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Avonmouth/Severnside Development Board: Flood Risk Sub Group Avonmouth-Severnside Flood Management Optioneering Technical Report Final November 2013

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Atkins Avonmouth-Severnside Flood Management Optioneering

Notice

This document and its contents have been prepared and are intended solely for Avonmouth/Severnside Development Board: Flood Risk Sub Group information and use in relation to Avonmouth-Severnside Flood Management Optioneering.

Atkins Limited assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents.

Document History

JOB NUMBER: DOCUMENT REF: 5120321/70/DG/002

Revision Purpose Description Originated Checked Reviewed Authorised Date

D1 Report structure P Canning 03/06/13

I1 Draft issue to client

P Canning A Cox T O’Shea A Cox

S Barge K Hills M Vaughan 19/06/13

I2 Issue to client

P Canning A Cox T O’Shea A Cox

S Barge K Hills M Vaughan 26/07/13

I3 Issue to client addressing final comments P Canning A Cox S Barge M Vaughan 06/09/13

I4 Issue to client addressing final comments A Cox P Canning S Barge M Vaughan 19/09/13

I5 Issue to client addressing final comments P Canning A Cox S Barge M Vaughan 24/09/13

I6 Issue to client updating Table 6-2 P Canning A Cox S Barge M Vaughan 07/11/13

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Table of contents

Chapter pages

1. Introduction 5

1.1. Scope and aims 5

1.2. Report Overview 5

2. Data Review and Gaps Analysis 7

2.1. Introduction 7

2.2. Available Data Sources 7

2.3. Data Review 9

2.4. Gaps analysis 20

3. Consolidation of Flood Risk Studies 22

3.1. Introduction and methodology 22

3.2. Tidal flood risk 22

3.3. Fluvial flood risk 22

3.4. Joint occurrence of flood risk sources 22

3.5. Climate change impacts 24

3.6. Description of existing and future flood risks 26

3.7. Summary 27

4. Option development 31

4.1. Introduction and policy context 31

4.2. Range of potential options 32

4.3. Further development of combined options 33

5. Option assessment 37

5.1. Introduction 37

5.2. Engineering design of options 37

5.3. Economic viability and affordability of options 39

5.4. Environmental constraints and opportunities of options 42

6. Flood hazard and option phasing 49

6.1. Introduction 49

6.2. Acceptable flood hazard and development 49

6.3. Option phasing response to flood hazard 50

7. Recommended option and scope for next stage 54

7.1. Recommendations 55

7.2. Business case 55

7.3. Site investigations 55

7.4. Engineering design 56

7.5. Environmental assessment 56

8. References 59

Appendix A: Flood Risk Interpretation 61

Appendix B: Cost estimates 65

Appendix C: FCRM GiA estimate 73

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Tables pages Table 2–1. Modelling data review. ................................................................................................................... 10 Table 2–2. Engineering and options data review. ........................................................................................... 12 Table 2–3. Economics and Development data review. ................................................................................... 16 Table 2–4. Environmental data review. ........................................................................................................... 19 Table 2–5. Modelling gaps analysis and future needs. ................................................................................... 20 Table 2–6. Engineering and Options gaps analysis and future needs. ........................................................... 21 Table 2–7. Economics and Development gaps analysis and future needs. .................................................... 21 Table 2–8. Environmental gaps analysis and future needs. ............................................................................ 21 Table 3–1. Comparison of tidal parameters. ................................................................................................... 22 Table 3–2. Comparison of combined wave and tidal parameters. .................................................................. 23 Table 3–3. Comparison of combined fluvial and tidal parameters. ................................................................. 23 Table 3–4: Comparison of recent studies findings .......................................................................................... 27 Table 5–1. Details of existing tidal defence improvements. ............................................................................ 39 Table 5–2. Details of secondary defence works. ............................................................................................. 39 Table 5–3. Details of strategic ground raising costs and funding potential. .................................................... 40 Table 5–4. Details of improving tidal defence costs and funding potential. .................................................... 41 Table 5–5. Details of secondary defence costs and funding potential. ........................................................... 41 Table 5–6. Combined assessment of options. ................................................................................................ 48 Table 6–1. Development types and flood hazard. ........................................................................................... 49

Figures

Figure 1-1. Spatial scope of study. .................................................................................................................... 6 Figure 3-1. Fluvial-tidal JPA curves for the 0.5% and 0.1% JP AEP events. .................................................. 24 Figure 3-2. Comparison of sea level rise predictions. ..................................................................................... 25 Figure 3-3. Comparison of rainfall and flow increase predictions. .................................................................. 25 Figure 3-4. Flood depths and extents for the approximate 1.3%AEP wave and tidal event, assuming continual maintenance and temporary asset breaches (translated from Atkins, 2006). ................................. 28 Figure 3-5. Flood depths and extents for the approximate 0.5%AEP wave and tidal event, assuming continual maintenance and temporary asset breaches (translated from Atkins, 2006). ................................. 29 Figure 3-6. Flood depths and extents for combined 0.5%AEP wave and tidal event, and 1%AEP fluvial event, assuming continual maintenance and no asset breaches (translated from Capita Symonds, 2011; technical report figures 7.19 and 7.3). ............................................................................................................................ 30 Figure 3-7. Flood depths and extents for combined 5%AEP fluvial and 5%AEP tidal events, assuming continual maintenance, and no asset breaches (translated from Capita Symonds, 2011). ............................ 30 Figure 4-1. Location of Severn Estuary SMP2 policy units (sourced from Atkins, 2010). ............................... 31 Figure 4-2. Extent and location of strategic ground raising (option 1). ............................................................ 34 Figure 4-3. Extent and location of improvements to existing, or construction of new, tidal defences with fluvial flood protection (option 2). ............................................................................................................................... 35 Figure 4-4. Extent and location of secondary defences, with fluvial flood protection (option 3). .................... 36 Figure 5-1. Key environmental features. ......................................................................................................... 44 Figure 6-1. Flood hazard summary. ................................................................................................................ 52 Figure 6-2. Indicative phasing of works. .......................................................................................................... 53

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1. Introduction

1.1. Scope and aims In March 2013, Atkins Ltd was commissioned on behalf of South Gloucestershire Council, Bristol City Council and Environment Agency, to undertake the Avonmouth-Severnside Flood Management Optioneering project, Phase 1. This commission forms part of a wider objective to enable the ‘Growth Development Model’, outlined in White Young Green (2012) and White Young Green/Amion (2012), in compliance with guidance and legislation relating to flood risk manamagement and environmental protection.

