic mitigation proposals are not currently thought to bring significant benefit to this site; however, this will be reviewed on completion of the Erosion and Flood Risks Management Strategy. 3.6.3 Key potential environmental aspects associated with the proposed site specific mitigation of flood risk at this location may include the visual impact on the townscape of the area arising from the raised ground levels. The slight change in elevation may impact on the views of the area and those to the conservation area from the east.
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3.6.4 If significant earthworks are to be undertaken as part of this elevation change, there may be the potential for adverse impact on buried archaeological or palaeoenvironmental remains as part of the local council designation of the region as an area of archaeological potential.
3.7 TOWN DEPOT
3.7.1 As the proposed development site is located in close proximity to the frontage of the River Test, it is likely that it would significantly benefit from the city wide strategic flood mitigation proposal. 3.7.2 Key potential environmental aspects associated with the site specific mitigation strategy are anticipated to be a temporary townscape impact resulting from the construction process. There may be a slight impact on the townscape from the raised ground levels, however this is not thought to culminate in a significantly adverse impact. 3.7.3 The Royal Pier and Town Quay are Listed Structures. There is the potential impact on the setting of these structures from both the construction and operation of the flood mitigation. The raising of the ground level may impact on views of the designations from surrounding areas and depending on the final design, reduce the visual amenity of the structures and their curtilage. 3.7.4 The proposed development site is located on the boundary of the Old Town West Conservation Area. Views of the area from the waters edge may be slightly impacted by the proposed flood mitigation strategy in terms of the elevated ground levels.
3.8 MERIDIAN AND DRIVERS WHARF
3.8.1 The proposed development site is located on the banks of the River Itchen in close proximity to the Itchen Toll Bridge. Due to its location, this site would benefit from the implementation of the city wide strategic flood mitigation strategy. 3.8.2 Site specific mitigation strategies may have the potential to impact on the townscape of the site and surrounding area due to the elevation of ground levels to approximately 0.5m above existing ground levels. However, this is not anticipated to constitute a significant impact due to the urbanised nature of the area. 3.8.3 There are 2 listed building designations within close proximity to the proposed development site. The Marsh Hotel and Royal Albert Hotel are both located on Albert Road South. There may be the potential for a temporary impact on the setting and general character of these buildings associated with the elevation of ground levels for flood mitigation. It is not anticipated that there will be a permanent impact on the setting or character of these buildings associated with the proposed flood mitigation strategy. 3.8.4 There are no conservation areas within close proximity to the proposed development site.
3.9 LOWER HIGH STREET
3.9.1 It is anticipated that this proposed development site would benefit from the protection offered by the strategic flood mitigation strategy. 3.9.2 Key potential environmental aspects associated with the incorporation of flood mitigation strategies into this proposed development site focus upon the impact on the Central Bridge (a Listed Structure) located to the West of the site. The setting of this structure may be
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impacted on a temporary basis from the construction process. There may be a slight impact on the setting of designation arising from the elevation of ground levels to mitigate the impact of flood waters, however this is not thought to be significant. 3.9.3 Due to the fact that the proposed development site is located within an area of archaeological potential as designated by SCC, there may be the potential for the disturbance of undiscovered archaeological features associated with the designation.
3.10 COLLEGE STREET
3.10.1 It is anticipated that this site would benefit from the strategic flood mitigation proposals for the city. 3.10.2 The proposed development site is located close to the frontage of the River Test to the north of the Town Depo proposed development site. 3.10.3 Site specific mitigation proposals are outlined within Table 1 . In terms of commercial development on the site, it is not thought that the environmental impact would be significant. There may be a temporary impact on the townscape of the site and surrounding area during the construction process to raise the ground levels, however, it is not thought that that will culminate in a significant permanent impact as the proposed elevation is not considerable. 3.10.4 The raising of pedestrian walkways may have slightly larger impact in terms of townscape due to higher walkway levels and associated lighting etc. The lighting has the potential to impact on a wider area due to its relative elevation. 3.10.5 The proposed development site is also located within an area designated as having archaeological potential. For this reason, there is the potential to disturb undiscovered archaeological remains as part of associated earthworks.
3.11 SOLENT SKY AREA
3.11.1 Due to its proximity to the banks of the River Itchen, it is anticipated that this site would benefit from the city wide strategic mitigation strategy. 3.11.2 Site specific flood mitigation proposals are anticipated to potentially have a permanent impact on the townscape of the area due to elevated ground levels. There may also be a temporary impact during the construction process, although this is not anticipated to be significant. 3.11.3 As the proposed site is also located in an area of archaeological potential, there is the potential for the disturbance of undiscovered archaeological remains as part of the earth works to create a sloping or stepped site to reduce the impact from flood events.
3.12 SUMMARY
3.12.1 Table 3.1 below summarise the above analysis. Potential constraints are shown as a X and potential opportunities as a . Whilst Table 3.1 does not rank the Options, (insufficient detail regarding the Options is available at this time to allow this to be completed accurately), it is supported by text identifying for example when a double ‘X’ is a potential significant constraint or where mitigation maybe available to minimise potential likely significant impacts.
