Borough of Poole Flood Risk Management Strategy
Borough of Poole
January 2011 Final Report
9V4473
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Document title Borough of Poole Flood Risk Management Strategy
Document short title Borough of Poole SFRM Status Final Report Date January 2011 Project name Poole SFRM Project number 9V4473 Client Borough of Poole Reference 9V4473/R/303581/Exet
Drafted by Duncan Riches / Rachel Bird
Checked by Steve Edwards Date/initials check …………………. …………………. Approved by Steve Edwards Date/initials approval …………………. ………………….
SUMMARY
This study has been produced by Royal Haskoning for the Borough of Poole (BoP), in consultation with the Environment Agency (EA). It is a strategic review of flood risk to 100 years beyond the end of the forthcoming Core Strategy (2126). The purpose is to provide a robust and defendable long term framework to inform decisions on flood risk issues relating to BoP planning policy. In this way the continued development of key sites in the Borough can help in the approach of obtaining contributions towards future improved flood defences.
Flood defence measures that meet the policies determined by the recently completed Shoreline Management Plan (SMP) have been identified and costed in outline to inform this study. Raised defences have been identified, as these meet the SMP policies and continue current practice. A number of other types of measures are discussed, and it may be that the current comprehensive Environment Agency Poole and Wareham Flood and Coastal Erosion Risk Management Strategy recommends a different approach at some locations when it reports in 2 years time.
The aim of this strategy study is to determine a realistic and workable option for defending the area both now and in the future from tidal flooding. The measures assessed within this report may not be the optimum solution but do provide a sensible estimate of the potential costs of a flood defence scheme for Poole. This allows the Borough of Poole to start planning and securing finances to ensure the funding is in place when defences are required.
The study area, as shown in Figure 1.1, is defined by the Borough’s administrative boundary which intersects with the coast in the east near Canford Cliffs and the west, mid way about Poole Harbour in Lytchett Bay. The low lying nature of the frontage means that many of the wards of the Borough of Poole are subject to flood risk.
Recent modelling for the Borough of Poole Strategic Flood Risk Assessment (SFRA) Level 2 study, completed in July 2008, highlighted that there are currently 504 properties at risk from tidal flooding from a 1 in 200 year event in Poole and that this number will increase to over 4,000 in the future due to sea level rise.
In addition to the existing properties, flood risk needs to be considered in terms of new developments. The Borough of Poole’s emerging Core Strategy covers the period up to 2026. When considering future development within the Core Strategy the requirements of Planning Policy Statement 25: Development & Flood Risk (PPS25) must be met. To do this the Borough needs to ensure that any development highlighted in the Core Strategy will not be at excessive flood risk during its entire lifetime. For residential development this is assessed to be 100 years. This means that the effects of climate change on flood defence issues need to be considered, at least up to the year 2126.
Current guidance from Defra suggests that sea levels could rise by up to 1.26m by 2126. This means that by 2126 there could be 4,039 properties at risk of tidal flooding in Poole. The indicative standard of protection for coastal areas is 1 in 200 years, as specified in Defra guidance. For this study we have consequently established that significant flood risk management measures will be required to provide a 1 in 200 year standard of protection to the whole area in 2126.
Currently flood defences are present at strategically important locations, including Poole Town Quay and Hamworthy. These assets are less than ten years old and were
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constructed by the Environment Agency. Defences at other locations around the harbour are a mixture of private and defacto defences (e.g. railway embankments and slipways).
Based on the sea level rise predictions this study has developed measures for the management of flood risk over the next 116 years. The study area has been split into discrete cells to determine the potential measures required to protect that particular cell. It does not look into the effects of fluvial or surface water flooding although the potential impact of these is considered in the development of measures.
An outline assessment of the cost of the proposed works was undertaken using information from recent completed schemes and the Environment Agency unit cost database as well as our own databases and engineering judgement. The total cost of the proposed flood risk management infrastructure for the entire study area equates to a capital cost of approximately £158 million.
An economic assessment was undertaken based on the number of properties at risk and average property damage values, using information from the Multicoloured Manual (MCM). This only focuses on the damage to the actual property and not indirect impacts on the area, such as loss of tourism. This is particularly relevant in this area and should be highlighted as part of a funding application.
In line with Defra and HM Treasury rules, the economic assessment only considers the existing land use for the area. Potential damages to planned developments are not taken into account.
Results indicate that over the lifetime of the Borough of Poole’s Core Strategy, present value benefits for a scheme protecting up to a 200 year flood event amounts to approximately £157 million in 2010. This value then increases significantly if the work is undertaken at a later date, rising to £1,128 million for a scheme constructed in 2126. This significant increase is a result of the predicted rise in sea level due to climate change, as both the quantity of houses affected and the depth of flooding is greater. This provides an overall benefit cost ratio of 1 currently, rising to 7.2 for a scheme in 2126. This suggests that in the future a scheme for the whole area would be economically viable.
This FRM strategy demonstrates that although considerable additional flood defence measures are required to provide a 1 in 200 year standard of protection in 2126, there is a compelling economic argument to do so. This means that development can be permitted and regeneration continue in the town centre despite the threat of sea level rise and that as sea levels continue to rise measures can be put in place as required.
The table overleaf provides details of the benefit cost ratio for potential flood defence schemes split across the different cells for five epochs. This helps to highlight that it is viable to undertake work in some cells sooner than others and therefore this can be a tool for prioritising the works required.
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Year the scheme is undertaken Cell 2010 2035 2060 2086 2126 1 0.3 0.8 1.8 3.4 5.4 2 1.5 2.5 4.0 6.0 7.9 3 2.1 4.6 10.6 20.1 30.0 4 1.0 1.9 3.7 6.1 8.7 5 0.8 1.5 3.0 5.0 6.9 6 0.3 0.5 0.9 1.5 2.1 All cells 1.0 1.8 3.2 5.2 7.2 Red indicates that cost is greater than potential benefit, orange indicates that the benefits outweigh the cost but that Defra funding is unlikely, whilst green highlights where it may be possible to obtain Defra funding for the works. Note – Defra funding opinion is based on current guidance.
Based on purely economic viability cell 3 looks to be the most appropriate area to focus initial work, although there are other factors that also influence the timing of when a scheme is undertaken. This includes the condition of the existing defences and the potential for regeneration in the area. Regeneration provides the possibility of obtaining developer contributions for the required works and therefore reducing the impact on the public purse. The potential to obtain such contributions may well bring the scheme to the fore.
No large redevelopment areas are proposed within cell 3 and therefore most of the funding would have to be from public sources, therefore it would not receive flood defence Gant in Aid (FDGiA) until well after 2035. Cells 2 and 4 both have large areas of proposed regeneration as well as being viable in terms of their benefit cost ratio.
Following the assessment undertaken for this study, we recommend that the main focus of any funding opportunities should be directed towards cells 2 and 4 as, in light of the potential for significant financial contributions, these are thought to provide the best value for money and will allow the continuing development of Poole town centre. Due to the low benefit cost ratio in cells 1 and 6, the planning policy for these areas should be considered to determine the best approach for future land use management in these areas, as flood risk will increase significantly, but without much possibility of attracting funding.
More specific recommendations have been split into three groupings: Understanding current risk: ‹ Undertake a structural condition survey of key flood defence assets, to better determine their residual life and potential to breach.
‹ Annual inspection of the assets should be undertaken. This will help determine when replacements are needed and highlight any urgent works.
‹ Undertake a Surface Water Management Plan (SWMP) to understand and start to address surface water flooding issues. This is particularly relevant following the Flood and Water Management Act which has removed the right to connect to an existing drainage network. A strategic approach is therefore needed for dealing with surface water issues.
‹ Engage with Wessex Water to explore how the potential flood defence measures and sea level rise impact on their facilities and operations.
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‹ Confirm the onset of flooding across the area taking into account both tidal levels and wave conditions where appropriate. This may require additional modelling work looking at both the current and future scenarios for various return period events.
Development of Potential Measures: ‹ Prior to the construction of the elements discussed in this report there would need to be a full appraisal carried out in co-ordination with the Environment Agency and other stakeholders.
‹ The issue of safe access and egress from these areas has been discussed and agreed in detail with the Environment Agency at a strategic level for the study area
‹ Consult with major land owners and employers in the area e.g. RNLI, Sunseeker etc, to determine whether or not they need to be included or excluded from any defence works, and what level of contribution would be appropriate.
‹ Maintain and develop the East side of Holes Bay as the principal route into and out of Poole Town.
‹ The railway currently acts as a low lying flood corridor in places. Options should be considered to address this issue, providing additional protection to the surrounding area where possible.
‹ Undertake a more detailed economic assessment focussing on cells 2, 3 and 4 as these are the most economically viable. This will involve a detailed threshold survey to enable a more accurate assessment of potential benefits using the more robust depth/damage approach.
‹ The visual issues of increased wall heights should be considered. Land raising of discrete land parcels, as part of a development, will be beneficial in the long term in terms of reducing both tidal and surface water flood risk. This will require safe access and egress routes to areas of high ground. In areas where land raising is not appropriate then use of undercroft parking could be utilised to ensure that the finished floor level of any more vulnerable land uses is raised above the predicted flood level.
Consider the wider issues for the area: ‹ All new development should meet the development protocol being agreed by the Borough of Poole and the Environment Agency. These will be based on the current Defra guidance until updated guidance is released.
‹ Undertake an SEA and/or Habitats Regulations Assessment (HRA) to consider the potential impacts on the SSSI, SAC, RAMSAR and AONB areas of the site.
‹ Monitor sea levels to aid the estimation of future sea level rise. This can then be used to revise the estimates for wall heights and the required finished floor levels of developments.
‹ Sandbanks is an area of land that plays a large role in the physical stability of the shoreline in the area. It protects the harbour area from wave action, maintains the transport links and helps maintain the vessel approach channels by keeping the ebb and flood velocities high. The impacts of this area of land should be thoroughly investigated when considering how this area should be managed in the future.
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CONTENTS
Page
GLOSSARY ix
LOCATION PLAN xii
1 INTRODUCTION 1 1.1 The Borough of Poole Flood Risk Management Strategy Study 1 1.2 Objectives 2 1.3 Existing Information 2 1.4 Methodology 3 1.5 Study deliverables 3
2 BACKGROUND INFORMATION 4 2.1 Poole Harbour 4 2.2 Study Area and Character 4 2.3 Transport Infrastructure 6 2.4 Notable Employers 7 2.5 Flood Risk Mapping 7 2.6 Flood Defence and Coast Protection 7 2.7 Local Development Framework, Development Control and the Strategic Flood Risk Assessment 10 2.8 Shoreline Management Plan Review (Draft) 11 2.9 Archaeology and Environmental Designations 12
3 STRATEGY DRIVERS 14 3.1 Planning Policy and Aspirations 14 3.2 Asset Management 14 3.3 Physical Environment 14 3.4 Summary of Strategy Drivers and Application 15
4 DATA COLLATION AND REVIEW 16 4.1 Regeneration 16 4.2 SMP Review Draft Policy for Poole Harbour 16 4.3 Existing Flood Defence Level and Residual Life 17 4.4 Freeboard and Wave Climate 19 4.5 Still Water Level (SWL) 22 4.6 Wave Climate 22 4.7 Sea Level Rise 22 4.8 Ground Levels 25 4.9 Flood Risk Mapping – fluvial and tidal 25 4.10 Surface Water and Ground Water Flooding 25
5 SPATIAL STRUCTURE AND UNIT DEPENDENCE 28 5.1 Determine cells and sub-cells 28 5.2 Links to SMP2 policies 31
6 PROPOSED MEASURES 32 6.1 Outline of Grouping of Strategy Measures 32 6.2 Determine and Summarise Proposed Strategy Options 32
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7 OTHER OPTIONS AND MEASURES REQUIRED FOR STRATEGY 38 7.1 Access / Egress 38 7.2 Coast Protection 38 7.3 Alternative Options 39 7.4 Coastal squeeze 40
8 OPTION COSTS 41 8.1 Asset database 41 8.2 Unit cost calculations 41 8.3 Options Costed 42 8.4 Asset Cost Inclusions and Exclusions 43 8.5 Calculated Cost (including optimism bias) 44 8.6 Possible contributions 45
9 ECONOMIC ASSESSMENT 47 9.1 Benefits Methodology 47 9.2 Assumptions 48 9.3 Benefits Results 49 9.4 Benefit / Cost assessment 50
10 PHASING 52
11 CONCLUSIONS 55 11.1 Approach 55 11.2 Findings 55 11.3 Other considerations 56
12 RECOMMENDATIONS 58
APPENDICES
APPENDIX A Draft SMP Policy Map APPENDIX B Freeboard Calculations APPENDIX C Flood Cell Additional Information APPENDIX D Proposed Defence Location Figures APPENDIX E Unit Cost Information APPENDIX F Indication of areas within Poole that will be below HAT at various horizons
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GLOSSARY
AAD Average Annual Damage is the average damage in pounds per year that would occur in a designated area from flooding over a very long period of time ABD Areas Benefiting from Defences AEP Annual Exceedance Probability. The estimated probability of a flood of given magnitude occurring or being exceeded in any year. AONB Area of Outstanding Natural Beauty is a precious landscape whose distinctive character and natural beauty are so outstanding that it is in the nation's interest to safeguard them. BoP Borough of Poole Catchment The area contributing surface water flow to a point on a drainage or river system (the area drained by that river, including areas away from the watercourse network). Can be divided into sub-catchments. CIL Community Infrastructure Levy Core Strategy A key compulsory Local Development Document specified in United Kingdom planning law. Every other Local Development Document is built on the principles it sets out, regarding the development and use of land in a Local Planning Authority's area. The principles should be in accordance with the Community strategy. DEM Digital Elevation Model Design Event A historic or notional flood event of a given annual flood probability, against which the suitability of a proposed development is assessed and mitigation measures, if any, are designed. DTM Digital Terrain Model EA Flood Zone 1 Low Probability of flooding EA Flood Zone 2 Medium Probability of flooding. Probability of fluvial flooding is 0.1 – 1% and probability of tidal flooding is 0.1 – 0.5% EA Flood Zone High Probability of Flooding. Probability of fluvial flooding is 1% (1 in 100 3a years) or greater and probability of tidal flooding is 0.5% (1 in 200 years) or greater. EA Flood Zone Functional floodplain. 3b Environment Non-departmental public body responsible for the delivery of government Agency (EA) policy relating to the environment and flood risk management in England and Wales. FAS Flood Alleviation Scheme FEH Flood Estimation Handbook. The Environment Agency approved method of estimating flood flows in the UK. Flood Defence A structure (or system of structures) for the alleviation of flooding from rivers or the sea to a specified design standard. Flood Estimation The Environment Agency approved method of estimating flood flows in the Handbook UK. Flood Risk The level of flood risk is the product of the frequency or likelihood of the flood events and their consequences (such as loss, damage, harm, distress and disruption). Flood Risk Considerations of the flood risks inherent in a project, leading to the Assessment development of actions to control, mitigate or accept them. Floodplain Any area of land over which water flows or is stored during a flood event, or would flow but for the presence of flood defences. Fluvial Pertaining to a watercourse (river or stream).