The technical scope of the Phase 1 commission is to:

Review existing data and analyse gaps.

Confirm flood risks and develop flood risk management (FRM) options.

Identify potential for various sources of funding (including FCRM GiA).

Assess and identify a preferred option.

The spatial scope of this project is predominantly defined by the tidal and fluvial floodplain, relevant to the proposed development areas. This is shown in Figure 1-1, including the proposed development sites and reference numbers. The temporal scope is defined by the proposed developments (generally phased between 2012 to 2030) and flood risks between 2010 to 2110.

1.2. Report Overview This report covers the technical scope items, and is structured as follows:

Section 2. Data review and gaps analysis. This section describes the review of data cross the modelling, engineering, economic and environmental streams, and an initial identification of further work required to support development of any recommended option.

Section 3. Consolidation of previous flood risk studies. This section describes the review of previous flood risk modelling studies, their strengths and weaknesses, and what they mean in light of the most recent extremes and climate change guidance updates.

Section 4. Development of FRM options. This section describes the initial consideration of possible options to enable the proposed development.

Section 5. Assessment of options and selection of a recommended option. This section describes the more detailed assessment of the main options and their sub-variations, on engineering, economic and environmental grounds.

Section 6. Identification of next stages. This section describes the tasks required in the next stage to progress the recommended option.

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Figure 1-1. Spatial scope of study.

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2. Data Review and Gaps Analysis

2.1. Introduction The data review and gaps analysis was undertaken to identify the current state of knowledge and understanding for the themes of modelling, engineering, economic and environment. Existing gaps that would need to be addressed prior to, or during, the detailed design stage are also identified.

2.2. Available Data Sources Data sources that were collected and reviewed consisted of:

Bristol Avon Catchment Flood Management Plan (Environment Agency, 2007).

Severn Tidal Tributaries Catchment Flood Management Plan (Environment Agency, 2007).

Severn Estuary Shoreline Management Plan 2 (SECG, 2010).

Severn Estuary Flood Risk Management Strategy (Atkins, ongoing).

SGC Strategic Flood Risk Assessment Level 1 (Scott Wilson, 2009).

BCC Strategic Flood Risk Assessment Level 1 (Halcrow, 2009).

BCC Preliminary Flood Risk Assessment (JBA Consulting, 2011).

Avonmouth to Aust Tidal Defence Scheme (Atkins, 2005-7).

Somerset North Coast Flood Warning Improvements (JBA, 2012).

Avonmouth/Severnside Strategic Flood Risk Assessment Level 2 (Capita Symonds, 2011).

Avonmouth/Severnside Outline Development Strategy (Amion and WYG, 2012).

Avonmouth/Severnside Integrated Development, Infrastructure, and Flood Risk Management Study (WYG, 2012).

BCC Adopted Core Strategy (June 2011);

Bristol Local Economic Assessment (March 2011);

Bristol Development Framework – Central Area Action Plan. Options Consultation (February 2012);

Growth Potential model outputs matrix (BCC, 2013);

Avonmouth / Severnside Integrated Development Strategy – Flood Risk. Flood Risk Management Study (Buro Happold, September 2011);

South Gloucestershire Council Economic Development Strategy 2012-2016;

South Gloucestershire Council Economic Development Strategy 2008-2015;

South Gloucestershire Economic Review (2007);

South Gloucestershire Local Economic Assessment (2011);

South Gloucestershire Council Strategy 2012-2016 (2012);

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South Gloucestershire Core Strategy; Planning for Future Development in South Gloucestershire (March 2010);

Site Allocations and Development Management Policies, Bristol Local Plan (March 2013);

Options for Site Allocations and Proposed Designations – Avonmouth and Kingsweston, Bristol Development Framework;

Site Allocations and Development Management Policies, Bristol Local Plan. Sustainability Appraisal Main Report (March 2013);

Central Area Action Plan, Bristol Development Framework. Options Consultations February 2012 (February 2012);

The Bristol 20:20 Plan; Bristol’s Sustainable City Strategy;

Bristol Local Economic Assessment (March 2011); and

Bristol Development Framework Core Strategy (Adopted June 2011).

Environmental GIS layers received from BCC and downloaded from the MAGIC website.

Cresswell Stage 1: Distribution of Wetland Birds within the Study Area.

Cresswell Stage 2: Review of Consent at Severnside and Avonmouth Impact Assessment.

Bristol Parks and Green Space Strategy.

Severn Estuary Rapid Coastal Zone Assessment Survey: National Mapping Programme.

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2.3. Data Review A summary of the findings of the data review related to modelling, engineering, economic and environmental studies is given in Error! Reference source not found. to Table 2–4.

Dataset Modelling Review findings

Bristol Avon CFMP The policy assessment was based on flood zone mapping, with no modelling. There is limited assessment of groundwater, surface water or sewer flooding.

Severn Tidal Tributaries CFMP

The policy assessment for Avonmouth-Severnside was based on flood zone mapping, with no modelling. Groundwater flooding was assessed via local/historic knowledge and GIS analysis. Although no exacerbating groundwater flood risks were found in the Avonmouth-Severnside area, it was noted that the groundwater table can be at or above ground level in the Avonmouth-Severnside area. Surface water flooding was also assessed via GIS analysis. This found that there was low to medium (no definition of meaning) risk for the Avonmouth-Severnside area.

BCC SFRA Level 1

Assessment of flood risk was based on Flood Zone maps and existing 1D and 2D flood risk modelling and mapping studies provided by the EA, the Bristol Avon CFMP, localised flooding information provided by key stakeholders. This was supplemented by an examination of considering flooding from surface water, reservoir overtopping or failure, sewers and groundwater sources. Flood risk to the study area is largely attributable to tidal, river and surface water sources. Flood risk from sewer and groundwater flooding was limited. Allowance for climate change increases flood risk significantly for a 1% fluvial/0.5% tidal AEP event. The number of properties at risk increased from 2,000 to 14,000.