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Table 1: Potential Environmental Opportunities and Constraints Flood Soil: Air Archaeology and Fauna and Landscape/ Noise and Mitigation Population Contaminated Water: Quality1 Quality Built Heritage Flora Townscape Vibration Option land Strategic Flood mitigation X X / X / X – X X X Option – City Wide. Option 1 – X X / X X – X – X Central Station Option 2 – MDQ X X / X / X – X – X Option 3 – Royal X X / X X – X – X Pier Option 4 – Fruit X X / X X – X – X and Veg Market
Option 5 – Town X X / X X – X – X Depot Option 6 – Meridian and X X / X X – X – X Drivers Wharf Option 7 – Lower High X X / X X – X – X Street Option 8 – X X / X X – X – X College Street Option 9 – X X / X X – X – X Solent Sky Notes: X potential constraint which may be managed through mitigation; XX potential constraint which may be a showstopper and /or mitigation is potentially difficult (timely, costly, technically etc) to achieve; potential opportunity which may be easily achievable; potential opportunity which may be harder to achieve. – no potentially significant opportunities or constraints identified. 1 In this initial assessment, water focuses on water quality. Flood Risk is considered elsewhere in this document, and water resource is not considered relevant to this study. 2 Flood Risk is discussed in greater detail in the main report. This column briefly summarises that information.
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4. Conclusion
4.1.1 The Scene Setting and Analysis undertaken above highlight that there are a variety of features of interest in the receiving environment. Depending on the Option selected, different features will be affected in different ways, with constraints and potential opportunities arising under most Options. 4.1.2 Environmental features need therefore to be carefully considered through Option design, construction and operation to minimise potential adverse effects. Nevertheless, it is important to recognise that the receiving environment is capable of accepting change and that opportunities for enhancement should not be missed. 4.1.3 The assessment undertaken within this document was completed based on desk-top data. In particular, the following limitations and constraints where identified:
Lack of available desk-top information regarding water quality, contaminated land, geomorphology, groundwater, archaeology, noise and population; Lack of Phase 1 Habitat Survey data; Limited information regarding Trees / TPO; Lack of river species survey data; Lack of information regarding the presence/absence of invasive species; and No Envirocheck Report available.
4.2 RECOMMENDATIONS
4.2.1 Coupled with the limited information available at this stage on the Options proposed, the above limitations mean there is a clear requirement for further assessment to be undertaken to support the design of Options as they develop. It is recommended that a more detailed assessment of environmental impacts is undertaken as the Options progress, to support the selection of a preferred Option, and the design, construction and operation of that Option. 4.2.2 It is likely that whichever Option is taken forward, site surveys will be required (particularly in relation to flora and fauna, archaeology and landscape issues) as well as consultation with various bodies in order to inform the assessment and Option design. It is recommended that the information limitations identified in paragraph 4.1.3 above should be collected as far as possible to support the process (notably through surveys and via data requests).
4.3 MITIGATION
4.3.1 At this stage, it is considered likely that mitigation will be available to reduce the significance of the potential impacts identified. This mitigation is likely to include alterations in design, recommendations for best practice construction (such as set out in BRE Guidance and Environment Agency Pollution Prevention Guidance), and be informed through consultation with relevant parties (such as DDC officers, English Heritage, Environment Agency and local residents).
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Site Strategic Hard Site Specific Environment Considerations
Central Station – transport Would benefit from strategic Option 1 – implementation of flood resilient infrastructure e.g. platforms/control rooms etc. Landscape/Archaeology interchange, pedestrian mitigation Option 2 – small wall constructed along site boundary on Western Esplanade (320m long and 4.1m AOD) – will only need to be constructed links to the city with after 2070. potential residential/hotel Option 3 – raising Western Esplanade – may impact offsite vehicle accesses accommodation. Residential and hotel ground floor accesses should be positioned 300mm above surrounding road levels. If hotel is likely to have a lifespan of 100 years – ground floor levels would need to be raised to 1.1-1.3m above surrounding ground levels. Commercial development ground levels should be raised 300m above ground level. Major Development Would benefit from strategic Commercial development – floor levels should be 300mm above existing road levels. In the Southern section of the site, these should be raised Landscape/Archaeology Quarter – residential, mitigation to 500mm above the surrounding ground. office and leisure site with Residential: possible CHP plant. Option 1 – development should be located in low risk areas. Option2 – raising West Quay Road to 0.7m-1m to provide safe access. Surface water attenuation may be required. Option 3 – creating a defense line along Southern boundary done as a ‘buffer strip’/ public open space – would need to be 4.8AOD and require land raising of 1.2-1.7m with a width of approximately 20m (potentially 50m at roads). Hotel – 60 year lifespan – ground levels should be raised 300mm above car park levels - 100 years – guest sleeping on upper floors only. Royal Pier – retail, Would benefit from strategic Option 1 – on site flood defense along River Test frontage to 3.9 AOD – approx. 