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Freeboard The difference between the design flood level and the lowest point on the flood defence. GIS Geographical Information System. A computer-based system for capturing, storing, checking, integrating, manipulating, analysing and displaying data that are spatially referenced. Groundwater Water occurring below ground in natural formations (typically rocks, gravels and sand). HAT Highest Astronomical Tide Hazard A situation with the potential to result in harm. A hazard does not necessarily lead to harm. Hydraulic model A computerised model of a watercourse and floodplain to simulate water flows in rivers to estimate water levels and flood extents. iSIS One-dimensional hydraulic modelling software. LiDAR Light Detection And Ranging - The Environment Agency’s digital terrain mapping data Listed buildings A listed building in the United Kingdom is a building which has been placed on the Statutory List of Buildings of Special Architectural or Historic Interest. It is a widely used status, applied to around half a million buildings. Localism A new Government initiative to move away from governmental decisions Agenda being taken at one central point. Instead they are dispersed to representative councils that have been elected by the people locally Main River Watercourses defined on a ‘Main River Map’ designated by DEFRA. The Environment Agency has permissive powers to carry out flood defence works, maintenance and operational activities for Main Rivers only. mAOD / mODN Metres Above Ordnance Datum. Elevations use Ordnance Datum Newlyn. MHWS Mean High Water Spring NFCDD National Flood & Coastal Defence Database. Environment Agency asset management system database. PPS25 Planning Policy Statement 25; ‘Development and Flood Risk’. Probability The likelihood of an event occurring. QMED Mean annual maximum flood, roughly equivalent to a 1 in 2.3 year flood event RAMSAR Ramsar sites are wetlands of international importance, designated under the Ramsar Convention. Residual Flood The remaining flood risk after risk reduction measures have been taken into Risk account. Return Period The average time period between rainfall or flood events with the same intensity and effect. RSS Regional Spatial Strategy SAC Special Area of Conservation and are defined in the European Unions Habitat Directive. They are to protect 220 habitats and approximately 1000 species. Shoreline A large-scale assessment of the risks associated with coastal processes and Management helps reduce these risks to people and the developed, historic and natural Plan (SMP) environments. Coastal processes include tidal patterns, wave height, wave direction and the movement of beach and seabed materials. SSSI Site of Special Scientific Interest which is a conservation designation denoting a protected area in the UK. SSSIs are the basic building block of site-based nature conservation legislation SLR Sea Level Rise. Standard of The level of flood that a defence is designed to protect against before it is protection exceeded.
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Strategic Flood In England and Wales, SFRAs are a required part of the local planning Risk process, as set out in Planning Policy Statement 25, produced by the Assessment Department for Communities and Local Government. SFRAs are primarily (SFRA) produced by local planning authorities, in consultation with the Environment Agency, and are intended to "form the basis for preparing appropriate policies for flood risk management" at the local level. At the regional level, the equivalent document is the Regional Flood Risk Appraisal and on smaller scales these SFRAs are used to inform site-specific Flood Risk Assessments (FRAs) for individual planning applications Surface Run-off Water flowing over the ground surface to the drainage system. This occurs if the ground is impermeable, is saturated or if rainfall is particularly intense. Sustainable A sequence of management practices and control structures designed to Drainage drain surface water in a more sustainable fashion than some conventional Systems (SuDS) techniques. Time to peak The time from the centroid of the total rainfall to the peak of the run-off hydrograph, i.e. the length of time it takes to convert rain into river flow. Topography The shape and form of the land, in terms of hills, steepness of slopes, or flat land. TUFLOW A 2-dimensional hydraulic modelling software package designed to model overland flow Water Level Water Level Management Plans (WLMPs) are required for all areas which Management have a conservation interest and where the control of water is important for Plans (WLMPs) the maintenance or rehabilitation of that interest.
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Figure 1.1: Location Plan showing study extent
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1 INTRODUCTION
1.1 The Borough of Poole Flood Risk Management Strategy Study
The Borough of Poole is the Unitary Authority for the historic coastal town of Poole which has local centres including Hamworthy and Creekmoor. The Borough is adjoined to the east by Bournemouth Borough Council and the west by Purbeck District Council.
The Authority’s coastal frontage is varied, including exposed coast, natural harbour, seaport and historic quay.
Commercial and urban development is present about the Northern and Eastern margins of Poole Harbour and many areas are at an appreciable present day risk of tidal flooding, including Hamworthy and Poole town centre.
Flood defences are present at strategically important locations, including Poole Town Quay and Hamworthy. These assets are both less than ten years old and were promoted by the Environment Agency. Defences at other locations about the harbour are a mixture of private and defacto defences (e.g. railway embankments and slipways). Forecast Sea Level Rise (SLR) will to increase the level of flood risk over the next century.
The Level 1 and Level 2 Strategic Flood Risk Assessments (SFRA) for the area, published between 2007 and 2009, highlighted the present day flood risk and determined that additional measures are required to enable future development at an acceptable risk of flooding, particularly in light of the anticipated rise in sea level.
The regeneration of areas of Poole Town and Hamworthy are an important element of the Boroughs Local Development Framework Core Strategy and whilst the existing flood defences provide some flood risk benefits, the existing schemes do not provide the protection required to enable development with a life expectancy of up to 100 years. The Core Strategy has a target of being delivered by 2026; whereas a strategy is required for flood risk management running to the year 2126 i.e. 100 years after the end the Core Strategy to account for the lifetime of residential developments. The areas affected by the Borough’s Core Strategy are detailed in Figure 1.1 and Figure 1.2.
The Environment Agency has primary responsibility for managing the existing flood risk although it is likely that the development and promotion of any flood alleviation scheme in this area will require a co-ordinated approach from all parties i.e. EA, BoP, Developers, Wessex Water and other stakeholders. Developer contributions from Section 106 agreements and Community Infrastructure Levy (CIL) contributions can be used to put finance towards the required works to meet the requirements of Planning Policy Statement 25 (PPS25), Development and Flood Risk, although it is expected that a degree of central or external funding will also be required.
This report sets out a Flood Risk Management Strategy for coastal flooding in the Borough of Poole to adopt to enable strategic development that is compliant with management policy from the Shoreline Management Plan (SMP) and PPS25, with a planning horizon of 116 years from the present day.
The study area is shown on Figure 1.1 and is defined as the areas at risk of coastal flooding within the Borough’s administrative boundary.
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Twin Sails Warehouse and Industrial Units
Power Station Site
Poole Town New ASDA Port of Quay Poole
Figure 1.2: Flood Risk in Poole Town Centre (BoP, 2009)
Figure 1.2 was taken directly from the SFRA and therefore developments that have been constructed recently may not be included on the map.
The project scope makes no requirement to consider the environment in establishing the strategy; however opportunities or limitations have been highlighted. It should be noted that detailed consideration of potential environmental and landscape impacts would need to be undertaken as part of the development of designs for potential flood defence improvement works.
Inland the River Stour bounds the northern limits of the Borough; however, relative to tidal problems at the coast the fluvial risk is less significant and is not considered as part of this study. Surface water problems are also not considered further by this report.
1.2 Objectives
The objectives of this study are to determine the flood risk management infrastructure required to provide a 1 in 200 year standard of protection for the study area in 2126, the associated costs of the required defences along with an assessment of when the defences will be required.
1.3 Existing Information
The Flood Risk Management Strategy for the Borough of Poole progresses the requirements and findings of three principal documents in relation to tidal flood risk:
• The Borough of Poole adopted Core Strategy, 2009 • The Level 1 and Level 2 Strategic Flood Risk Assessments, 2008
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The development of options and assessment was guided further by:
• The South West Report of Regional Extreme Tide Level, 2003 • Planning Policy Statement 25 and Practice Guide, March 2010 & December 2009 • Areas Benefiting from Defences Mapping, 2008 • Shoreline Management Plan, 1999 (SMP) • Shoreline Management Plan Review, Draft 2009 (SMP2)
1.4 Methodology
The principal stages are: • Gather Background Data • Establish Strategy Drivers • Collate and Review Strategy Data and additional data required • Develop Spatial Framework • Develop Strategic Options • Calculate Scheme Costs • Economic Assessment • Phasing • Discussion • Conclusions
1.5 Study deliverables
The deliverables include:
• plans of discrete flood risk units with an attributed Flood Risk Management Strategy • costed programme for implementation • identification of the required flood defence measures • assessment of the changing economic viability of defences with climate change
The high level cost estimates presented by this report will enable the Borough of Poole to begin to establish the flood risk element of the intended Community Infrastructure Levy from potential developers.
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2 BACKGROUND INFORMATION
2.1 Poole Harbour
Poole Harbour is a large (40km2), shallow enclosed estuary with double sandy spits forming its mouth. It was formed by a breach in the barrier beaches created by the last Holocene sea level rise and acts as the estuary of the rivers Frome and Piddle.
The harbour has lagoon-like characteristics due to the double high water tidal regime. The main channel of the estuary runs along the more developed northern side of the harbour, whereas the southern and western side is more natural, comprising extensive mudflats and salt marsh. Other notable features include a number of islands that characterise the western limits of the Harbour as well the large bays to the North and West that are reached by narrow channels cut into the perimeter of the Harbour.
The Harbour’s outer, exposed frontage running between Sandbanks and Canford Cliffs faces south east and is largely protected from the prevailing south westerly storms by the Isle of Purbeck and St Albans Head. Locally Handfast Point controls the accumulation of sediment at the west end of Poole Bay.
At a range of 1.6m the tidal regime for Poole is classed as microtidal. The mean tidal prism (the difference in volume of water for high and low tides in the harbour) has been calculated by Halcrow (2004) as being 30,400m3, however on a spring tide this can rise to over 45,800m3 dramatically increasing the velocities through the main Harbour entrance, as well as the channels to Lytchett Bay and Holes Bay. Velocities in the harbour entrance are noted in reports to reach and exceed 3m/s (ABP, 2009).
Tide levels of interest include:
Description Level (mODN) Level (mCD) Highest Recorded (17/12/89) 1.66 3.06 Highest Astronomical Tide (HAT) 1.40 2.8 Mean High Water Spring (MHWS) 0.80 2.2 Mean Tide Level (MTL) 0.20 1.6 Mean Low Water Springs (MLWS) -0.80 0.6 Chart Datum (mCD) -1.40 0 Lowest record level (20/02/96) -1.98 -0.58 Table 2.1: Tide Levels of Poole Harbour
Inland the coastal area of Poole Harbour is generally low lying and includes areas of land reclamation.
2.2 Study Area and Character
The study area as shown in Figure 1.1 is defined by the Borough’s administrative boundary which intersects with the coast in the east near Canford Cliffs and the west, mid way about Poole Harbour in Lytchett Bay. The low lying nature of the frontage means that many of the wards of the Borough of Poole are subject to flood risk, as shown in Figure 2.1. Note that this image has been taken from the SFRA report and therefore recent developments may not be included in the mapping.
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A35 Holes Bay Poole Town
Poole Park Waterloo / Weymouth Mainline
Penn Hill
Lytchett Bay
New ASDA Canford Cliffs Hamworthy Bridge
Shore Road Port of Poole
Legend: Brownsea Sanbanks Chain
……. Flood Zone 3 Island Ferry and Harbour ……. Flood Zone 2 Entrance
Figure 2.1: Borough of Poole Flood Risk – Zone 2 and 3 (BoP Level 1 SFRA, HR Wallingford, July 2009)
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Figure 2.1 shows the present day flood zones, the ward boundaries and other features of interest. Note also the density of housing at coastal margins and the number of properties with private marine frontage. The risk areas mapped are for 1 in 200 year and 1 in 1000 year flood events in red and yellow respectively. The Still Water Levels (SWL) for these events are 1.95mODN and 2.12mODN respectively.
The geology of the area is predominantly soft (sands, silts and clays), ranging from the sand spit of Sandbanks to the soft cliffs of Ham Common and Canford Cliffs, as well as the low lying margins of the harbour perimeter. Historically active and subject to natural processes these features have become less mobile as a result of the development of roads, marinas and agriculture.
With the exception of higher ground at Ham Common, Penn Hill and Canford Cliffs the coastal zone generally lies between 1.4mODN and 2.6mODN.
2.3 Transport Infrastructure
Poole is serviced by the Waterloo to Weymouth railway line. In addition to Poole Station, there are local stations at Hamworthy, Parkstone and Branksome which provide essential commuter links to Poole and Bournemouth as well as regional and national links to Southampton and London. In terms of local processes the line is relatively important, providing a “fixed” point in the frontage at Poole Park, crossing Holes Bay and bridging the entrance to Lytchett Bay. The route of the line from Poole Station forms a physical barrier, dividing parts of the town centre from the old town at a relatively low level.
The road network within the Borough is well developed. Trunk routes include the A35 which provides links towards Dorchester (west), the A3049 which bypasses the major urban areas of Poole towards Bournemouth (east) and the A350 which links to the centre of Poole, the local bus station and Poole Rail Station (south).
The A35 skirts the northern flanks of Lytchett Bay and Holes Bay, whilst the Holes Bay roundabout (A35/A3049 and A350) sits in a relatively low area at the North West of Holes bay. The A350 forms the eastern edge of Holes Bay.
Poole is serviced by the Port of Poole which manages a variety of goods, including bulk materials and RoRo traffic. In addition ferry services are available to France and the Channel Islands.
For Hamworthy there are two further transport links of note; a rail spur linking the Port of Poole to the main line at Hamworthy Station, and the Blandford Road (B3068) which runs north east from Hamworthy to the A35 at Upton.
There are two notable crossings, the most important being Poole Bridge, linking Poole Town to Hamworthy at the entrance to Holes Bay. A chain ferry also operates at the entrance to the Poole Harbour, linking Sandbanks to Studland.
Construction is underway to build a new bridge in Poole Harbour to link Poole Town and Hamworthy, which will be known as Twin Sails Bridge. Twin Sails Bridge is expected to be completed by the end of 2012. The location of Poole Bridge and proposed Twin Sails Bridge are noted on Figure 1.2.
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2.4 Notable Employers
The relatively benign conditions of Poole Harbour are an asset and the accessible frontage has attracted a variety of specialist operators and employers including: • Poole Harbour Commissioners (wider responsibility for the harbour and Port of Poole operator) • RNLI (frontage on Holes Bay) • Sunseeker (frontage opposite Poole Town Quay and Holes Bay) • Royal Marines (frontage between Hamworthy Park and Rockley Park) • Poole to Cherbourg Ferry • Condor Ferry to St Malo
This highlights the importance of the harbour to the local economy and the need to maintain the operating capacity. Whilst each will require an ongoing waterfront access the details are too specific for a study of this scale to consider further.