Avonmouth / Severnside SFRA Level 2

The SFRA was broken down into two stages: a baseline assessment to assess current and future flood risk for existing conditions and a decision support stage to provide additional information to support strategic planning and decision making into the future. Assessment was based on hydraulic modelling using a 1D-2D ESTRY-TUFLOW model. Groundwater was not considered as a source of flood risk due to a lack of available data. Anecdotal evidence suggests that pluvial/groundwater may influence the severity and frequency of flooding in the study area. Modelling indicates tidal flooding from the Severn Estuary represents the main flood risk to Avonmouth / Severnside. Projected sea level rise associated with climate will not impact flood extents significantly but flood depths will be greatly increased.

SGC Strategic Flood Risk Assessment Level 1

Assessment of flood risk was based on EA Flood Zone mapping (predominantly JFLOW) with no additional modelling. Concerns were raised over the accuracy of NFCDD data and the extent of Flood Zone 3b. Flood Zone 3b had not been mapped by the EA and a conservative approach for its delineation was agreed based on NFCDD data. This was considered to be a conservative approach. Assessment of surface and groundwater flooding was based on recorded incidences as documented in the Bristol Avon CFMP. The Severn Tidal Tributaries CFMP suggested the highest risk from surface water flooding was mainly in urban areas due to tide locking. The focussed assessment at Severnside indicated 2% of the Area is located within Flood Zone 3b, 82% in Flood Zone 3a and 4% in Flood Zone 2 with one historic flood event in 1976 at Severn Beach.

Avonmouth to Aust Assessment of wave, tidal and fluvial overtopping, damage and breach. Defined by EWLs sourced from ABPmer (2005), similar to EA

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Tidal Defence Scheme (2011); wave climate sourced from ABPmer (2005), still valid. JPA carried out using JOIN-SEA (waves and tides) and Defra Simplified Method (flows and tides); both still valid. Analyses carried out for year 2005, 2030, 2055 and 2105, using now superseded Defra (2006) climate change guidance. Inundation for combined wave-tidal overtopping and breach using HYDROF (Atkins in-house) 2D model.

Severn Estuary Flood Risk Management Strategy

Assessment of wave, tidal and fluvial overtopping, damage and breach. Defined by EWLs sourced from ABPmer (2005), similar to EA (2011); wave climate sourced from ABPmer (2005), still valid. JPA carried out using JOIN-SEA (waves and tides) and Defra Simplified Method (flows and tides); both still valid. Analyses carried out for year 2010, 2030, 2060 and 2110, using EA (2011) climate change guidance; still valid. Inundation for combined wave-tidal overtopping and breach using TUFLOW 2D model.

Somerset North Coast Flood Warning Improvements

Assessment of wave overtopping inundation, using SWAN to transform offshore waves to the coast. Extreme tide levels were sourced from EA (2011), and are still valid. Wave climate was generated by applying annual significant wave parameters at the offshore boundary, propagated landward by variable wind conditions (Force 6, 8 and 10 winds; 240, 270 and 360 degree wind directions). No JPA carried out, and therefore the analysis represents very extreme events.

Avonmouth/Severnside Integrated Development, Infrastructure, and Flood Risk Management Study

No additional modelling undertaken, summary of previous studies. The recommendation was for a technical study to establish the feasibility of improving the coastal flood defences. This will need to include modelling to asses the impact of flood defence options to properly assess their potential benefits. This work would need to be undertaken using the same modelling that has been used to inform the SFRA 2 to ensure consistency.

Table 2–1. Modelling data review.

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Dataset Engineering and Option Review findings

Bristol Avon CFMP Recommended Policy Option 3 – Continue with existing or alternative actions to manage flood risk at the current level, relevant to Avonmouth village and docks. Flood risk is predicted to increase in the future through climate change but the effective decrease in the standard of protection that this will bring is not expected to have significant social or economic implications.

Severn Tidal Tributaries CFMP

Recommended Policy option 4 - Take further action to sustain the current level of flood risk into the future (responding to the potential increases in risk from urban development, land use change and climate change), relevant to the general Avonmouth and Severnside area.

Severn Estuary SMP2 Recommended Hold The Line between 2010-2110. Policy decision supported by more detailed assessment in the Severn Estuary FRM Strategy; see below.

Severn Estuary FRM Strategy

Recommended maintenance and improvements to the existing tidal FRM assets. Range of options included design types of earth embankment, rock armouring, revetments and vertical/wave recurve walls, and alignments seaward and landward of Avonmouth Docks. Outline design carried out at c100m-1km resolution. Outline hydrodynamic (wave/tidal/fluvial overtopping) design of options using EuroTOP (EC, 2007). Geotechnical failure modes taken from A-A TDS, relevant to the River Avon bank. No structural (wave/tidal/fluvial loading) design undertaken. Foreshore evolution assessment based on hybrid 1D-regime model; influence included in outline hydrodynamic design. Outline health and safety risks highlighted.

SGC SFRA Level 1 No assessment of potential engineering or options to manage flood risk.

BCC SFRA Level 1 Range and detail of options assessed is limited and cursory, consisting of the use of SuDS and urban drainage. No other engineering assessments.

Bristol City PFRA No assessment of potential engineering or options to manage flood risk. However, useful information on past ground water and surface water flood events is given.