600m long and 1m above existing ground levels behind the Landscape/Archaeology/Surface Water heritage and residential mitigation defense. Finished floor levels should be 300mm above road and car parking levels. Management/Ecology areas. Option 2 – raise floor levels to 3.9 AOD and raise minor walkways by 2-300mm. Option 3 – raise floor levels to 3.4.AOD Residential Option 1 – combination of Option 1 above and options 2/3 for MDQ. Option 2 – ground floor levels should be raised to 4.3m AOD. Fruit and Veg Market – Limited benefit from strategic Raised ground levels to 300mm. Landscape/Archaeology – Stone residential, hotel and flood mitigation If the Hotel is to have a lifespan of more than 100 years – ground levels should be raised 0.5m above existing. Age/Medieval Town Walls. Site associated car parking surrounded by 4 conservation sites and number of listed buildings. Trees? Town Depo – leisure, Would benefit from strategic ‘Safe accesses through the site could be provided by raising the access route to 3.3 AOD whereas residential development would need to be Landscape/Archaeology office and residential mitigation – likely that strategic raised to 3.95AOD. facilities flood defense line would be Marine uses could be maintained if ground levels were raised to approximately 3.3AOD (0.8M above current ground levels). located within this site For commercial use, ground levels would need to be raised by 0.6-0.8m above existing ground levels but would still leave the development site exposed to a low flood risk. In order that the buildings remain completely ‘safe’ and ‘dry’ floor levels would need to be raised to 3.8m AOD. Residential options: a) Ground levels should be raised to 3.95AOD and finished floor levels to 4.5AOD to ensure property protection. b) Raise ground levels to 3.95 AOD and finished floor levels to 4.0AOD with residential property on the second floor only. c) Raise ground levels to 4.2AOD and finished floor levels to a level of 4.5AOD to allow for ground floor habitable rooms.
Meridian and Drivers Would benefit from strategic Commercial development would benefit from raised ground levels to 3.3mAOD whereas residential development would require raised ground Wharf – industrial mitigation levels of 3.95mAOD commercial and To provide access between Meridians Wharf and Drivers Wharf a raised promenade would need to be constructed at approximately 4.2mAOD. residential. Marine industrial users are expected to require continued usage of the River Itchen frontage and in order to maintain this it is anticipated that ground levels should be raised to approximately 3.3mAOD (0.5m) Commercial development would require ground levels to be raised to 3.5mAOD. . In order that the buildings remain completely ‘safe’ and ‘dry’ floor levels would need to be raised to 3.8m AOD Residential options: a) Ground levels should be raised to 3.95AOD and finished floor levels to 4.5AOD to ensure property protection. b) Raise ground levels to 3.95 AOD and finished floor levels to 4.0AOD with residential property on the second floor only. c) Raise ground levels to 4.2AOD and finished floor levels to a level of 4.5AOD to allow for ground floor habitable rooms.
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Lower High Street – Would benefit from strategic All development should be 300mm above normal ground levels. Landscape/Archaeology/Contamination residential and commercial mitigation Canute Palace/Quilters Vault Area – floor levels are to be raised 4.2 AOD. from flood water/temporary townscape units. Option 2 – raised to 3.9m AOD impact Option 3 0 allow flooding College Street – Would benefit from strategic Commercial Landscape/Archaeology commercial frontage and mitigation Option1 – finished floor level to 3mAOD which is approximately 300mm above normal flood levels. residential Option 2 – raise floor levels to 2.45 AOD – this would mean ground floor levels would flood to about 250mm during a floor event. accommodation. Off-site Safe Access Option 1- the Central Bridge leading onto the Itchen Bridge could be raised to approximately 4.1 AOD to provide a safe, dry access route Option 2 – raising land off site from the intersection of Threefield Lane and College Street by 350-400mm would provide ‘dry’ access for the life of the development. Option 3 – raising land off intersection by approximately 150-200mm would mean the access would have a limited flood depth of 250mm. Solent Sky – mixed use Would benefit from strategic Commercial development should be raised 300mm above normal road levels. Landscape/Archaeology redevelopment with mitigation Residential Musuem. Option 1 –can provide a safe basis for residential development. Where residential property is located, ground levels should be raised to 4.2m AOD. Finished floor levels should be approximately 300m above the 4.2AOD ground floor levels. Option 2 - creating a site that slopes or steps up from existing levels on Canute Rd. Raising ground levels to 3.95 AOD would provide safe access but leave development prone to flood levels of 250mm in the event of a flood. Finished floor levels of residential development should be located at 4.5mAOD (i.e. 300mm above flood levels), which would make them approximately 600mm above the surrounding site levels. Basement Car park is prone to flood risk, making a slope from the existing ground level up to 4.2AOD would prevent flood water entering the basement car park. Option 3 – providing commercial space on the ground floor would eliminate the need for extensive ground raising. Safe pedestrian routes should be raised to 3.95 AOD. If ground floor levels are only raised to 4.2 AOD they may still be subject to flooding – no residential units should be located on the ground floor in this instance.
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