2.5 Flood Risk Mapping
Flood Maps are published by the Environment Agency and for coastal flooding these identify risk areas for two return period events, the 1 in 200 year tidal (Flood Zone 3) and 1 in 1000 year (Flood Zone 2). The present day mapping for Poole Harbour is based on a Royal Haskoning mapping exercise completed in November 2007, based on the 2002 levels, and the resulting flood zones are shown on Figure 2.2. The project used a 2D model and digital terrain model (DTM) of Poole to map the areas at risk from a Still Water Level (SWL) of 1.95mODN (0.5%) and 2.12mODN (0.1%) with no formal defences present. The methodology was repeated for 100 years of Sea Level Rise (SLR), i.e. 2102.
In 2008 the Tidal Flood Zone 3 extents were further divided into areas of residual risk and Areas Benefiting from Defences (ABD). Maps of this detail were only produced for the present day 0.5% event. In Poole this mapping was completed by Royal Haskoning and included location specific wave overtopping models.
Noting the wider requirement for the Authority to understand future flood risk as part of the Core Strategy, the Borough commissioned Royal Haskoning to repeat the Flood Zone Mapping exercise for 2126 Flood Zones.
2.6 Flood Defence and Coast Protection
The frontages of Poole Harbour are varied. The exposed coast is characterised by a nourished beach, controlled by a system of rock groynes between Sandbanks and Canford Cliffs.
Within the harbour there are a variety of frontages ranging from natural saltmarsh (Holes Bay and Lytchett Bay) and soft cliff (Rockley Park) through to reveted areas of open amenity parkland (Hamworthy Park, Baiter Park, Whitecliff Recreation Ground) and formal quays (Poole Town Quay, Port of Poole, Cobb Quay and the old Power Station Site).
In addition there are marinas protected by rock breakwater (Poole Yacht Club, Poole Town Quay, Salterns Marina and Sandbanks Marina) as well a variety of minor private frontages.
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Figure 2.2: Borough of Poole Current and Future Flood Risk
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2.6.1 Coast Protection and Beach Management
The Sandbanks frontage provides protection for the Sandbanks Peninsular and acts as an important amenity for the residents and visitors alike. Beach material has historically been controlled by a system of groynes and beach recharge. The southern system of timber groynes was recently replaced with 9 principal rock groynes. The beach was also recharged at the same time with arisings from a maintenance dredge of Poole Harbour in recent years. It is estimated that this would need to be repeated roughly every 10 years to maintain the beaches at a sufficient width to protect the sea walls and prevent recession of the soft cliffs along this section of cost.
To better understand the beach system, and to establish a strategy for its long term maintenance the Borough commissioned an HR Wallingford Study. Published in August 2008 the report (HR Wallingford EX5763, 2008) recommended that a further 6 rock groynes be constructed between Shore Road and Canford Cliffs, with a likely requirement for recharge at 10 year intervals.
2.6.2 Existing and Future Defences - Summary
Whilst significant areas of Poole are subject to flood risk, the properties that are subject to greatest risk are defended. Raised defences are present along Poole Town Quay, Green Gardens and Hamworthy Park. See Table 2.2 for details. Note that these schemes are connected in places and also work in conjunction with the Poole Harbour Commissioners Breakwater Scheme. When they were designed they had a standard of protection of 1 in 200 years.
Scheme Location When completed? Poole Town Quay Sea Defence Poole Quay – near Poole April 2001 (Phase 1) Pottery Poole Town Quay West Poole Quay April 2004 Poole Quay Sea Defences Poole Quay April 2004 (Phases 1 & 2) Green Gardens Sea Defence Near the Quay breakwater 2007 Table 2.2: Existing Flood Defence Schemes
Other pockets of property remain at risk from flooding across the Borough. The viability of defending these remaining properties has been investigated and detailed by the Poole Harbour Pre-Feasibility Report (Atkins/PBA, January 2009) and the Holes Bay Pre-Feasibility Report (Atkins/PBA, 2008). The areas investigated by the studies are shown in Table 2.3, together with an assessment of their economic viability. None of the areas stated as being at flood risk are viable and hence are not considered further here.
Average No of Present Annual Cost benefit Viable Area Properties Day SoP Damages ratio Scheme (1 in 300yr) (AAD) 0 (due to Marshes 4 lack of flap £836 N/A No End valve) Falconer 0 1 in 50 yr £73 N/A No Drive
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Average No of Present Annual Cost benefit Viable Area Properties Day SoP Damages ratio Scheme (1 in 300yr) (AAD) Woodlands 0 1 in 100 yr £0 N/A No Avenue Lower Not Defra 51 1 in 5 yr £679,000 ~1 Hamworthy funded Not Defra West Quay 83 1 in 5 yr £532,000 <1 funded Blue Lagoon 9 1 in 200 yr £231 N/A No Luscombe 2 1 in 100 yr £122 N/A No Valley Shore Road 22 QMED £79,738 <1 No Panorama 0 1 in 300 yr £85 N/A No Road Table 2.3: Poole Pre Feasibility Study Details
2.7 Local Development Framework, Development Control and the Strategic Flood Risk Assessment
The Borough of Poole is the responsible planning authority and produces a range of statutory planning documents including the Local Development Framework (LDF), which will replace the Poole Local Plan First Alteration (adopted in March 2004).
To implement the LDF the Borough of Poole has developed and adopted a Core Strategy (adopted 2009).
2.7.1 Strategic Flood Risk Assessment
To support PPS25, Planning Authorities complete a Strategic Flood Risk Assessment (SFRA) to inform the spatial plans and applications for specific developments. The Borough of Poole Level 1 SFRA (HR Wallingford, July 2007) drew together the available flood risk, flood defence and historic flood event information for the whole Borough, highlighting to the Authority the principal areas at risk.
The Level 2 study (HR Wallingford, July 2008) went on to model flood risk for areas at the focus of the Core Strategy and produced a variety of hazard and vulnerability products for a limited study area, for the present day, 2086 and 2126. The study area subject to the sequential test is detailed in Figure 2.1.
Noting the increase in flood risk and the intended Core Strategy regeneration areas the Level 2 SFRA draws a number of conclusions and makes a number of specific recommendations for the regeneration areas at risk of tidal flooding.
Present Day Risk • Breaching of existing defences is not considered a significant risk • The onset of flooding in the Power Station Site occurs at a 1 in 20 year event with extensive flooding from the 1 in 1000 year event. • Extensive flooding in West Quay Road site will occur at a 1 in 20 year event.
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Development and Future Flood Risk • For the sites within the flood risk areas in the future, basements should not be used for habitation; commercial applications require a level for 1 in 200 year event + SLR + 300mm. Note this allowance of 300mm has been determined by the Environment Agency. • Undercroft parking could be justified to provide habitable accommodation over. • New developments will require a freeboard of up to 600mm (above the present day 1 in 200 year level) • Defences will need to be raised by SLR; 0.7m by 2086 and 1.3m by 2126 • New defences require a freeboard allowance of approximately 200mm • Both Regeneration Areas require defences • Defences are required outside of the regeneration areas to prevent spreading from adjoining areas • Surface and foul pumping arrangements should consider the implications of SLR
2.7.2 Development Management
The Development Management planning requirements have been agreed between the Environment Agency and the Borough of Poole. The residual flood risk to Poole is that risk remaining from either overtopping or breach of defences. Together with the delivery of the FRMS the residual flood risk should remain constant under current climate change scenarios, though this may change with emerging evidence.
The following options, or combination of options, can be used to reduce residual flood risk in Poole:
• Raise finished floor levels • Build in resistance measures (barriers/stop boards etc.) and structural soundness, against flooding loadings to new buildings • Build in resilience measures – raise electrics, water resilient plaster and flooring etc • Lower risk uses of ground floors – offices, shops • Town centre flood warning/evacuation plan.
2.8 Shoreline Management Plan Review (Draft)
Since 2008 the Shoreline Management Plan (SMP) for Poole and Christchurch Bays has been subject to wholesale review. The first Shoreline Management Plan (SMP1) was published in 1999 and identified policies over a single 50 year horizon, the SMP review (SMP2) identifies policies over three epochs covering a 100year horizon: 2005 to 2025, 2025 to 2055 and 2055 to 2105.
The Poole and Christchurch Bay SMP2 is at a draft stage with the policies subject to consultation with the Operating Authorities, Interest Organisation and the general public. The final document will be published in 2010.
Prior to proposing the Policy Option for each Policy Unit the SMP Review assesses two strategic options: 1. No active intervention 2. Present Management Scenario
Having considered the possible impact of each Strategic Option over the three epochs on a variety of features the SMP Review recommended one of 4 management options:
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• No active intervention (NAI): a decision not to invest in providing or maintaining defences or natural coastline. • Hold the line (HTL): maintain or upgrade the level of protection provided by defences or natural coastline. • Managed realignment (MR): manage the coastal processes to realign the ‘natural’ coastline configuration, either seaward or landward, in order to create a future sustainable shoreline position. • Advance the line (ATL): build new defences seaward of the existing defence line where significant land reclamation is considered.
The policies for the Borough of Poole are discussed in section 4.2.
With regard to morphological development the Draft SMP2 makes a number of points regarding Sandbanks, Poole Harbour and the harbour mouth, these can be summarised as: • The Bournemouth and Poole Bay frontage is already subject to considerable coastal squeeze. Limited realignment will not achieve a more sustainable position. • Coastal squeeze in Poole Harbour is likely to result in a lowering of the foreshore, but this will largely depend on the availability of sediments for deposition. • Sandbanks spit is a low lying feature (4-6mODN in places) and its continued presence is reliant upon the seaward sandy foreshore being nourished and stabilised by seawalls and groynes. Its exposure is limited by the ebb tide delta feature at the mouth of the harbour. • The Studland spit is a natural accumulation of material protected by Handfast Point and the harbour mouth training works, underpinned by the chain ferry slipway. • Rising sea levels will increase erosion pressure on the open coast frontage • Rising sea levels may increase the tidal prism of Poole Harbour, putting pressure on the Sandbanks and Studland spits. • The development of a new harbour entrance would be sustained by natural processes if it occurred on the Sandbanks spit, rather than Studland where accretion takes place more readily. The processes could result in dramatic changes to the location of sand banks from outside, to inside the harbour dramatically increasing the exposure of the open coastline.
2.9 Archaeology and Environmental Designations
In addition to the listed buildings shown in the SMP Review, the following features and locations were identified as having a high archaeological value: • Poole Old Town • Lower Hamworthy (Roman military site and port) • The Northern Heathland • Upper Hamworthy (Rockley Sands, Turlin Moor and Upton Park • The shores and bodies of Poole Harbour including Lytchett, Holes and Parkstone Bays • The Poole Bay Littoral • The Roman Road and its environment.
The Borough of Poole, and Poole Harbour in particular, are subject to a variety of National and European designations. Whilst not considered in this study, an overview map (Figure 2.3) is provided below for context.
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Figure 2.3: Listed Buildings and Environmental Designations in the Borough of Poole Borough of Poole SFRM 9V4473/R/303581/Exet Final Report - 13 - January 2011
3 STRATEGY DRIVERS
The principal requirement of the Flood Risk Management Strategy Study is to determine the cost and phasing of specific measures that will enable the Borough of Poole’s development aspirations whilst satisfying the requirements of the Planning Policy Statement 25: Development of Flood Risk (PPS25) and other relevant management policies for the period 2010 to 2126.
Drivers for strategy fall into three categories: • Planning • Asset Management • Physical Environment The individual drivers are identified in the following sections in bold and summarised in section 3.4.
3.1 Planning Policy and Aspirations
To ensure the ongoing vitality of the Borough the Strategy will determine the assets required across all coastal frontages to enable developments and regeneration. However, the priority for the Borough is the Core Strategy regeneration sites which are subject to a delivery timetable and consequently act as a specific driver to the Strategy’s development.
The application of PPS25 is determined by the Environment Agency Development Control team and discussed in greater detail in Section 2.7.2.
Planning at the coast is subject to the agreed Shoreline Management Plan (SMP) which, having considered a great variety of factors, sets out a Strategic Policy for discrete units of coast.
3.2 Asset Management
Flood Defence and Coast Protection Assets alleviate the risk of flooding. One of the principal values of interest to this study is the existing defence level, as it controls the onset of flooding, particularly from an event Still Water Level (SWL).
In addition to defending against a SWL, defences must also have a provision for overtopping as a result of uncertainty in determining an event SWL, wave overtopping, settlement and degradation. These four elements of flood risk management are described as freeboard.
For longer term planning a better understanding is required of an asset’s structural condition. For this study structural life has been assessed and will be important in developing the asset replacement programme.
3.3 Physical Environment
The coastal environment is subject to large transfers of energy; the movement of water, driven by waves, tides and storm surge shape the environment and cause flooding.
The Borough of Poole has a variety of frontages including exposed coast, natural harbour, adapted harbour, seaport and sheltered creeks; the finished levels (defended or ground) will control the onset and nature of flooding (hazard from flows, ponding etc) from either an elevated still water level (SWL) or wave overtopping from a storm event
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(wave climate). A change in the magnitude or frequency of either will alter the accepted standard of protection. The resulting risk associated with present day and future events is mapped to produce a variety of flood risk maps that help spatial planners and others manage flood risk.
The sensitivity of a frontage to either still water events or wave action is largely dependent upon exposure; exposed frontages with deeper water and greater fetch will be susceptible to larger waves, whilst sheltered, shallower environments with a restricted tidal range such as Poole Harbour, will be more sensitive to elevated still water levels. Fetch and water depth limit the development of larger waves.
Sea Level Rise (SLR) is the general term given to the upward trend in mean sea level resulting from a combination of local or regional geological movements and climate change and is expressed as a rate of change.
Where defences are not present the topography of the coast will control the onset of flooding, for this study these levels are referred to as ground level; the same level is also important to understand the height of defences present or required.
3.4 Summary of Strategy Drivers and Application
The drivers will affect three principal elements of the Strategy: • Strategy Measure (type of defence or management action) • Cost of Strategy Measure • Phasing of Strategy Measure
Table 3.1 below, sets out links between the strategy drivers and strategy elements and notes whether or not the driver could be expected to change significantly over the study period, 2010 – 2126.
Significant Change in Driver Strategy Element period 2010 / 2126 Regeneration Measure / Phasing Yes SMP policy Measure / Phasing No Existing Defence Level Cost / Phasing No Structural Life Phasing Yes Freeboard Cost No SWL Cost Yes Wave Climate Measure Yes SLR Cost / Phasing Yes Ground levels Measure / Cost No Flood Risk Areas Cost Yes Table 3.1: Strategy Option Drivers - Summary
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4 DATA COLLATION AND REVIEW
The collation of data for each strategy driver is detailed in the following sub sections.
4.1 Regeneration
The Regeneration driver is based on the requirements of the Borough of Poole Core Strategy, which states that the Town Centre is the principal focus of change and that the intention is to increase the mix of land use and associated densities. The principal development areas are detailed in Figure 4.1.
Figure 4.1: Intended Regeneration Areas (BoP, February 2009)
Important dates for the development of the strategy are:
• 2002 – baseline year for 1 in 200 year tide level • 2012 – period start for Community Infrastructure Levy (CIL) contributions • 2026 – Core Strategy Plan Period • 2086 – end of commercial development design life (60 year design life) • 2126 – end of residential development design life (100 year design life)
The character of the regeneration is important to the Core Strategy. Water is particularly important and vertical quayside is anticipated for the West Quay Road and Power Station Sites.