Avonmouth to Aust TDS

Recommended maintenance and improvements to the existing tidal FRM assets, and new assets through or around Avonmouth Docks. Range of options included design types of earth embankments, rock armouring, revetments and vertical/wave recurve walls, and alignments seaward, landward and within Avonmouth Docks. Early view from Avonmouth Docks Technical Director that a route within the dock perimeter would not be desirable. Outline design carried out at c100m resolution, but not completed. Outline hydrodynamic (wave/tidal/fluvial overtopping) design of options included. EWL (EA, 2005), climate change (Defra, 2006) and wave overtopping (Besley, 1999) guidance now superseded. Intrusive geotechnical investigations (trial pits, cable tool percussion boreholes, windowless sampler holes) were undertaken along existing tidal FRM assets. Laboratory testing of samples including geotechnical parameters and contaminants. Interpretative report identified engineering issues due to geotechnical parametersGeotechnical failure modes (slip circle failure, seepage) assessed along River Avon banks between Avonmouth and the M5 bridge. No structural (wave/tidal/fluvial loading) design undertaken. Topographic survey was undertaken along the existing tidal FRM assets (cross-sections and spot heights) and also within the Avonmouth Docks area (ground levels and building thresholds). Foreshore evolution assessment based on LiDAR and historic OS mapping and aerial photography; influence included in outline hydrodynamic design. Early Contractor Involvement was limited to

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headlining of key construction issues.

Avonmouth/Severnside SFRA Level 2

Conceptual options identified included improve existing FRM assets, land use management, strategic and local ground raising, new/improved access routes, individual property flood resilience, flood warning and rhyne network improvements. These conceptual options were short-listed for 8 broad areas within the Avonmouth-Severnside region.

Avonmouth/Severnside Outline Development Strategy

Limited reference to the options discussed in the Avonmouth/Severnside Integrated Development, Infrastructure, and Flood Risk Management Study.

Avonmouth/Severnside Integrated Development, Infrastructure, and Flood Risk Management Study

An initial assessment of overground, and major underground oil/gas pipelines, statutory undertaker services was undertaken. Identification of possible options to implement strategic land raising on a site by site basis, or raising the existing or new defences to 10.74mAOD or 12.74mAOD. Consideration of various flood risks and mitigations are also identified.

Table 2–2. Engineering and options data review.

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Dataset Economics and Development Review findings

Severn Estuary Flood Risk Management Strategy

Provides cost benefit assessment for study area in line with FCERM-AG and MCM methodologies, with a base date of 2012. Elements of options, costs and benefits considered could be used to feed into this study.

Avonmouth to Aust Tidal Defence Scheme

Provides detailed cost benefit assessment for study area in line with FCERM-AG and MCM methodologies, but with base data that needs updating. Elements of options, costs and benefits considered could be used to feed into this study.

Avonmouth and Severnside Integrated Development, Infrastructure and Flood Risk Study

Notes that a substantial amount of the area is at risk from fluvial flooding and falls within the Functional Floodplain (Zone 3b) and thus proposed schemes need to take this into account, whilst accepting that the biggest risk is tidal from the Severn Estuary. As with other reports confirms that the standard of protection provided is only as high as the lowest or weakest length of defence.

Raises the issue of land raising in the 57/58 permission zone and potential impacts of adjacent areas, and on the area and distribution of the Functional Floodplain. Appear to have assumed that some development can go ahead within the Functional Floodplain, subject to appropriate mitigation.

Provides high level assessment of different funding sources, concluding that limited potential to raise funds through S106 agreements or CIL. States Infrastructure Levy from the IDB as £25k per hectare of new development, this charge excludes development within the 57/58 permissions.

Provides high level risks related to the areas potential for tidal and fluvial flooding.

Has assumed future development types are a mix of B1/B2, B8 and sui generis land uses, and used historic development rates to inform future rates. Indicates that a similar area of land to that developed (for greenfield) will need to be set aside for ecological mitigation (see the Cresswell study). Suggests a total of 350ha of greenfield land has potential for development. Cost for allowing this development through provision of appropriate infrastructure estimated as £110M.

Avonmouth Severnside Outline Development Strategy

Considers two defence heights, 12.74mAOD and 10.74mAOD, with estimate costs of £200-300M and £56.8M (though quoted as £59.8M in latter sections) respectively (lacks any detail regarding these values however). Indicates that latter option is preferable due both to costs and environmental impacts. Notes that the 10.74mAOD level would still require mitigation for tidal flooding in the future and fluvial flooding.

They have calculated benefits based on three development scenarios using varying mixes of development types, focusing on GVA and employment over a 40-year period, and gives return on investment figures ranging from 13.2:1 to 23.4:1; well above the benchmark of 8:1 for property related regeneration projects. Includes a high level assessment of risks facing the area, split down into procurement, project specific, client specific, environment and external influences categories.

Reviews a list of potential funding sources which may be viable, noting that TIF (Retention of business rates and Tax Increment Financing) is the most appropriate at this stage. Indicates that £160M of additional rates may be generated and may be fully retained by

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the Local Authorities. This would leave a balance of £33.8M needed to be found from other sources (infrastructure funding of £106.8M could require total cash costs in excess of £200M over 25-years).

Notes that all costs and benefits require further work, and that lower cost alternatives are being formulated. They have included an allowance of £450k per annum to manage delivery of the development opportunities.

Growth potential model outputs

Matrix providing information on each land parcel proposed for development, giving details of phasing, area, existing planning permission and flood risk source and how this changes over the 100-year time horizon. Additional detail is provided for the South Gloucestershire sites in terms of depths of flooding. It is not clear the source of the flood risk information and how current this is. This matrix will form the basis of the proposed development scenario, and define when developer contributions will be accrued.

Avonmouth / Severnside Integrated Development Strategy

This document is a Flood Risk Strategy prepared for BCC and SGC developed in accordance with PPS25, and informed by the Level 2 SFRA, the SMP and the Severn Estuary Strategy. The report quotes tidal scheme costs for raising defences to 10.74mAOD, derived partly from the SFRA, from what appears to be a detailed cost approach (in that the cost per metre varies for a variety of defence lengths). The details behind these calculations do not appear to be available however. It notes that no allowance for fluvial flood alleviation schemes had been included in the SFRA, for which they provide high level estimates, but note that these are “based on engineering judgement” and would need to be revisited. Fluvial costs range from £0.2M to £4M, but no breakdown is provided or indication if these are whole life costs or not. States cost of £280M for a defence height of 12.4mAOD.

Early indication of the potential cost of ecological mitigation related to development, suggested as £3.2M.

States that a phased approach to improving tidal defences along discrete lengths of defences may not be feasible as lower defences may be outflanked, but goes on to say that a strategic solution to raise all of the defences in increments may provide value. No mention of benefits available or potential contributions.

States need to include consideration of fluvial flood risk and agree appropriate mitigation works to account for climate change with agreement from the IDB.

States that “With regard to the development of greenfield land within the study area, even with a strategic tidal defence solution in place, a sequential approach will be required as the area will still be in flood zone 3a.”, “Bristol’s recently adopted Core Strategy proposes only the redevelopment of existing brownfield land and indicates that additional Greenfield land will not be allocated for development in the study area during the plan period.”

Suggests a FHRC MCM compliant economic appraisal should be completed as part of further study.

Appendices provide details on proposed development sites, areas (including detailed breakdown of development types and areas, Appendix 6-10), commitments and consents and flood risk.

Avonmouth / Severnside Level 2

Developed initial costs for a range of mitigation options for each parcel of land; these may provide a basis for this study. Includes a high

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SFRA level assessment of impacts of large scale land raising on flood levels.

Recommends that areas within the “breach hazard bandwidth” area in the “less vulnerable” category should also pass the Except ion Test, meaning all existing brownfield sites in this area would need to be re-developed in advance of using the available greenfield land.

South Gloucestershire Council Economic Development Strategy 2012-2016

Strategy for the entire Council area, focussing on key economic areas; highlights Avonmouth / Severnside as an industrial location of international significance.

South Gloucestershire Council Economic Development Strategy 2008-2015

High level strategy which sets a targets and objectives for the area; limited mention of the Avonmouth / Severnside area.

South Gloucestershire Economic Review

Review of the local economy, key employers, sector splits, employment rates and business news. No direct relevance to this study.

South Gloucestershire Local Economic Assessment

Report on existing and planned economic activity, aiming to inform policy for future strategic development, identifying strengths, challenges and opportunities. This document highlights the Avonmouth / Severnside area as an important development site but does not contain detail relevant to this study.

South Gloucestershire Council Strategy 2012-2016

High level Council Strategy identifying key measures of success for each topic area (e.g. health); no information directly relevant to this study.

South Gloucestershire Core Strategy; Planning for Future Development in South Gloucestershire

Core Strategy is the key document in the portfolio of Local Development Framework planning documents. Highlights opportunities and constraints for future development of Severnside, and sets out a policy (CS35) for the area to address these issues.

Site Allocations and Development Management Policies, Bristol Local Plan

Details site allocations and policies for development; confirms Avonmouth as an Enterprise Zone. Avonmouth and Kingsweston Levels subject to its own policy (DM18).

Options for Site Allocations and Proposed Designations

Summarises the Core Strategy policy context for the Avonmouth and Kingsweston Neighbourhood Partnership area – and seeks to promote area as development/employment centre, whilst protecting environmental assets. Sets out appraisal of series of options for each

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– Avonmouth and Kingsweston, Bristol Development Framework

proposed development site.

Site Allocations and Development Management Policies, Bristol Local Plan. Sustainability Appraisal Main Report

The purpose of a sustainability appraisal is to promote sustainable development through better integration of sustainability considerations into the preparation and adoption of plans. Report documents the sustainability appraisal of proposed development locations and type within the Avonmouth-Kingsweston area. May provide useful supporting or background information.

The Bristol 20:20 Plan; Bristol’s Sustainable City Strategy

High level targets in terms of sustainability; no direct reference to study area.

Central Area Action Plan, Bristol Development Framework. Options Consultations February 2012

Covers Central Bristol and not relevant to the study area.

Bristol Local Economic Assessment

Provides details on local economy to give evidence base to Council strategy and policy. Notes Avonmouth / Severnside as a key economic driver in the area, and notes threat of flooding to the site.

Bristol Development Framework Core Strategy

Avonmouth and Bristol Port identified as priority area for industrial and warehouse development, and highlights need to protect environmental assets and for development to contribute to both strategic and local infrastructure required to allow development. Core Strategy Policy (BCS11) will seek to secure agreements and contributions for works to address the site issues/constraints. Identifies need to work with key partners in producing a strategic solution. Indicators of success of policy set out.

Confirms that the sequential approach will be delivered through the site allocations process and through the development management process on unallocated sites. The requirement for mitigation measures will be delivered through the development management process, by means of the requirement for on-site works or the use of planning obligations or a Community Infrastructure Levy (CIL) for off-site works. The requirement for SUDS will be delivered through the development management process, by means of Policy BCS13’s requirement for Sustainability Statements and the implementation of Policy BCS15. The council will work with the Environment Agency to consider responses to flood risk including measures to secure flood defences.

Table 2–3. Economics and Development data review.

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Dataset Environmental Review findings

Avonmouth and Severnside Integrated Development, Infrastructure and Flood Risk Management Study, WYG (February 2012)

As there is a flood route in from between the two Severn crossings our project study has extended to include this area as well as the development area.

No detail on protected species.

Chapter 6 – Contamination Brief overview of the situation, which concludes by stating that most of the area is likely to be affected by some form of contamination. Contamination should not stop development but be seen as an opportunity to remediate.

Chapter 7 – Green Infrastructure (GI) Need to consider cycle routes and other GI that may be proposed in Core Strategies/ Local Plans. Any new development will need to be co-ordinated to retain existing and provide new & enhanced GI.

Chapter 8 – Landscape Area is not the subject of any local, regional or national landscape designations. However, is within NE Severn and Vales CA and South Gloucestershire Council has published a Landscape Character Assessment.

Landscape impacts likely to be more significant in central green field area – fields contain a ridge and furrow landform (also a heritage issue- see below).

Chapter 9 – Archaeology/Heritage The Severn Levels, including the study area, is an area of high archaeological potential due to preservation of archaeological and paleo-environmental remains within waterlogged silt deposits and peat layers.

Central part of study area contains important historic landscape features – historic field boundaries, drainage channels and important hedgerows and the increasingly rare ridge and furrow field earthwork remains of medieval cultivation.

Other baseline info included e.g. two SMs (Anti-Aircraft Battery and Mere Bank sea defences) and numerous listed buildings and Sites and Monuments Records (SMR).

Includes a brief overview of English Heritage’s Rapid Coastal Zone Assessment (2009).