4.2 SMP Review Draft Policy for Poole Harbour
The draft policies for Poole from the draft SMP2 are shown in Table 4.1. A supporting map is included in Appendix A. Further information about the Shoreline Management Plan is detailed in section 2.8.
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Area 2025 2055 2105 Branksome Chine to HTL HTL HTL/ATL Sandbanks (PUs: G4, H1) Sandbanks inner beach to Poole Town Quay (PUs: HTL HTL HTL H2, H3, I1, I2) Holes Bay (PUs: I3) HTL HTL HTL Holes Bay – Upton House NAI NAI NAI (PU:I3a) Port of Poole to Ham HTL HTL HTL Common (PUs: I4, I5) Ham Common and Rockley MR MR NAI Sands (PU: J1) Lytchett Bay (PU: J2) NAI NAI MR Lytchett Bay – Turlin Moor MR HTL HTL (PU: J2a) Table 4.1: Draft SMP Review Policies for Borough of Poole (RH, 2009)
In general the policies set out by the Draft SMP Review require this Strategy to propose management options that maintain the status quo. Only at limited sections of coastline (Rockley Sands, Lytchett Bay and Holes Bay) where there is “space” between the present day frontage and existing permanent property and infrastructure does the SMP make the case for realignment, either through natural or managed processes.
The SMP Review details the importance of Sandbanks to maintaining the present workings of Poole Harbour. Whilst ceasing efforts to manage beach foreshore is unlikely to result in the total loss of Sandbanks and its material at a process level the impact on the present day infrastructure would be dramatic, as would the development of a second or alternative entrance nearer to Canford Cliffs. The ongoing security of Sandbanks as a feature is therefore important, if only at a process level.
4.3 Existing Flood Defence Level and Residual Life
The Borough’s long coastal/tidal frontage includes a considerable number of assets (approximately 26km) maintained and owned by a variety of private owners as well as the Environment Agency and the Borough of Poole. A full assessment was not included in the scope of this study; instead data has been gathered from the Environment Agency’s National Flood and Coastal Defence Database (NFCDD) as well as from staff at the Borough of Poole. This information is shown on Figure 4.2.
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Figure 4.2: Existing Defences
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The Environment Agency’s National Flood and Coastal Defence Database for the Borough of Poole shows frontage data for approximately 90 assets. This study selected data from the following fields:
• Reference • Type • Maintainer • Level • Spatial data
In general the data was found to be inconsistent with important elements of data missing, for example: • Only 16 of the 90 assets had an effective defence level • No assets had a value for toe level i.e. base of defence • Approximately only half of the asset records had a construction date – many apparently based on decadal estimates • Some of the spatial data was poor with a number of “straight line” assets, i.e. represented as a straight line between asset start and end points.
The Residual Structural Life is an assessment of the predicted remaining life of the asset, taking into account the structural condition i.e. an indication of when the risk of the asset collapsing becomes unacceptable. This therefore depends on the material used and construction techniques. Data regarding the residual life of the asset was not available from the NFCDD dataset, it was also not comprehensively available from the Borough of Poole for coast protection assets.
In order that the variable quality of data could be treated as consistently as possible, all existing assets were assigned to one of five bands. These bands were based on a subjective on-site assessment of structural life and/or comments made in the draft SMP. Allocation to one of the bands reflects a requirement to carry out capital works within the following time frames:
• 2015 (i.e. immediately) • 2026 • 2086 • 2126
Figure 4.3 shows the bands that were assigned to the existing defences in the study area. Where new/renewal of assets are proposed as part of the strategy they are considered to have a 60 year design life.
The Effective Defence Level has been measured from a variety of sources, including NFCDD, LiDAR topographic data and previous investigations. Given the variety of sources used the defence levels quoted in the study should be assumed to be accurate to no more than 200mm.
4.4 Freeboard and Wave Climate
Freeboard has been calculated from adding design still water level uncertainty, wave height, settlement and degradation. Still Water Level (SWL) Uncertainty The design SWL as stated in section 4.5 has an uncertainty band of ±0.2m.
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Figure 4.3: Residual life of existing defences 9V4473/R/303581/Exet Borough of Poole SFRM January 2011 - 20 - Final Report
Wave Overtopping The aim of the wave overtopping allowance is to reduce rates and/or volumes to an acceptable value. Whether a volume is acceptable will depend on how sensitive a structure or risk area is, e.g. a publicly accessible quay in an urban location is likely to be more sensitive to overtopping than a remote revetment for reasons of public safety, whilst a bank is more vulnerable than a revetment.
Based on a table of “critical overtopping discharges and volumes” (Allsop et al, 2005) the following assumptions of acceptable overtopping rates are made.
Vertical Walls (inc quays) – are largely in publicly accessible areas with relatively high exposure: 0.01 m3/s/m Banks and Revetments – are more susceptible to damage from overtopping but are in less accessible areas: 0.05 m3/s/m
Values for the required freeboard were calculated for three example frontages, including a plain vertical wall, a simple armour slope and an embankment using one of two methods; Eurotop Manual / W178 or AMAZON (RH software), for both “sensitive” and “non sensitive” locations. Details of the calculations are included in Appendix B and the results included below as Table 4.2. In general these calculations are reasonably conservative, walls and banks are assumed to have no foreshore and that the incident waves are normal to the structure in question.
Minimum freeboard required (m) Critical Non- Overtopping Overtopping overtopping Meaning sensitive sensitive rate 3 Year Year Year Year m /s/m 2026 2126 2026 2126 Unsafe for driving at low Plain vertical 0.01 speed, unsafe for trained 0.8 0.8 0.7 0.7 wall staff
Revetment seawalls start Simple 0.05 to get damage, unsafe for 0.3 0.3 0.1 0.1 amour slope driving at low speed
Embankment seawalls get Embankment 0.05 damage even if fully 1.1 1.1 0.7 0.7 protected Table 4.2: Wave Freeboard Calculation
The totals for overtopping for sensitive locations range between 0.1m and 1.1m depending upon location. For non-sensitive locations the range is between 0.1m and 0.7mOD.
Settlement Settlement is location specific and no information is available on which to make a judgement. However, for inclusion a value of 0.05m will be applied to all locations.
Degradation Degradation is assumed not to be a concern, the perimeter of the harbour is not grazed within the study area, should the grazing areas be required the study assumes that fencing would be installed to limit the chance of damage.
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Summary
This strategy study is a high level assessment. Uncertainty (±200mm), Settlement (50mm) and Degradation (0mm) are fixed values, whilst critical overtopping rates will vary depending on fetch, aspect and defence type (0.1 – 1.1m); a total of between 0.350mm and 1.350m if all elements are coincident.
Given the variety of locations in the study and an assessment of the relative sensitivities of each structure and risk areas the study will assume a total freeboard requirement above the SWL of 0.7m: • locations with a greater sensitivity are generally protected by hard, vertical structures (Poole Town Quay to RNLI, Hamworthy Park) • locations defended by banks are generally more sheltered with open areas landward, or lower density housing (Lytchett Bay and Holes Bay)
This is thought to provide a sensible representation of the freeboard allowance required for the whole study area. When any of the works are in the design process then a more detailed assessment will be required for that specific defence in terms of the freeboard required. This will be particularly relevant in the more exposed areas.
4.5 Still Water Level (SWL)
In 2003 Royal Haskoning published a regional Extreme Tide Level report for the South West peninsular. The work was commissioned by the Environment Agency and the levels derived have been accepted as a regionally consistent source for the recent and future mapping and strategic development projects throughout the Region.
The levels were based on the year 2002, and for Poole the 1 in 200 year (or 0.5% probability) event was estimated as 1.95mODN. A subsequent review of the dataset (RH, 2008) reduced the 1 in 200 year value for Poole Harbour to 1.84mODN; the confidence limits remained unchanged at 200mm.
For the purposes of this report and for consistency with the SFRA and Draft SMP a 1 in 200 year level of 1.95mODN has been used.
4.6 Wave Climate
The ABP Mer Estuary Processes Assessment, conducted as part of the SMP2 indicates that the most energetic parts of Poole Harbour are the Northern and Eastern parts, where extreme wave heights will vary between 0.5m and 1.2m for a 1in100year event, largely dependant upon wave fetch.
The wave climate was also accessed by the “Areas Benefiting from Defences” mapping exercise which used a significant wave height of 0.7m, with a period of 6s for mapping overtopping of defences. These values have been used for the freeboard calculation.
4.7 Sea Level Rise
Over the last 150 years an increasingly comprehensive and accurate record has been kept of wave climates and still water levels; these show that changes are happening to the natural environment and that further change can be expected in the future.
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Regardless of the cause of these changes, they must be anticipated in order that appropriate plans can be put in place.
It is reasonable to recognise that sea levels are rising when estimating future extreme coastal water levels. Relative sea levels along the south coast of England are known to have been rising for millennia, and there is no evidence to suggest that this will stop. Monitoring over the last 20 years has shown a global annual rate of sea level rise of 3.4mm which is twice as fast as the previous 90 years. There is also a broad consensus amongst scientists who observe and model ocean dynamics that global sea level rise will accelerate. Moreover, we are required by Defra to account for rising sea levels.
Increased sea level rise is one of the more certain effects of climate change, because the physical explanation for it is quite simple. Rising atmospheric temperatures (regardless of their cause) gradually heat the oceans. Any substance that becomes warmer expands. Because the oceans are so deep (the Atlantic is around 3.3 km deep on average) even a small increase in volume causes a significant increase in sea level (e.g. 1% increase in volume would derive a rise of around 33 metres). In addition warmer atmospheres melt glaciers and ice caps, currently on land, which then flow into the sea further raising its level.
Figure 4.4 below is taken from “The Copenhagen Diagnosis, 2009: Updating the World on the Latest Climate Science”. This is a summary document prepared in the run up to the Copenhagen Climate Conference of 2009, it shows how global mean sea levels have been rising, and possible future increases. Guidance has not yet been released as to how these estimates can be used and therefore the current Government Guidance regarding sea level rise is provided in Planning Policy Statement 25 (PPS25): Development and Flood Risk. The values for the South West are shown in Table 4.3. To see how the UKCP09 predictions compare with the Defra predictions see Figure 4.5. This shows that in the short term the predictions are relatively similar.
Administrative Net Sea Level Rise (mm/yr) Relative to 1990 Region 1990 to 2025 to 2055 to 2085 to 2025 2055 2085 2115 South West 3.5 8.0 11.5 14.5 Source: Table B.1 Planning Policy Statement 25: Development and Flood Risk Table 4.3: Predicted sea level rise per year in the South West
Figure 4.4: Past and future sea-level projections (Copenhagen, 2009)
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Example Sea Level Rise Data - South West
1.400 UKCP A1F1 - 95 1.200 UKCO A1F1 - Mid UKCP A1F1 - 5 Defra 1.000
0.800
0.600
0.400 Sea Level Rise (m)
0.200
0.000 1980 2000 2020 2040 2060 2080 2100 2120 2140 -0.200 Date
Figure 4.5: Comparison of UKCP09 and Defra predictions
The predicted water levels for this area have been taken from the Environment Agency South West Extreme Tide Level Report, produced in 2003 by Posford Haskoning. These are shown in Table 4.4 below. Based on the current sea level rise guidance as shown in Table 4.3 above, the predicted extreme tide levels for 2010 and selected years up to 2126 have been calculated and are also shown below. Note that neither Defra nor the advisory body UKCIP provide for SLR guidance beyond 2115, so for consistency with the SFRA and other flood risk mapping studies this Strategy will take forward the 2115 rate of SLR to 2126.
Poole only has a small tidal range and therefore based on the current predictions the increases due to sea level rise mean that extreme events today will soon become much more common. For example the current 1 in 200 year event will be equivalent to a 1 in 50 year event in 2035, a 1 in 10 year event by 2060, a 1 in 1 year event by 2086 and less than MHWS by 2126. Note that this does not account for any wave action, which will also increase as incident waves will be less ‘depth limited’.
Return Predicted water levels (mOD) for various horizons Period (yrs) 2002 2010 2035 2060 2086 2110 2126 MHWS 0.8 0.83 0.96 1.18 1.48 1.83 2.06 HAT 1.2 1.23 1.36 1.58 1.88 2.23 2.46 1 1.39 1.42 1.55 1.77 2.07 2.42 2.65 5 1.56 1.59 1.72 1.94 2.24 2.59 2.82 10 1.63 1.66 1.79 2.01 2.31 2.66 2.89 25 1.73 1.76 1.89 2.11 2.41 2.76 2.99 50 1.8 1.83 1.96 2.18 2.48 2.83 3.06 100 1.88 1.91 2.04 2.26 2.56 2.91 3.14 200 1.95 1.98 2.11 2.33 2.63 2.98 3.21 1000 2.12 2.15 2.28 2.50 2.80 3.15 3.38 Table 4.4: Predicted future Extreme Tide Levels for Poole
As a result of the effects of climate change over the next 116 years, tidal flood risk to Poole is expected to increase owing to a predicted rise of mean sea level of 1.26m. This will put approximately 4000 properties at risk compared to approximately 500 under current conditions.
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4.8 Ground Levels
The study area benefits from the availability of LiDAR data. Gathered in 2007 the mapping has a 1m horizontal resolution with a quoted vertical accuracy of ±0.400m.
Review of 2006 LiDAR data by Royal Haskoning for the Flood Zones report (RH, 2008) found that at a number of sample locations the average difference between the LiDAR data and ground surveys was -0.070m, the maximum difference was ±0.150m. At other locations along the coast the 2006 LiDAR data was found to be generally accurate to 0.100m.
For the purposes of this report the LiDAR levels will be assumed to be accurate to within ±0.200m. Figure 4.6 shows the LIDAR provided for the area.
4.9 Flood Risk Mapping – fluvial and tidal
Table 4.5 below summarises the mapping available, notes the suitability of coverage and details the periods that the extents are applicable to.
The most appropriate mapping for use in this study is highlighted in bold.
Borough Published Title FZ 3 (0.5%) Deliverables Wide Date Coverage Tidal Flood Zones 2002 @ 1.95mODN Compliance Main RH, Nov 2007 V V 2102 @ 2.86mOD Stage Wessex tidal 2002 @ 1.95mODN areas benefiting RH,Oct 2008 V V
from defences HR Flood 2002 @ 1.95mODN Modelling to HR, May 2008 V 2086 @ 2.65mOD U Support Poole 2126 @ 3.25mOD SFRA Level 2 Future Tidal Flood Zone RH, Oct 2008 V 2126 @ 3.21mOD V Modelling in Poole Table 4.5: Summary of available flood risk mapping
4.10 Surface Water and Ground Water Flooding
Along with fluvial and tidal flooding, surface water and ground water flooding also need to be considered, both now and in the future. Detailed modelling of the surface water and ground water flood risk has not been undertaken although a review has been made of historic incidents of flooding and the Environment Agency Areas Susceptible to Surface Water Flooding (AStSWF) Maps have been considered. The topography of the area has also been assessed to highlight areas which may be at risk in the future due to sea level rise.