Chapter 10 – Ecology International, national and local designated areas within and adjacent to study area. European Sites include: Severn Estuary SPA, SAC and Ramsar site. Based on a review of planning applications produced for the area, it was identified that protected species are likely to be within the study area.

Overview of the Cresswell study on European sites is included and is the basis of the ecology section. Cresswell study also includes

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information on habitats within the area but no detail on protected species.

Severnside/Avonmouth Wetland Habitat Project Stage 1: Distribution of Wetland Birds within the Study Area, Cresswell (2010)

Baseline study of wetland areas and habitat (including a site walkover) to identify the feasibility of restoring or enhancing sub-optimal wetland habitat and to inform the process of identifying a series of target sites for habitat creation. Study area does not cover the whole of Avonmouth-Severnside FMO study area.

Summary table and figures detailing areas with potential for habitat creation.

Severnside and Avonmouth Wetland Habitat Project Stage 2: Review of Consent at Severnside and Avonmouth Impact Assessment, Cresswell (Dec 2011)

A review of the 1957/58 Severnside Planning Consent (as required under The Conservation of Habitats and Species Regulations, 2010) and an assessment of development impacts at Avonmouth which could occur through development at Severnside alone, or in combination with other plans and projects. Concluded impacts could give rise to potentially significant effects upon the integrity of the SPA and Ramsar site.

Mitigation strategy identified that: 21,990m2 at Severnside and 41,000m

2 at Avonmouth would be required to mitigate impacts predicted in

relation to gadwall and the other wildfowl species forming part of the SPA Qualifying Assemblage. In addition, 465,900m2 (Severnside)

and 275,500m2 (Avonmouth) have been identified as necessary to offset impacts on waders forming part of the SPA Qualifying

Assemblage. The report identified a number of potential sites. Some initial work has been done to identify suitable sites, but further wok is necessary to determine the suitability of these sites.

It is anticipated this information will be used by the competent authorities (SGC/BCC) to undertake an Appropriate Assessment for the Local Development Framework. Appendices I and II include citations and a detailed table showing waterfowl abundance and distribution at Avonmouth and Severnside (derived from Cresswell Surveys 2010).

Bristol Parks and Green Space Strategy

Information on BCC’s vision and strategy for parks and green space.

SFRA Avonmouth/ Severnside: Technical Report, Capita Symonds (February 2011)

Very brief environmental section (6.6) with high level statements.

Bristol Avon CFMP – SEA ER (March 2009)

High level baseline information on CFMP study area.

Bristol City SFRA (March 2009)

No environmental information.

Avonmouth to Aust Sets out a baseline dataset of known archaeological remains, paleo-environmental evidence and historic landscapes and buildings.

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TDS - Historic Environment DBA (2006)

Consideration of this dataset in light of the proposed preliminary options resulted in highlighting of the likely impacts related to each option.

Good baseline knowledge for study area but some information may have been superseded or be more detailed within the Rapid Coastal Zone Assessment (EH).

Avonmouth to Aust TDS - Land Contamination DBA (2006)

Provided a baseline dataset of past and present contaminative activity. It identified areas of contamination, with information on the nature, extent and date of the activity where possible. The potential impact the options would have on these contaminated areas was subsequently assessed, with signposting of the likely way forward for further land contamination studies.

Avonmouth to Aust TDS - Landscape DBA (2006)

Provided preliminary information on the landscape and visual context of the study area. A baseline assessment of possible impacts for each of the preliminary options was completed, highlighting potentially sensitive areas that are likely to require further examination.

Avonmouth to Aust TDS - Ecology DBA (2006)

The ecology DBA determined the nature, extent and importance of the ecological resources within the study area. Subsequent to this, a preliminary impact assessment was carried out, highlighting areas of risk that are likely to require further study.

Severn Estuary Rapid Coastal Zone Assessment Survey: National Mapping Programme (2008).

The following documents are available:

Severn Estuary Rapid Coastal Zone Assessment Survey: National Mapping Programme

http://www.english-heritage.org.uk/publications/severn-estuary-rczas-phase1/

http://www.english-heritage.org.uk/publications/severn-estuary-rczas-phase2a/

MAGIC website and Nature on the Map (Natural England)

Baseline data retrieved, including NE condition assessments of SSSI units.

Table 2–4. Environmental data review.

http://www.english-heritage.org.uk/publications/severn-estuary-rczas-nmp/

http://www.english-heritage.org.uk/publications/severn-estuary-rczas-phase1/

http://www.english-heritage.org.uk/publications/severn-estuary-rczas-phase2a/

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2.4. Gaps analysis The data reviews for each stream were considered in relation to the existing gaps and future needs for Phase 2 of the Avonmouth/Severnside FMO project. The findings of this, and an initial identification of likely required themes and extents of work, are given in:

Table 2–5, relating to modelling.

Table 2–6, relating to engineering and options.

Table 2–7, relating to economic assessment.

Table 2–8, relating to environmental assessment.

Theme Gaps and future needs

Hydrology Phase 2 would need to update design flow inputs to reflect improved estimation techniques.

Climate change Climate change guidance used in a number of the modelling projects has been updated. In Phase 2, model inputs (flows, tide boundaries etc.) will need to be revised to reflect any changes.

Wave and tide climate

The national Coastal Flood Boundary dataset was released in 2011; this supersedes data used in previous modelling in the study area. In Phase 2 tidal inputs to models and any joint probability assessment will need to be updated to reflect this.

Topographical data In Phase 2, 2D model domains may need to be updated to include new or higher resolution LiDAR data flown since their production.

Recent Development

In Phase 2 a check on any significant development in the floodplain would be required. This may impact flood risk in the study area, and need to be incorporated into the 2D model domain.

Flood Defence Infrastructure

In Phase 2 information on improvements to any formal flood defence infrastructure will need to be included as part of the model update procedure.

Groundwater and surface water

Additional information on the potential mechanisms and locations of groundwater and surface water flooding is required outside of recorded incidences documented in the Bristol Avon CFMP.

Table 2–5. Modelling gaps analysis and future needs.