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Figure 4.6: Topography
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The figures in Appendix F highlight the areas below HAT at various horizons over the 116 years this study is focussing on. This therefore represents the areas that could be affected by both surface water flooding and a higher ground water table. By 2026 approximately 120 hectares will be below the predicted HAT level, whilst by 2126 this value increases to approximately 350 hectares, highlighting the need to also consider the drainage network and ground levels in this area.
Pumping is already undertaken in parts of the study area. With sea level rise tide locking will occur more often and for longer, therefore the pumping requirement will need to increase. This is a direct impact of climate change rather than any flood defence options in the area. The Borough of Poole will need to coordinate with Wessex Water regarding their current and further pumping requirements.
The AStSWF maps were procured as a preliminary national output to provide Local Resilience Forums (LRFs) with an initial indication of areas that may be susceptible to surface water flooding. The maps highlight areas that range from less to more susceptible to surface water flooding. Note that the guidance associated with the AStSWF maps stated that due to the assumptions made and the modelling undertaken the information should not be viewed at a scale less than 1:50,000. These are therefore designed to be used as a guide rather than highlight individual properties and roads at risk.
There are areas classified as intermediate susceptibility across most sub-cells, excluding 1C, 4B, although in some subcells the areas at intermediate risk are small isolated pockets. Areas classified as more susceptible to surface water flooding include Nuffield Industrial Estate (3A), across A3049 into Stanley Green (3A), sewage works (3B), north east of the railway in the school playing fields (4A), Sandbanks Road (5A) and in isolated areas adjacent to the railway (5A). Due to the pumping requirements and the known tide locking issue, more investigation is recommended into the surface water flooding issues both now and in the future. This could be undertaken by the Borough of Poole in the form of a Surface Water Management Plan (SWMP) in partnership with the Environment Agency and Wessex Water.
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5 SPATIAL STRUCTURE AND UNIT DEPENDENCE
5.1 Determine cells and sub-cells
To enable the development of the Strategy a spatial structure was developed to manage and compare the findings. Based on a review of the present and future Coastal Flood Zone maps, the area was divided into 6 cells and 25 sub cells representing discrete elements of the current and future risk extents, based roughly on the SMP coastal zones.
Below are some general descriptions and details of the cells and sub-cells. The location of the sub-cells, along with the flood risk within each sub-cell is shown on Figure 5.1. More information on the sub-cells can be found in Appendix C.
5.1.1 Cell 1
This cell has been split up into four areas: • 1a - Upton / Allens Lane Industrial area; • 1b - Turlin Moor East; • 1c - Turlin Moor West; and • 1d - Rockley Sands.
Generally the cell covers residential areas, although there is also a sewage pumping station, school, sailing school and caravan park within the cell boundary. Currently there are no defences across the majority of this area, just a natural earth bank frontage, except within sub-cell 1d which is protected by cliffs and a sea wall.
5.1.2 Cell 2
This cell has been split up into five areas: • 2a - Ham Common; • 2b - Lower Hamworthy / Hamworthy Park; • 2c - Port of Poole; • 2d - Former Power Station Regeneration Site; and • 2e - Holes Bay (west).
Generally this cell covers a residential area along with industrial areas, e.g. the Port and the Power Station site, a caravan park, marine base, residential marina and recreational area. Sub-cell 2d covers one of the proposed major development sites within the Borough of Poole. There are walls in places protecting sub-cells 2a, b, c and d although these are not all formal flood defences. Some are just quay walls. The frontage of sub- cell 2e is a salt marsh.
5.1.3 Cell 3
This cell has been split up into three areas: • 3a – Stanley Green; • 3b – Creekmoor; and • 3c – Holes Bay (north) This is generally an industrial area with some residential housing. It includes Pergins Island and Upton House and Country Park (3c). There are no existing defences around cell 3.
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5.1.4 Cell 4
This cell has been split into five sub-cells: • 4a – Sterte; • 4b – West Quay Road Regeneration site; • 4c – Poole Old Town West; • 4d – Poole Old Town East; and • 4e – Baiter Park & Poole Park.
This cell covers a mix of uses including light industry, commercial buildings, residential housing as well as schools, a stadium and a fire station. There are also recreational areas including a boating lake and cricket ground. In particular sub-cell 4c is the historical centre of the town.
Existing defences exist in the form of sections of rock revetment, flood walls and quay walls.
5.1.5 Cell 5
This cell is split into two sub-cells: • 5a – Whitecliff; and • 5b – Blue Lagoon frontage.
This area is a residential with recreational facilities including a sailing club and park. It includes the Blue Lagoon. There are sections of rock and concrete revetments as well as various privately owned walls within this cell. There is also a wall across the lagoon.
5.1.6 Cell 6
This cell is split into six sub-cells: • 6a – Luscombe Valley; • 6b – Sandbanks; • 6c – Panorama Bay; • 6d – Old Coastguard Road; • 6e – North Haven Point; and • 6f – Sandbanks coastal frontage.
This is primarily a residential area, with a golf course, nature reserve, and marina. It includes the harbour entrance and the Brownsea Island ferry terminal. There are sections of concrete walls and private defences / walls, along with groynes. Beach recharge is also undertaken within sub-cell 6f.
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Figure 5.1: Strategy Study Cells and Sub Cells
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5.2 Links to SMP2 policies
The cells and sub-cells are tabulated below (Table 5.1). The sub cells do not fit exactly with the SMP Policy Units as flood risk areas do not necessarily follow the changes in management policy required by the SMP Units. The table below therefore provides guidance on the overall policy for each sub-cell.
SMP 2 – units and draft policies Sub Cell Name / Area Policy To To To Cell Unit 2025 2055 2105 1 A Upton J.2a NAI NAI MR 1 B Turlin Moor (east) J.2a NAI NAI MR 1 C Turlin Moor (west) J.2a NAI NAI MR 1 D Rockley Park J.2 NAI NAI MR 2 A Ham Common J.2 / J.1 MR MR NAI 2 B Hamworthy Park J.1 / I.5 HTL HTL HTL 2 C Port of Poole I.4 HTL HTL HTL 2 D Power Station Regeneration Site I.4 / I.3 HTL HTL HTL 2 E Holes Bay (west) I.3 HTL HTL HTL 3 A Stanley Green I.3 HTL HTL HTL 3 B Creekmoor I.3 HTL HTL HTL 3 C Holes Bay (north) I.3a HTL HTL HTL 4 A Sterte I.3 HTL HTL HTL West Quay Road Regeneration 4 B I.3 / I.2 HTL HTL HTL Site 4 C Poole Town Quay (west) I.2 HTL HTL HTL 4 D Poole Town Quay (east) I.2 HTL HTL HTL 4 E Baiter and Poole Park I.2 HTL HTL HTL 5 A Whitecliff I.1 HTL HTL HTL 5 B Blue Lagoon I.1 HTL HTL HTL 6 A Luscombe Valley I.1 / H.3 HTL HTL HTL 6 B Whitely Lake H.3 HTL HTL HTL 6 C Panorama Bay H.2 HTL HTL HTL 6 D Old Coastguard Road H.2 HTL HTL HTL 6 E North haven Point H.1 HTL HTL HTL 6 F Sandbanks G.4 HTL HTL ATL Table 5.1: Strategy Study Cells and Sub Cells
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6 PROPOSED MEASURES
6.1 Outline of Grouping of Strategy Measures
Taking the SMP policy option as a lead, this report considers the means by which the policy option could be developed to a scheme by proposing that five options be considered:
• Relocate – wholesale spatial transfer of all elements of commerce, industry, transport and housing stock to an area of reduced flood risk. • Advance / Reclaim – infill and reclaim areas to a design level. • Raise – Raise areas at risk to the required design level. Both defending and reducing the impact of SLR on surface water discharge and other utilities. • Defend – Maintain and/or renew existing defences to reinforce/raise the present flood defence boundaries either by way of hard or soft defences. To be constructed locally (e.g. hard or soft walls) or at an area level (e.g. a barrier). • Adapt – the use of an area should be altered to better suit an increasing risk of tidal inundation. The extent to which this option can be taken will be dependent on agreeing what level of vulnerability is acceptable; i.e. annual flooding to 300mm would not be acceptable to a residential development, but may be compatible with some port operations, and would almost certainly be compatible with an appropriately developed nature reserve.
Table 6.1 below shows the relationship between the proposed FRM Strategy Options and the SMP2 Draft Policies.
FRM Strategy SMP Policy Option Nature of Likely Works Option Maintain, construct or reconstruct defences in Defend order that the existing flood defence alignment Hold The Line is maintained. (HTL) Raise ground levels to avoid the need to Raise defend. Alter usage of an area to activities that are Managed Adapt less vulnerable to flooding, possibly within Realignment (MR) existing boundaries.
Demolition of existing urban features with Managed development at a suitably low risk brown or Realignment (MR) Relocate green field site. Existing area to be used for (cont) alternative land uses e.g. biodiversity No Active Stop existing operational and maintenance Intervention Do Nothing activities. May require initial health and safety (NAI) works to eliminate safety hazards Reclaim an area of foreshore or infill an Advance The Line Advance existing harbour to enable development or (ATL) other Strategic Option such as Relocation. Table 6.1: FRM Strategy Option and SMP Compatibility
6.2 Determine and Summarise Proposed Strategy Options
The decision on whether to undertake flood defence works in the long term is based on SMP policy for the area. We have therefore used the SMP policy decision within each
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sub cell and determined the most efficient measure for achieving the required outcome (such as Hold The Line and Managed Retreat).
Table 6.2 below details what the proposed strategy is within each sub-cell for the short and long term to deliver the SMP policy.
Cell Short term strategy (2026) Long term strategy (2126) Consider current SoP of Turlin Moor Adapt or implement scheme for climate 1a industrial estate and implement scheme if change as required (linked to area outside viable(linked to area outside BoP) of BoP) Implement a scheme to defend school at 1b No action required (no assets at risk) Turlin Moor Implement a scheme to defend western 1c No action required (no assets at risk) end of Turlin Moor Consider need to defend sailing school. Adapt for climate change (or accept 1d Maintain existing frontage change in SoP) Adapt frontage for climate change 2a MOD to maintain frontage as required considering potential links with 2b Evaluate effectiveness of existing Hamworthy Park Scheme (comments re Adapt scheme for climate change as 2b completeness at western end of scheme). required (linked to 2c) Monitor and maintain scheme to continue SoP Consider scheme to defend industrial units Adapt and extend scheme to defend 2c to north of area and vunerability/resiliance entire area (note links to cells 2b and 2d of waters edge business to south of cell Implement scheme to defend entire Consider minor works to prevent flooding frontage form climate change water level 2d to north of regeneration site rise which will threaten regeneration area (linked to 2c) Implement scheme to defend residential Consider flood risk at Falconer Drive and 2e properties (much wider area than short implement scheme if required term) against climate change No active intervention (no assets at risk). 3a No active intervention (no assets at risk) Accept change in SoP Investigate current flood risk and Adapt scheme or implement scheme to 3b implement scheme to defend if viable adapt for climate change Investigate current flood risk and Adapt scheme or implement scheme to 3c implement scheme to defend if viable adapt for climate change Monitor frontage (maintain any existing Implement scheme to defend Sterte to 4a assets) address climate change Maintain defences and adapt for climate Improve SoP to defend existing assets change (if required since new defences 4b and release land for development likely to be installed to final levels). Potential link to cells 4c & 4d Maintain recently constructed defences to Adapt for climate change (linked to cell 4c provide continued SoP 4d) Maintain recently constructed defences to Adapt for climate change (linked to cell 4d provide continued SoP 4c) Maintain frontage through Baitor Park Adapt for climate change to defence 4e (property not currently at flood risk) houses beyond Baitor Park
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Cell Short term strategy (2026) Long term strategy (2126) Adapt for climate change including Consider (need for) improvements to 5a significant lengths of new / improved defend boatyard defence in whitecliff harbourside park Improve Frontage to provide 1 in 200 SoP Adapt (and significantly extend around 5b (mainly around marina) blue lagoon) for climate change 6a Improve Frontage to provide 1 in 200 SoP Adapt for climate change 6b Improve Frontage to provide 1 in 200 SoP Adapt for climate change EA/BoP frontage adapt for climate change Riparian owners to maintain private (link to private frontage) Private frontage - 6c frontage. EA/BoP frontage to be Encourage improvements and climate maintained as existing change adaptation through planning. Consider EA adoption or FAS Encourage improvements and climate No BoP/EA action. Riparian owners to 6d change adaptation through planning. maintain Consider EA adoption or FAS Encourage improvements and climate No BoP/EA action. Riparian owners to 6e change adaptation through planning. maintain Consider EA adoption or FAS Continue existing practice (inc. beach Due for major refurbishment/replacement 6f recharge and monitoring and maintenance expected ~2050 of groynes) Table 6.2 – Proposed strategy per sub-cell for the short and long term
In terms of raised defences we have determined the most appropriate type of defence based on the space available, existing defences and the general cost for different types of defences. For raised defences we therefore have proposed embankments where possible i.e. the space is available and there is not an existing wall, and then new walls or raising of existing walls in the other areas. Figures in Appendix D show the location and type of defence proposed to provide the 1 in 200 year standard for the area by 2126.
In a number of locations the flood defence wall will need to sit on top of the quay wall. The quay wall therefore also needs to be replaced to ensure the stability of the flood defence wall on top. Generally it has been assumed that this will utilise sheet piles, although revetments have been proposed in two locations where more space is available.
Table 6.3 provides details of the measures required e.g. wall or embankment to fulfil the proposed strategy. Brief reasoning is also included as to why a particular defence type or location was chosen. Figures showing the location of the proposed defences can be seen in Appendix D.
For selected areas a second option is proposed which involves raising the ground levels to above the predicted 1 in 200 year water level in 2126. This is only economically realistic for areas that are to be cleared for re-development. Should development go ahead, then the ground raising option should be adopted and made a condition of planning approval.
This second option means that any new development will be above the predicted 1 in 200 year level by 2126 and therefore out of the risk area. This also removes the surface water drainage and any ground water flooding that could occur due to a rising water table and means that the area is not surrounded by high walls, which could impact on
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the character of the area. This is assuming that the whole site is raised, and not just the building footprints.
The use of walls or embankments is the only option that has been considered in detail as part of this study. It may not necessarily be the optimum scheme for the whole area but this is a realistic and workable solution that can provide an indication regarding the likely costs of the works and therefore the contributions required. The Environment Agency’s Poole Bay, Poole Harbour and Wareham Flood & Coastal Erosion Risk Management Strategy is in the process of assessing alternative options e.g. a tidal barrier. An advantage of the walls / embankment option is that the defences can be provided as discrete packages to protect individual cells as and when the funding is available for that particular cell.
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Existing defence Sub arrangements Basis of measure Cell Proposed Option 1 Basis of measure selection Proposed Option 2* Cell (anticipated remaining selection structural life) No formal defence / earth 1 A New embankment Space available and cheap - - bank No formal defence / earth 1 B New embankment Space available and cheap - - bank No formal defence / earth 1 C New embankment Space available and cheap - - bank Cliffs, sea wall and 1 D slipway around sailing No defence proposed None needed - - school New wall along a section Only short section required 2 A Marina - - (~300m) therefore fit in with existing Sections of concrete flood Raise wall and section of new 2 B wall, embankment and Precedent of existing defence - - wall (~500m) wave return wall Quay walls but no formal Replace quay and provide new 2 C Precedent of existing quay - - defence wall Long term benefits from reduced surface water Wave return wall and Replace quay and provide new Replace piling and 2 D Precedent of existing quay interaction. Possible as quay walls wall raise ground area proposed for development No defences, salt marsh Varies depending on space 2 E New wall and embankment - - in front of houses available 3 A None New embankment Space available and cheap - - 3 B None New embankment Space available and cheap - - 3 C No defence proposed None needed - - Most appropriate based on 4 A Rock revetment New wall - - existing defence
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Existing defence Sub arrangements Basis of measure Cell Proposed Option 1 Basis of measure selection Proposed Option 2* Cell (anticipated remaining selection structural life) Long term benefits from Pontoons and quay wall reduced surface water around Sunseeker & RNLI Replace quay and 4 B Replace quay and raise wall Precedent of existing quay interaction. Possible as area, stone revetment raise ground behind area proposed for around new development development Quay wall and EA flood 4 C Replace quay and raise wall Precedent of existing quay - - wall 4 D EA flood wall Replace quay and raise wall Precedent of existing quay - - Section of new embankment 4 E Rock revetment Space available and cheap - - (~650m) Rock and concrete Section of new embankment 5 A Space available and cheap - - revetment (~500m) Wall across lagoon and 5 B New wall in places Matches with current defence - - private walls Most appropriate based on 6 A Concrete wall New wall and revetment - - space available Most appropriate based on 6 B Concrete wall New wall and revetment - - space available Most appropriate based on 6 C No formal defence New wall - - space available Most appropriate based on 6 D Private defences New wall - - space available Private wall behind rock 6 E Replace quay Precedent of existing quay - - revetment Concrete sea wall, groyne 6 F Raise wall Precedent of existing wall - - field and recharged beach Table 6.3 – Defences proposed for each sub-cell *Note that two options have only been considered in areas highlighted for development due to the work required to raise ground levels.
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7 OTHER OPTIONS AND MEASURES REQUIRED FOR STRATEGY
7.1 Access / Egress
PPS25 requires that developments are safe in terms of flood risk for their entire life time, i.e. 100 years for residential properties and 60 years for commercial developments. For the developments highlighted within the Core Strategy, that equates to the flood risk in 2126. To be safe the Finished Floor Level (FFL) should be above the 1 in 200 year level in 2126 plus an Environment Agency freeboard allowance of up to 600mm. There should also be access and egress routes from the development to an area outside of the flood risk zone. To help to ensure this ideally access and egress routes should also be raised to the same level as the finished floor levels. In urban areas this is extremely difficult to undertake. Therefore it has been agreed that in general where a raised defence is present Development control does not require raised access, so long as floor levels for new residential development are raised or under croft parking is provided. The Borough of Poole and the Environment Agency have put together an agreement regarding development requirements which provides further details as to what will and will not be allowed in terms of new development. See section 2.7.2 for details.
However, we recommend that the Borough review the level of principle routes in order to plan for emergency management of an exceedance event. Whilst not serving all properties the raising of levels of certain routes could provide convenient corridors to high ground to aid residents’ safety and to give developers a safe route to connect to for access and egress for their development.
Note that the lowest point on the Twin Sails Bridge is at approximately 2.7mOD. This could therefore be at risk from a 1 in 200 year event in approximately 100 years time based on current sea level rise predictions.
The railway line to the port also acts a flood corridor. Works in this area could be undertaken to prevent this flow route, either in the form of raising or using a barrier in times of flood.
The routes proposed are detailed in Figure 7.1 below.
Strategically, in areas of existing development any re-surfacing or renewal works should involve raising of the ground level if possible. Any new infrastructure projects should be at a minimum of 3.21mOD i.e. above the predicted 1 in 200 year water by 2126.
7.2 Coast Protection
The SMP Review details the importance of Sandbanks to maintaining the present workings of Poole Harbour. Whilst ceasing efforts to manage beach foreshore is unlikely to result in the total loss of Sandbanks and its material at a process level the impact on the present day infrastructure would be dramatic, as would the development of a second or alternative entrance nearer to Canford Cliffs. The ongoing stability of Sandbanks as a feature is therefore important, if only at a process level.
As part of the SMP a strategic decision was therefore made to hold the Sandbanks Peninsular, whilst allowing the Studland Peninsula to adapt. Therefore the cost of the Boroughs existing coast protection strategy needs to be considered as there will be cost savings from stopping maintenance of the Studland Peninsula and allowing it to adapt.
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Figure 7.1: Possible areas where roads could be raised for safe access & egress
7.3 Alternative Options
Two barrier options were also discussed during this assessment, located at Hamworthy Bridge and at the Harbour Entrance. These options were not progressed further for the following reasons:
• Around the quay, maintenance, and in some cases replacement, of the quay walls will still be required due to the structural stability. This is a large cost in addition to any costs of installing and maintaining the barrier. It is therefore more cost efficient to raise the quay walls as they need replacing. • In some areas the current standard of protection is less than the required 1 in 200 years. Work is therefore required in the short term to increase the standard of protection to existing properties. If a barrier were installed in the short term then it is likely that before 2126 it would need to be replaced or undertake significant maintenance, which could be very costly. • A barrier could have an effect on the anticipated morphological development within the harbour area. • Due to sea level rise a barrier would need to be closed more frequently and for increasingly long periods of time in the future. This would therefore have a
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significant impact on the shipping traffic in the area, particularly if a barrier was installed across the harbour entrance. This could therefore have a negative economic impact on the Borough of Poole. • Another result of sea level rise is likely to be the increased risk of surface water and ground water flooding due to more frequent tide locking of outfalls and a rising water table. A barrier would not help solve these issues. • Potential significant adverse environmental impacts on SAC and RAMSAR sites are also possible from the installation of a barrier, which would be a major issue.
As stated previously, pumping is already undertaken and the pumping requirement will increase with sea level rise. A coordinated approach to this by the Borough of Poole and Wessex Water needs to be undertaken to try to provide a sustainable regime both now and in the future. Pumping requirements have not been assessed as part of this study although it is recommended that monitoring is undertaken regarding the existing pumping to aid planning for the future requirements.
7.4 Coastal squeeze
As highlighted by the SMP, this area of Poole is already subject to coastal squeeze. All of the proposed new defences are either along the existing defence alignment or at the edge of a developed area. The new defences will therefore not increase the coastal squeeze in the area, as the habitats would not have been able to migrate into these areas due to the existing defence and / or development. Sea level rise will still result in some habitat loss due to coastal squeeze. This will be particularly relevant in Holes Bay and Lychett Bay. Currently there are areas of marsh land in these bays. With sea level rise, these areas will eventually be covered by water at all times. Based on the current MHW line on OS mapping and the predictions for sea level rise up to 2126, there could be approximately 23 hectares and 79 hectares of habitat lost in Holes Bay and Lychett Bay respectively by 2126 due to coastal squeeze. Compensatory habitat should be considered where possible elsewhere in the borough.
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8 OPTION COSTS
8.1 Asset database
The Borough’s coastline is protected from erosion and flooding by assets that are owned and managed by a variety of authorities and organisations. Taking the flood defence asset data, tables were populated with the following information:
• Reference details • Defence type (current and future) • Defence length • Ground level • Design water levels (current and future) • Effective crest level • Freeboard • Structural life
From these data the following further details were then derived:
• Required defence levels to provide 1 in 200 year standard in 2126 • Defence height (current and future) • Future cross section/volume (where applicable) • Requirement for associated works such as foreshore revetment.
8.2 Unit cost calculations
In order that asset cost can be calculated in a consistent manner, data on flood defence scheme construction costs was taken from the Flood Risk Management Estimating Guide – Unit Cost Database 2007.
The database is based on over 300 Environment Agency schemes and enables differentiation between not only defence types, but scales of construction, e.g. wall height, new or raising etc. To help to verify the results and to provide an idea of how the costs were split i.e. construction, design, supervision etc. benchmarking was undertaken using a number of recent schemes.
During this assessment we have considered the following defence types:
• New walls; • New embankments; • New quay walls; • Raising of existing walls; • Raising of existing embankments; and • Rock revetment.
Appendix E details the price per metre for each defence type listed above. These prices vary depending on the defence height, length and location. Included is a table which details the unit costs used, the selection being based on the existing and likely types of assets required, as well as notes regarding benchmarking against specific known schemes. Where adjustments were considered necessary notes justifying the change are made. All costs were adjusted to November 2009 prices using an appropriate price index to bring them in line with current day prices at the time of the assessment.
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It is important to appreciate that there is a very wide spread in the data sets used to determine these average costs, and so these figures can only be used as a general guide.
8.3 Options Costed
Costs have been developed for the raised defences on the current alignment option, as this meets the policy recommendations of the SMP, and continues the current practice. This may not emerge from the current comprehensive Poole and Wareham Flood & Coastal Erosion Risk Management Strategy as the optimum measure, but it is a realistic and workable solution that provides an indication of the likely costs of the works, and consequently the scale of potential contributions from developers or other funding routes.
When considering what defences are required there are two options in terms of provision of defences: either upfront or adaptive. Upfront means that any defences built should be constructed to the required level to provide a 1 in 200 year standard of protection in 2126, whilst adaptive means that defences are built at a lower level e.g. the predicted 1 in 200 year water level in 2060, with the potential for raising the crest level to the 2126 level in the future as sea levels rise further. The first option provides the lowest cash cost estimates as capital works are only required once. However, in terms of discounted costs over time, phasing of defences may offer lower present value costs, although this will be dependent on numerous local site conditions including ease of access, environmental impact, disruption, and form of construction.
For the purposes of this assessment we have therefore focussed on the upfront option and the costs detailed in this report assume the assets have been designed to have a 100 year lifetime and are sufficiently high when constructed to provide a 200 year SoP in 2126. This means that they would offer a much higher standard in the short term, before the accumulated allowance for climate change has been realised.
The cost for quay walls used within this report assumes the use of a vertical steel sheet pile wall with tie bars into the retained ground providing structural support. To avoid percolation beneath the sheet piling we have included for a cut-off to an impermeable bed. This therefore means that the sheet piles are relatively long and the length and thickness (to provide a 100 year design life) of the sheet piling does mean that the capital costs for these elements are higher than for standard sheet piles.
The costs also include for the replacement of promenade surfacing adjacent to the piles. Note that the cost of any additional flood wall above the quay wall level is accounted for separately. Figure 8.1 shows an example of quay wall layout around Poole Harbour.
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Figure 8.1: Example quay wall
8.4 Asset Cost Inclusions and Exclusions
Details of what has been included or excluded from the cost assessment is shown below:
The inclusions are: • Contractors direct costs • Direct overheads such as preliminaries, method related charges, site establishment, staff costs, profit, insurances etc • The costs also include an allowance for other associated construction works, which are not separately identified as measured elements: • Minor associated works such as fencing, drainage, minor repairs, such as road resurfacing • Temporary works such as access tracks, pumping, cofferdams, river diversions • Variations / compensation events / delay costs where they aren’t specific to any particular element. • Maintenance costs – based on experience this was assumed to be approximately 10% of the construction cost estimate
The exclusions are: • VAT • External costs such as consultants, Environment Agency costs, land, compensation payments • Any costs associated with contaminated ground • Environmental Impact Assessments and associated required works / mitigation.
This is a strategy study and therefore costs are outline only. As a result a number of assumptions have been made when determining the likely costs of the works and therefore there is a degree of uncertainty in the estimates produced. It is therefore a HM Treasury recommendation that for high level cost estimates such as these a 60% ‘optimism bias’ is applied. This is as a result of a number of schemes in the past resulting in a significant over spend than originally predicted in the outline stages due to unforeseen costs. The 60% optimism bias is therefore designed to help give a more realistic picture of what the final costs may be, taking into account unforeseen costs and
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complications. All of the costs stated in this report therefore include the 60% optimism bias.
8.5 Calculated Cost (including optimism bias)
Table 8.1 details the capital costs (including 60% optimism bias) for the proposed work within each cell, split by defence element. Each defence element has been given a unique id based on the cell it falls into and then a consecutive number. The location of each of the individual defence elements is shown in Appendix D. These defence elements were determined based on the existing defences, the space available and type of defence required. Note that these costs are based on defences being constructed to the 1 in 200 year 2126 level rather than staged raising of the defences i.e. the upfront approach and having a 100 year lifetime.
Flood Cell Defence ID Capital Cost 1G1 £ 1,010,000 1G2 £ 840,000 1 1G3 £ 1,380,000 Cell 1 Total £ 3,220,000 2G1 £ 1,720,000 2G2 £ 3,990,000 2G3 £ 7,170,000 2G4 £ 560,000 2 2G5 £ 3,590,000 2G6 £ 16,750,000 2G7 £ 14,270,000 2G8 £ 2,330,000 Cell 2 Total £ 50,380,000 3G1 £ 1,200,000 3 3G2 £ 670,000 Cell 3 Total £ 1,870,000 4G1 £ 2,650,000 4G2 £ 1,580,000 4G3 £ 3,770,000 4G4 £ 2,270,000 4G5 £ 20,480,000 4 4G6 £ 5,580,000 4G7 £ 21,360,000 4G8 £ 1,970,000 4G9 £ 1,600,000 Cell 4 Total £ 61,260,000 5G1 £ 1,070,000 5 5G2 £ 6,790,000 Cell 5 Total £ 7,850,000 6G1 £ 15,980,000 6G2 £ 16,150,000 6 6G3 £ 1,050,000 Cell 6 Total £ 33,180,000 Grand Total £ 157,760,000 Table 8.1: Costs
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These costs are based on the upfront costs. All of the defence elements will be required by 2126 to provide a 1 in 200 year standard across the whole area therefore the total costs of the works is approximately £157.8 million. The majority of the cost is the initial construction cost. As the costs are dominated by the initial construction costs there has been no need to discount them to produce a present value cost, so the costs are presented as ‘cash’ costs. An allowance for ongoing maintenance and long term replacement has been included by adding 10% to the construction cost estimate. This has been tested against a present value cost analysis for a section of sheet piled wall, assuming a major maintenance intervention every 20 years.
The adaptive approach to the defences was determined to be significantly more expensive and therefore not included in this report. The adaptive approach can also mean that obtaining funding can become complicated when the second stage of works is needed. The initial works reduce the probability of flooding and therefore reduce the damage estimates for the future works, therefore impacting on the benefit cost ratio for the second stage of works.