Options Engineering options analysed to date consist of improvements to the existing tidal FRM assets and seaward/internal/landward of Avonmouth Docks (A-A TDS, SEFRMS) to an outline design level of detail. Further options of land use management, strategic and local ground raising, new/improved access routes, individual property flood resilience, flood warning and rhyne network improvements (Avonmouth-Severnside SFRA Level 2) have been analysed at a conceptual level. This previous work represents a wide range of options. Further development of the options is particularly needed for those not focussed at improving the existing FRM assets, to an outline design level of detail. Phase 1 of this study will develop general form, location and timing of works. Further analysis within Phase 2 would consist of detailed design.

Hydrodynamic A significant amount of outline hydraulic design has been carried out for options related to improving the existing tidal FRM assets. Hydraulic design of alternative

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Design options has been limited to that carried out in the Avonmouth/Severnside SFRA, with conceptual heights of 10.74mAOD and 12.74mAOD. No details on the proposed fluvial mitigation measures are provided in the existing studies. Further analysis within Phase 2 would consist of updating of the hydraulic design analysis generally with new extremes and climate change guidance, as well as giving significantly more detail and understanding for any option not focussed at improving the existing FRM assets.

Structural Design A limited amount of structural design has been carried out on all options. Phase 2 would need to cover the full range of structural design analysis from outline to detailed design.

Geotechnical Design

Some conceptual geotechnical design has been carried out in relation to maintain and improving the existing tidal FRM assets (assessing piping, slip circle, and settlement issues). Phase 2 would need to collect further intrusive geotechnical data for any option not focussed at improving the existing FRM assets.

Table 2–6. Engineering and Options gaps analysis and future needs.


Contributions Further information regarding the economic model from the Amion study would be beneficial for Phase 1. Ideally a single, defined development strategy with contributions could be provided so there is a single “scenario” for assessment. Phasing and range of potential contributions would also be helpful to inform options.

Scheme costs Scheme costs have been assessed based on conceptual options. Phase 1 will identify outline design costs, applying unit cost rates with percentage uplifts for non-construction elements. Phase 2 will need to develop detailed costings, with potential for Early Contractor Involvement.

Table 2–7. Economics and Development gaps analysis and future needs.


Human Beings PRoW data was based on previous studies. Definitive locations and routes will be required for future assessment.

Ecology Desk study information from Avonmouth to Aust TDS is detailed but is out of date. Updated Bristol Regional Environmental Record Centre (BRERC) is required for further assessment.

The Cresswell studies provide detailed information of bird usage. WeBS data has been provided and can support subsequent assessment of the preferred scheme option.

Confirmation of the boundary of Hallen Marsh will be required in Phase 1.

Heritage There is a good baseline heritage data in the Avonmouth to Aust TDS reporting, however this is over seven years old and more recent local authority data is required for further assessment.

GIS SNCI layers obtained for BCC area; details of reasons for designation requested from BCC and required for further assessment. SNCI layers for SGC part of study area requested and required for future assessment.

Table 2–8. Environmental gaps analysis and future needs.

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3. Consolidation of Flood Risk Studies

3.1. Introduction and methodology The wide range of predecessor studies (listed in section 2) have analysed flood risk under various extreme parameter and climate change guidance. The previous regional or national guidance documents are:

Climate change guidance; Defra (2006) and Environment Agency (2011, referring to UKCP09 outputs). The latest guidance now explicitly identifies the spread of potential emissions scenarios and their hydrodynamic consequences. However, the guidance notes that the medium 95%ile emissions scenario forms the core dataset that should be used for analysis and design, with the other emissions scenarios supporting development of adaptive designs.

Extreme parameters guidance; Environment Agency (2005) and Environment Agency (2011). The latest guidance now explicitly identifies a confidence interval to inform more precautionary design or analysis. The discussion below refers to the datasets without confidence intervals included.

To enable a consistent dataset to inform option design, assessment and selection, previous studies have been translated to the latest guidance; further details are given in Appendix A.

3.2. Tidal flood risk Studies that have assessed tidal flood risk consist of Atkins (2006; ongoing), Capita Symonds (2011) and JBA (2012). The Atkins (2006; ongoing) studies, and the Capita Symonds (2011) study that used the outputs from that work, applied extreme water levels that are now superseded by the Environment Agency (2011) guidance. The JBA (2012) tidal element of work applied the Environment Agency (2011) guidance and is therefore current. Comparison of these studies is given in Table 3–1, indicating an approximate equivalent AEP. From this comparison it is apparent that there is limited variation in the attribution of %AEP to EWLs.

However, it is notable that a relatively small change in EWL could result in a large change in %AEP. The consequence of this is that the probability (%AEP) and extent of predicted flooding in the Avonmouth-Severnside area is very sensitive to minor changes in EWL.

Extreme water levels* (mAOD) at Avonmouth

8.6 8.9 9.0 9.1 9.4

%AEP in Environment Agency (2011) and JBA (2012)

10 1.3 1 0.5 0.1

Approximate %AEP in Atkins (2006; ongoing) and Capita Symonds (2011)

5 2 1 0.5 0.3

*whilst EWLs are given generally to 2 decimal places in guidance and studies, only 1 decimal place is shown as this is the extent of confidence in the datasets.

Table 3–1. Comparison of tidal parameters.

3.3. Fluvial flood risk Studies that have assessed fluvial flood risk predominantly relate to Capita Symonds (2011). Extreme fluvial flows are generally defined in specific studies, rather than national or regional guidance, and therefore the Capita Symonds (2011) analysis remains current.

3.4. Joint occurrence of flood risk sources

3.4.1. Wave and tidal events

The only studies with rigorous joint probability assessments (JPA) are the Avonmouth to Aust TDS (Atkins, 2006) and Severn Estuary FRM Strategy (Atkins, ongoing). The JPA in these studies quantified that extreme

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wave and tidal events have a medium likelihood of joint occurrence (defined via the detailed JOIN-SEA method). There are no recent updates on these studies, and therefore they remain current.