The costs stated above cannot be directly compared with the costs detailed in the pre- feasibility reports for a number of reasons.
o The areas protected are different and therefore not easily compared. o The costs for the pre-feasibility study did not take into account sea level rise, and therefore the proposed defence levels are much lower than those proposed as part of this study. o The pre-feasibility only considered the raising of existing defences. They assumed that the existing walls are strong enough and in the required condition to support the additional defence height.
8.6 Possible contributions
The piecemeal protection of individual regeneration sites is not the preferred approach to future flood risk management as it cannot realise potential benefits that a more comprehensive scheme could bring to a wider area, by reducing flood risk to existing properties. It is also less sustainable and will leave key areas with potentially unsafe access as a result of increased tidal inundation. Therefore a holistic and phased approach should be adopted. For the various areas considered in this report, the land has a number of owners. As this is only at the initial strategy stage and as the viability of any proposed works is still uncertain, landowner discussions have not been held and therefore contributors have not been identified at this stage.
Construction of flood risk management infrastructure provides benefits to both existing development and new regeneration areas and therefore if any schemes or works are taken forward, then contributions from all parties should be considered. Additionally, in public areas, contributions should be discussed with the local authority where public areas may be enhanced as part of the works.
The cost of future flood risk management infrastructure could be met through either private or public investment and can be viewed in a similar way to other major infrastructure requirements (such as highways) as they are essential for the development of key areas as identified in the localism agenda. Due to the costs involved a funding framework may need to be developed as part of the Delivering Poole’s Infrastructure Development Plan Document (DPD) which is to set out the overarching planning contributions and infrastructure delivery framework for Poole.
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Wherever possible and reasonable, opportunities for contributions from developers should be maximised.
Private Funding Contributions from the new development areas are required to achieve the development obligations under PPS25. These can be raised through a variety of sources including Section 106, Community Infrastructure Levy (CIL) and Community Benefit funds.
The exact method for determining the funding requirements will need to be determined through the Delivering Poole’s Infrastructure DPD and any necessary supporting Supplementary Planning Document, which will need to take into account economic viability.
Developer contributions will also vary for the different options. For example, it is expected that the ground raising of large developments should be paid for by the developer, as they will benefit significantly from the land being made available and safe for development, in addition it would also be reasonable to secure contributions to more general flood defence works which improve egress conditions in extreme events.
Public Funding for Regeneration Government funding through various regeneration mechanisms such as Homes and Communities Agency, Regional Funding Allocations and capital borrowing could be appropriate to support future regeneration. Key flood risk management infrastructure could be funded in this way. This is subject to current changes in the funding mechanisms for regeneration activities.
Public Funding for future flood risk management measures Flood Defence Grant in Aid (FDGiA) is distributed by the Environment Agency on behalf of Defra. Funding provided from third party sources can alter the priority of FDGiA allocations. Funding is generally aimed at schemes that provide the greatest reduction in flood risk for the lowest public contribution. Table 9.3 gives the benefit/cost ratios for the proposed works identified as part of this strategy.
An alternative funding route for part of the costs could be the Housing and Communities Agency (HCA) via their Local Investment Planning process, subject to the review of regeneration funding mechanisms. One of the aims of the HCA is to help to fund works required to provide affordable housing. For example, currently North Somerset Council have submitted an application to HCA for funding for flood defence capital works in the Weston-super-Mare area. These works are required to allow residential developments within the Weston Development Area. This could be an area for further investigation by the Borough of Poole, notwithstanding the current uncertainty with regard to central funding mechanisms.
Bournemouth, Dorset and Poole have developed a Local Investment Plan for the period of 2010 – 2026. This plan reflects a number of challenges to the area, including dealing with the effect of climate change.
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9 ECONOMIC ASSESSMENT
9.1 Benefits Methodology
A number of methods were considered to assess the economic benefits of various options for flood management within Poole. It was decided, in view of the level of this study, to modify a method successfully undertaken on other Royal Haskoning projects to estimate flood damage costs and hence potential benefits from generalised property flood damage values. This is based on the Multi-coloured Manual (MCM) strategic level approach. Note this does not account for any damage to roads or services, or traffic/travel disruption. In addition, the impact to the local economy, tourism, loss of business etc is not included in this assessment. These will be an important factor for the Borough of Poole and should be taken into account when funding bids are submitted.
Flood damage costs have been estimated using the method outlined below to provide an indication of potential benefits against which to compare the cost of the strategy. As recommended for Strategy level assessments in the MCM Handbook weighted Annual Average Damage (WAAD) values were used to assess flood damage on a strategic level, for residential and commercial properties. These values were updated to November 2009 using the current consumer price index.
For residential properties the WAAD value was used and applied to the number of properties at risk for each timeframe. For commercial properties damage values are provided per square metre. The floor area for each commercial property was determined using the Local Land and Property Gazetteer (LLPG) dataset and then the commercial property assigned a type based on the coding in the MCM.
A discount factor was applied to the damage values for each year, as required under government investment guidelines - the HM Treasury Green Book (3.5% for present day to 30 years time, 3% for 31 to 75 years time and 2.5% for 76 years and beyond), in order to estimate present value damages. The overall value represents the total damage that could be caused and hence represents the maximum potential benefit. The degree of benefit realised for any scheme depends on the design life of the works and the standard of protection offered. The higher the standard of protection of the scheme, the closer to the maximum available benefit the scheme financial benefit is At a 200 year SoP the scheme benefit is very close to the maximum available, but there always remains a residual risk of flooding for even more extreme events. Financial benefit is damage averted.
In order to identify the number of properties at risk, the LLPG was analysed in combination with the 200 year tidal flood extents for 2126. Using GIS the properties within the flood extent were highlighted. The modified property dataset was then divided into residential and commercial properties for both current and future scenarios. It should be noted that flats have not been excluded from this assessment because it was too complex to remove from the dataset those not on ground level without conducting an extensive survey. The inclusion of all flats could lead to overestimation of benefits. Although only the ground floors of these properties are likely to be directly affected by flooding, residents on higher levels will experience some level of damage due to the loss of access and services during and after a flood event, potentially leading to additional temporary housing costs should the flats become uninhabitable. A more detailed study would ascertain the number of ground floor flats following a site survey and produce more accurate benefit figures.
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Based on the topography for the area GIS was used to determine whether each property could be expected to flood at a 200 year tidal flood event, over a number of epochs. These were based on the predicted water levels for the 1 in 200 year event in 2010, 2035, 2060, 2086 and 2126. To ascertain whether a property actually flooded during an event, a threshold allowance was added to the topographic data of 100mm for commercial properties and 300mm for residential properties. These event epochs were chosen to equally split the 100 year time frame. The predicted sea level rise over the next 16 years up to 2026 is only minimal (approximately 60mm) and therefore the flood extent does not change significantly. It was therefore decided that 2035 (130mm increase from 2010) would be a more useful horizon to consider the changing damage values.
MCM WAAD costs vary according to the standard of protection (SoP) assumed to exist for each property or group of properties. For the purpose of this assessment it was assumed that defences currently exist in the study area but do not protect all properties within the area at risk from flooding. It is also assumed that not all properties at risk will experience flooding during every flood event. Based on the model outputs, a general 1 in 50 year standard of protection was assumed for current conditions during a 1 in 200 year event. It is accepted that some areas have a greater or a lesser standard of protection than 1 in 50 years but this was assumed to be representative for the study area. This approach is suitable when property counts only are used rather than depth- damage calculations.
The average standard of protection was then reduced for each period of time into the future to represent the effects of climate change where increased storminess and rising sea levels could be expected to increase both the number of properties at risk and the frequency of the flooding. By the year 2126 it is estimated that existing defences will offer no protection, i.e. they will be exceeded at least once per year.
9.2 Assumptions
A number of assumptions have been made as part of the benefit assessment. These are standard assumptions but they could have an impact on the damage values calculated. The values stated below therefore provide a guide to the potential damages although the assumptions made during this assessment would need to be investigated as part of a more detailed economic assessment for specific areas.
The assumptions made during the damage calculations are:
• Properties are assumed not to have basements. • All flats have been included in the assessment whether they are on the ground floor or higher levels because it was considered problematic to remove non-ground floor flats without extensively surveying the area at risk. Additionally if the ground floor of a property is flooded, services and access are likely to be cut off for residents on higher floors therefore incurring some level of damage. Approximately 30% of the properties within the study area are flats. • The November 2009 price base has been used throughout the assessment unless otherwise stated. (Note that there has been a very low increase in RPI since then). This correlates with the price date for the cost estimates. • Not all of the properties at risk will experience flooding during every flood event. This is taken into account by applying a standard of protection value.
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• Threshold allowances are the same for all properties (100mm for commercial and 300mm for residential) • The commercial properties have been put into one of five categories and a set damage value per square metre assigned to each category. • Any listed buildings in the dataset have not been accounted for therefore costs may subsequently be higher than estimated. • Saltwater damage costs to building fabric are 10% higher than fresh water (source: MCM). • An extra 10.7% has been added to the total damage cost to account for the cost of emergency services during a flood event (source: MCM). • When undertaking a depth-damage calculation the damage value per property increases with depth. The damage value needs to be capped at the estimated property price to ensure the damages are not overestimated. This is known as capping. Because WAAD values have been used in this assessment capping is not required as the damage value is already limited to a set figure. • MCM values have been used to determine AAD values for both residential and commercial properties • It is standard economic practice that when determining the available benefit estimates only the current land use can be considered. This means that no future development has been accounted for when undertaking the assessment of the number of properties at risk at each horizon. This is because there are no guarantees that the development will go ahead and in line with PPS25 any new development should be designed to be safe from flooding for its lifetime.
9.3 Benefits Results
Table 9.1 below shows the number of residential and commercial properties at risk of flooding for the various epochs taking into account sea level rise. Table 9.2 then provides the PV damages for each cell at the various epochs. This represents the increasing potential maximum scheme benefits.
Number of properties at risk Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Total Residential 0 32 2 220 1 79 334 2010 Commercial 0 26 0 120 12 12 170 Total 0 58 2 340 13 91 504 Residential 0 53 20 421 8 103 605 2035 Commercial 0 29 0 185 15 15 244 Total 0 82 20 606 23 118 849 Residential 0 116 39 828 78 135 1196 2060 Commercial 2 39 0 255 16 22 334 Total 2 155 39 1083 94 157 1530 Residential 18 230 64 1125 147 185 1769 2086 Commercial 4 50 8 355 21 32 470 Total 22 280 72 1480 168 217 2239 Residential 61 817 199 1826 221 261 3385 2126 Commercial 10 62 18 498 26 40 654 Total 71 879 217 2324 247 301 4039 Table 9.1: Number of residential and commercial properties at risk
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PV Damages by Year the scheme is undertaken Cell 2010 2035 2060 2086 2126 1 £1,090,000 £2,420,000 £5,720,000 £11,080,000 £17,280,000 2 £75,870,000 £123,850,000 £202,730,000 £300,510,000 £397,790,000 3 £3,990,000 £8,550,000 £19,870,000 £37,530,000 £56,160,000 4 £61,000,000 £116,840,000 £225,090,000 £374,550,000 £532,520,000 5 £6,110,000 £11,990,000 £23,860,000 £39,330,000 £54,410,000 6 £8,720,000 £16,250,000 £31,010,000 £50,800,000 £70,500,000 Total £156,780,000 £279,900,000 £508,280,000 £813,800,000 £1,128,660,000 Table 9.2: Present value damages for 5 different scheme construction years
When undertaking the benefit assessment a standard of protection has been chosen to represent the general onset of flooding for the area based on the modelling undertaken. It was estimated that currently the standard of protection is approximately 50 years, falling to 25 years in 2035, 10 years in 2060, 5 years in 2086 and no standard of protection by 2126, as determined from analysis of anticipated increases in sea level. These standards are thought to be representative of the whole area. Some areas will have a lower standard, whilst others have a higher standard but that distinction is too detailed for this strategy level study.
The results show that damages can be expected to increase for each period of time into the future. This is expected because as a result of climate change, increased storminess and rising sea level will put more properties at risk as the extent and depth of flooding increases for each of the timeframes. Results indicate that over the lifetime of the Borough of Poole’s Core Strategy, present value damages from a 1 in 200 year event (including the effects of climate change) amounts to approximately £156.8 million in 2010. This value then increases significantly if the work is undertaken at a later date, rising to £1,128 million for a scheme constructed in 2126.
It should be noted that these results are indicative and should be considered in combination with the assumptions detailed above. In particular, the assumption regarding basements, flats and listed buildings could have a significant impact on the estimated damage value.
9.4 Benefit / Cost assessment
Table 9.3 shows the benefit / cost ratio for schemes constructed at the five key dates considered. This is presented by flood cell and in combination. Note that the cost estimates used for this table include a 60% optimism bias as detailed in Section 8.4.
Table 9.3 assumes that all the proposed works within a cell are completed at the same time. The red cells indicate non-financially viable schemes i.e. where the costs of the works are greater than the damage estimates and therefore the benefit / cost ratio is less than 1. The orange cells indicate financial viability, but unlikely to attract Defra grant aid i.e. a ratio of between 1 and 6, whilst the green cells indicate the schemes with a good chance of securing Defra grant aid i.e. the benefit significantly outweighs the costs and the ratio is greater than 6. Note that these colours are just an indication of the chance of securing funding. Currently Defra funding requires a benefit cost ratio of about 8 or more and this is likely to rise in the future. If alternative funding routes can be
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secured e.g. developer contributions, then those can be deducted from the total costs and therefore the benefit cost ratio will increase, improving the likelihood of obtaining Defra funding for the remaining costs.
Year the scheme is undertaken Cell 2010 2035 2060 2086 2126 1 0.3 0.8 1.8 3.4 5.4 2 1.5 2.5 4.0 6.0 7.9 3 2.1 4.6 10.6 20.1 30.0 4 1.0 1.9 3.7 6.1 8.7 5 0.8 1.5 3.0 5.0 6.9 6 0.3 0.5 0.9 1.5 2.1 All cells 1.0 1.8 3.2 5.2 7.2 Table 9.3: Benefit / cost ratio summary
For example if an area has a potential damage value of £10 million and a scheme will cost £2 million. That would have a benefit cost ratio of 5 which is still slightly too low to attract Defra funding. Whereas if a developer and alternative funding routes could provide £700,000 then the revised benefit cost ratio for the Defra funding would be 7.7 and therefore much more likely to obtain grant aid.
Currently only work in cells 2, 3 and 4 are economically viable, with the cost of works in cells 1, 5 and 6 equating to more than the benefit available. By 2035 cell 5 is economically viable, whilst cells 1 and 6 are not economically viable until approximately 2060, and are unlikely to attract Defra funding in the foreseeable future.
Based purely on Table 9.3, defence works should be focussed on cells 3, 2 and 4 as they provide the best return on the investment. Although flood risk is increasing, it does not become economic to improve flood defences for cells 1 and 6 in until after 2060. Therefore the planning policy and strategy needs to be considered to determine the best way to manage the land in these areas and manage future development.
Note that this assessment has used the latest guidance regarding sea level rise. Monitoring of sea levels would be required to provide a more accurate picture of when any of the work becomes economically viable.
The economic assessment undertaken as part of this study is not directly comparable to the pre-feasibility study due to the return period and time frames considered. The pre- feasibility study determined the benefits based on a 1 in 300 year flood event currently, rather than a 1 in 200 year with climate change as used for this study. In addition, this study has determined all of the benefit for each cell rather than just a small area of the cell. This is due to the need for all of the proposed defences in the cell to be constructed to allow the maximum benefit to be taken. The pre-feasibility studies, on the other hand, looked at much smaller areas.
The time frames for the SMP2 were also different and therefore cannot be directly compared, particularly as sea levels are predicted to rise significantly in the period between 2102 and 2126. The benefit areas used for the SMP2 were also different to the cells used for this strategy.
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10 PHASING
The timing of any works is a sensitive issue in this area. This section details some of the drivers and elements that may have an impact on the phasing of any works. The date for investment in a particular flood defence element is dependent on a number of factors including the height of the existing defence, the residual life of the existing asset, the potential to obtain Defra funding, regeneration pressures, and the potential to obtain developer contributions.
There are also a number of other drivers for implementing a scheme. For example political pressures to ensure the town can continue to grow and develop, therefore increasing the economy of the area, as well as the loss to the local economy due to commercial properties flooding and therefore reduced tourism. These are important factors but due to current economic practices they have not been accounted for in this assessment. These factors may be considered in more detail when a funding bid is submitted and more detail is known about the potential wider economic impacts.
For all of the proposed works sea level rise needs to be monitored and the phasing adapted to match any changes to the sea level rise predictions. Currently the Proudman Oceanographic Laboratory (POL), part of the Natural Environment Research Council, is responsible for measuring sea levels across the globe. They regularly report on their findings, which will be included in the UK Climate Impacts Programme (UKCIP). This is an on-going programme, and the most recent advice is currently detailed in UKCP09. Any changes as a result of this programme should therefore be taken into account when assessing when work needs to be completed in the future. Figure 4.5 in Section 4 showed the current Defra predictions for sea level rise compared to the various UKCP09 predictions. This highlights that for the initial stages the predictions are relatively close to each other, it is only in the long term when they diverge. Monitoring of how sea levels are actually changing will therefore help to reduce that uncertainty.
Phasing is achieved by consideration of the following factors which can be split into elements that drive the need for a scheme and elements that help to enable a scheme:
Comparison of the existing defence levels and the predicted water levels to determine the year in which the existing defence will be overtopped. A freeboard allowance is applied to the water level to account for any uncertainty. Drivers An assessment of the residual life of the structure, this would preferably be based on a structural inspection however a visual assessment is often the only available information. Table 10.1 shows how the residual life of existing defences has been classified. The desired regeneration and development areas, and availability of potential Enablers contributions from developers or other funding routes The increasing economic viability of the schemes as sea level rise increases.
Residual When it may life Description Timescale fail* group 1 No existing defence - No defence 2 In poor condition now 2010 - 2015 Now 3 Likely to fail before the end of the Core Strategy 2010 – 2026 Short term 4 Likely to fail after the delivery of the Core Strategy 2026 – 2086 Medium term 5 Likely to fail before the need for 2126 defence level 2086 - 2126 Long term Table 10.1: Classification of residual risk * This is classed as when the risk of failure of the defence becomes unacceptable
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Table 10.2 below shows the different drivers for undertaking improvement works in each cell and subcell, and an approximate date when each of the drivers may apply. Note that the Core Strategy highlights possible regeneration opportunities in sub-cells 2D and 4B and the proposed defences are shown in Appendix D. The bodies likely to promote schemes are then able to see when they are likely to require funding and can plan accordingly. This information has then been used to prioritise where work would be more beneficial.
Drivers Enablers Proposed Residual Cell Subcell Sea level rise** Economics*** Regeneration defence life band* 1A 1G1 Below SoP now Nothing 1 1B 1G2 No defence 2080 2060 planned 1C 1G3 2015 2A 2G1 No defence Below SoP now 2B 2G2 2010 – 2015 Below SoP now 2B 2G3 2026 - 2086 2070 2B 2G4 2026 - 2086 2070 2 2010, 2086 2026 2B / 2C 2G5 No defence 2015 2C / 2D 2G6 2010 - 2026 2010 2D 2G7 2010 - 2026 2015 2E 2G8 No defence 2010 3A 3G1 Below SoP now Nothing 3 No defence 2010, 2060 3B 3G2 2025 planned 4A 4G1 2026 – 2086 2080 4A 4G2 2026 - 2086 2015 4A / 4B 4G3 2026 - 2086 2010 4B 4G4 2026 - 2086 2015 4 4B 4G5 2010 – 2015 Below SoP now 2010, 2086 2026 4C 4G6 2026 - 2086 2010 4C 4G7 2010 - 2026 Below SoP now 4C / 4D 4G8 2026 - 2086 Below SoP now 4E / 5A 4G9 No defence Below SoP now 5A 5G1 No defence 2015 Nothing 5 2035, 2126 5B 5G2 2010 - 2026 Below SoP now planned 6A / 6B 6G1 2010 - 2026 Below SoP now Nothing 6 6C/ 6D/ 6E 6G2 2010 - 2026 2010 2086 planned 6C/ 6D 6G3 2010 - 2026 2080 Table 10.2: Drivers impacting on phasing of the works * Residual life split is shown in Table 10.1. ** Note that the sea level rise estimates are based on the current guidance and are approximate. Where there are variations in the height of the defence the earliest year is stated. *** This is the year from Table 9.3 when the benefit is greater than the cost. Colours are based on Table 9.3
Based on Table 10.2 it is clear that work is needed in some areas in the immediate future. Due to the high cost of protecting the whole area the cells need to be prioritised to ensure the best value for money is obtained. The priorities for the Borough of Poole are most likely to be cells 2 and 4. This is based on the economic assessment, potential developer contributions through regeneration opportunities as well as sea level rise. These cells are financially viable and also likely to receive developer contributions due
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to the large areas of regeneration planned. They also both have areas of defence that are either below standard now or will be in the near future.
Cell 3 is also financially viable but there are no major developments planned for this area and therefore contributions will be unlikely. This means that benefit cost ratio to obtain Defra funding will be a key element to the timing of these works. Cells 1, 5 and 6 also have no major development planned, although their economic viability is lower. A more detailed assessment regarding the condition of the existing defences in these cells may be required, followed by maintenance works to increase the residual life of existing development. Planning policy decisions then should be considered as to whether new development should be located within these cells or whether alternative land uses would be more appropriate in these areas.
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11 CONCLUSIONS
11.1 Approach
The Borough of Poole currently has approximately 500 properties at risk of tidal flooding from the 1 in 200 year tidal event, which is expected to increase to over 4,000 properties by 2126. This increase is due to a predicted sea level rise of approximately 1.26m based on current Defra guidance.
The SMP2 policy for this area is Hold the Line in most locations. Therefore the aim of this study was to determine the flood risk management infrastructure requirements to provide a 1 in 200 year standard of protection from coastal flooding to the Borough of Poole up to 2126, to enable the informed consideration of flood risk issues in planning policy.
The area was split into six discrete cells and then further split into 25 sub-cells. Based on the required SMP2 policy and current flood defence measures for each sub-cell the most appropriate flood risk management infrastructure was determined to continue the provision of a 200 year standard of protection to 2126.
The option of raised walls and embankments was investigated as part of this assessment. This may or may not be the optimum scheme for the area but is a realistic and workable option that provides an indication of the potential costs of the works, and therefore the contributions required. The Poole Bay, Poole Harbour and Wareham Flood and Coastal Erosion Strategy is in the process of addressing a wider range of options for the area.
11.2 Findings
An outline assessment of the cost of the proposed works was undertaken using information from recent completed schemes and the Environment Agency unit cost database. The total cost of the proposed flood risk management infrastructure to provide the 1 in 200 year standard in 2126 for the entire study area equates to a capital cost of approximately £157.8 million, including a 60% optimism bias. Table 8.1 in Section 8.5 shows these costs split up for each defence element required and then totalled for each cell. Note that for a cell to be protected all of the defence elements associated with that cell will be required by 2126.
An economic assessment was undertaken to determine the viability of undertaking the proposed works within each cell. This was based on a 100 year scheme life with various years for the scheme implementation, therefore highlighting that if a scheme is not currently economically viable then it may be in the future once climate change is taken into account. Based on a scheme in 2010, the present value damages (and hence the maximum available benefit) are £156.8 million, which suggests that the proposed works have a benefit cost ratio of 0.99, showing a scheme for the whole area is not currently economically viable. By 2035 the overall benefit / cost ratio increases to 1.8, showing it to be economically viable, but very unlikely to attract Defra grant aid. By 2060 the overall ratio is 3.2.
It is unlikely that all of the works will be undertaken at the same time and therefore is useful to look at the benefit cost ratios for each cell, as shown in Table 9.3. The table shows that currently cells 2, 3 and 4 have a benefit cost ratio of greater than 1, whilst for cell 5 it is not viable until 2035, cell 1 until 2060 and cell 6 just after 2060. Note that
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currently Defra funding requires a benefit cost ratio of approximately 8 or more and this is expected to rise.
Although the exact timings of the works are not certain this study has shown that there is a viable and realistic option for the protection of Poole up to 2126. The argument is stronger for work in some areas (cells 2 and 4) than others and these should therefore be the initial focus of any works and funding. The remaining areas can then be reassessed in the future once there is more certainty regarding sea level rise and other factors become more significant.
11.3 Other considerations
Due to sea levels rising, the heights of any defences also needs to rise to ensure that the required standard of protection is maintained. In some areas the wall heights may in the future start to affect the visual landscape, with walls that are eventually too high to see over. Alternative options may therefore need to be considered in the long term to adapt the defences and areas at risk. These long term adaptations should be considered when designing any defences now to ensure that any future changes are possible. This will also depend on how the areas behind the defences are used and what the vision for the Poole area is in the future.
For areas identified as having significant development potential an additional option has been considered which involves raising the land behind the defence line, therefore removing the need for a raised defence. This would mean that the area is out of the tidal flood risk area and would also alleviate surface water drainage and ground water flooding problems that are expected to increase due to sea level rise. The raising of land requires the demolition of existing buildings and all the utilities and infrastructure to be raised. This is therefore not realistic for existing urban areas where only infill development is expected, hence this has only been considered for the major re- development areas. The cost of ground raising should be included as part of the development and therefore the costs for this option are included as a guide but have not been carried forward into the economic assessment.
Funding sources for the required works will need to be identified. It is likely this will be made up of public funding along with developer contributions. Sea levels will also need to be monitored and any changes to the trends highlighted by UKCIP. The current projections are detailed in UKCP09 but the guidance has not yet been up dated on how to use these projections. The latest guidance should be taken into account when considering the timing of any of the works.
This assessment has only provided an initial investigation into the possible options for protecting the existing coastal areas at risk within the Borough of Poole, as well as enabling further development. Due to the timescales and funding available at this stage a large number of assumptions have had to be made and only concepts have been considered. More detailed work including ground investigations and outline design is required to determine how the option highlighted here could be developed and executed.
An alternative option of a tidal barrier was considered briefly during this assessment, but would require a considerable amount of study in technical, environmental, and economic aspects to assess its viability.
Due to the poor condition of the much of the quay wall, maintenance and repair is expected to be needed in the near future. This will be required to maintain the operation
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of the harbour area irrespective of whether the walls were also acting as a flood defence. Therefore this cost would be incurred even if a tidal barrier was installed. In addition, due to sea level rise a barrier would be required to be closed on an increasingly frequent basis therefore impacting on the operation of the harbour. This could have a major adverse impact on the economy of the area.
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12 RECOMMENDATIONS
Following the assessment undertaken for this study, we recommend that the main focus of any funding opportunities should be directed towards cells 2 and 4 as, in light of the potential for significant financial contributions, these are thought to provide the best value for money and will allow the continuing development of Poole town centre. Due to the low benefit cost ratio in cells 1 and 6, the planning policy for these areas should be considered to determine the best approach for future land use management in these areas, as flood risk will increase significantly, but without much possibility of attracting funding.
More specific recommendations have been split into three groupings:
Understanding current risk: ‹ Undertake a structural condition survey of key flood defence assets, to better determine their residual life and potential to breach.
‹ Annual inspection of the assets should be undertaken. This will help determine when replacements are needed and highlight any urgent works.
‹ Undertake a Surface Water Management Plan (SWMP) to understand and start to address surface water flooding issues. This is particularly relevant following the Flood and Water Management Act which has removed the right to connect to an existing drainage network. A strategic approach is therefore needed for dealing with surface water issues.
‹ Engage with Wessex Water to explore how the potential flood defence measures and sea level rise impact on their facilities and operations.
‹ Confirm the onset of flooding across the area taking into account both tidal levels and wave conditions where appropriate. This may require additional modelling work looking at both the current and future scenarios for various return period events.
‹ Prior to the construction of the elements discussed in this report there would need to be a full appraisal carried out in co-ordination with the Environment Agency and other stakeholders.
‹ The issue of safe access and egress from these areas has been discussed and agreed in detail with the Environment Agency at a strategic level for the study area
‹ Consult with major land owners and employers in the area e.g. RNLI, Sunseeker etc, to determine whether or not they need to be included or excluded from any defence works, and what level of contribution would be appropriate.
‹ Maintain and develop the East side of Holes Bay as the principal route into and out of Poole Town.
‹ The railway currently acts as a low lying flood corridor in places. Options should be considered to address this issue, providing additional protection to the surrounding area where possible.
‹ Undertake a more detailed economic assessment focussing on cells 2, 3 and 4 as these are the most economically viable. This will involve a detailed threshold survey
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to enable a more accurate assessment of potential benefits using the more robust depth/damage approach.
‹ The visual issues of increased wall heights should be considered. Land raising of discrete land parcels, as part of a development, will be beneficial in the long term in terms of reducing both tidal and surface water flood risk. This will require safe access and egress routes to areas of high ground. In areas where land raising is not appropriate then use of undercroft parking could be utilised to ensure that the finished floor level of any more vulnerable land uses is raised above the predicted flood level.
Consider the wider issues for the area: ‹ All new development should meet the development protocol being agreed by the Borough of Poole and the Environment Agency. These will be based on the current Defra guidance until updated guidance is released.
‹ Undertake an SEA and/or Habitats Regulations Assessment (HRA) to consider the potential impacts on the SSSI, SAC, RAMSAR and AONB areas of the site.
‹ Monitor sea levels to aid the estimation of future sea level rise. This can then be used to revise the estimates for wall heights and the required finished floor levels of developments.
‹ Sandbanks is an area of land that plays a large role in the physical stability of the shoreline in the area. It protects the harbour area from wave action, maintains the transport links and helps maintain the vessel approach channels by keeping the ebb and flood velocities high. The impacts of this area of land should be thoroughly investigated when considering how this area should be managed in the future.
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