The Capita Symonds (2011) study also applied the Atkins (2006; ongoing) JPA outputs. The JBA (2012) study did not carry out a specific JPA process, however numerous combinations of extreme wave and tidal parameters were modelled. The JBA (2012) wave and tidal parameters have been compared with the JOIN-SEA JPA work to identify the approximate equivalent JP AEP. This is shown in Table 3–2. This clearly shows that the JBA combinations of extreme wave and tide parameters represent very extreme events, outside of the normal range of FCRM design. However, the study does provide useful information on maximum potential impacts of temporary wave overtopping, for precautionary purposes (such as emergency planning).

Extreme water levels* (mAOD) at Avonmouth used in JBA (2012)

8.6 8.6 8.6 9.0 9.0 9.0

Extreme wave heights* (m) at Avonmouth used in JBA (2012). Variation depends on wind direction between 240 and 300° N.

1.1 to 1.2

1.7 to 1.8

2.4 to 2.5

1.1 to 1.2

1.7 to 1.8

2.4 to 2.5

Approximate %JP AEP of wave-tide pair based on Atkins (2006; ongoing)

0.1 Greater than 0.1

Greater than 0.01

Greater than 0.1

Greater than 0.01

*whilst EWLs are given generally to 2 decimal places in guidance and studies, only 1 decimal place is shown as this is the extent of confidence in the datasets.

Table 3–2. Comparison of combined wave and tidal parameters.

3.4.2. Fluvial and tidal events

The only studies with rigorous joint probability assessments (JPA) are the Avonmouth to Aust TDS (Atkins, 2006) and Severn Estuary FRM Strategy (Atkins, ongoing). The JPA in these studies quantified that extreme tidal and fluvial events have a modest to high likelihood of joint occurrence defined via the Simplified Method (Defra, 2003). There are no recent updates on these studies, and therefore they remain current.

The Capita Symonds (2011) study applied various combinations of extreme fluvial and tidal parameters. These combinations have been compared with the Simplified Method JPA work to identify the approximate equivalent JP AEP. This is shown in Table 3–3. This comparison indicates that, in an over-arching sense, the combinations are effectively assessments of variable fluvial-tidal pairs along the 0.5-0.1%JP AEP curve, as indicated in Figure 3-1 (please note both axes are logarithmic for visual clarity).

Tidal %AEP used in Capita Symonds (2011) 50 50 5 0.5 0.1

Fluvial %AEP used in Capita Symonds (2011) 1 0.1 5 50 50

Approximate %JP AEP of fluvial-tide pair based on Atkins (2006; ongoing)

0.5 0.1 0.5 0.5 0.1

Table 3–3. Comparison of combined fluvial and tidal parameters.

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Figure 3-1. Fluvial-tidal JPA curves for the 0.5% and 0.1% JP AEP events.

3.5. Climate change impacts

3.5.1. Sea level rise and storminess

A comparison of the amount of sea level rise predicted by the various guidance documents is given in Figure 3-2. Of particular note is that:

Predicted sea level rise over the next 20 years is predominantly predicted with certainty, between different emissions scenarios, approximating to 0.1m at this location.

Predicted sea level rise over the next 20-100 years is predicted to vary more and has less certainty, with the latest guidance explicitly identifying the potential variation due to different emissions scenarios being between 0.4m to 1.7m at this location.

Predicted storminess increase has remained uncertain and relative low, identified as being between a 0-10% increase of the next 100 years.

Consideration of these above points identifies that previous modelling of flood risk in the present day and next 20 years would broadly remain unaffected by guidance updates, but that previous modelling of any subsequent period would be affected.

3.5.2. Rainfall and flow increases

A comparison of the amount of rainfall or flow increase predicted by the various guidance documents is given in Figure 3-3. Of particular note is that the predicted flow increase over the next 100 years has remained predominantly the same, for the core guidance, approximating to a progressive 10% to 20-25% increase. However, predicted flow increase is uncertain over the whole 100 year period.

Consideration of these above points identifies that previous modelling of flood risk in the present day and next 20 years would broadly remain unaffected by guidance updates, but that previous modelling of any subsequent period would be affected.

0.01

0.1

1

10

100

0.1 1 10 100 1000

Tid

al %

AEP

Fluvial %AEP

0.5%JP AEP curve

0.1%JP AEP curve

Capita Symonds (2011) combinations

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Figure 3-2. Comparison of sea level rise predictions.

Figure 3-3. Comparison of rainfall and flow increase predictions.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 2110

Sea

leve

l ris

e (

m)

Year

EA (2011): UKCP09 Low emissions scenario 50%ile

EA (2011): UKCP09 Medium emissions scenario 95%ile

EA (2011): Upper end scenario

Defra (2006)

-10

10

30

50

70

90

2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 2110

% c

han

ge i

n f

low

Year

EA (2011): UKCP09 Low emissions scenario 50%ile

EA (2011): UKCP09 Medium emissions scenario 95%ile

EA (2011): Upper end scenario

EA (2011): High ++

Defra (2006)

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3.6. Description of existing and future flood risks Flood risk modelling outputs from Atkins (2006) and Capita Symonds (2011) are shown in Figure 3-4 to Figure 3-7. The approximate revised %AEP in relation to recent guidance updates is identified, with events shown of interest to FCRM (1.3%AEP) and development (0.5%AEP) focussed guidance. It should be noted that the information shown occurs under continued maintenance of the existing FCRM system, under the medium 95%ile emissions scenario. Whilst not explicitly shown in the maps, between 2030 and 2060 the frequency of over-design tidal events would increase and become regular enough for continued maintenance to be impractical i.e. improvements would be required to continue the function of the tidal defences.

The present day mechanism of flooding is indicated as:

Avonmouth Dock gates and non-continuous coastal, variably engineered assets allowing regular tidal flooding and potentially breaching in the 10%AEP wave-tidal event. The flood extent would initially be constrained by relatively high made ground levels in the docks, but by the 1.3%AEP event would increasingly flood Avonmouth village, Chittening Warth Industrial Estate and Hallen Marsh (as shown in Figure 3-4, Year 2010).

The embankment along the River Avon potentially breaching in the 1%AEP tidal event. This would add to the flooding caused by breaching of the Avonmouth Dock coastal assets, with flooding extending further into Avonmouth village, Chittening Warth Industrial Estate, Hallen Marsh and Crooks Marsh (as shown in Figure 3-5, Year 2010).

The embankment between the Severn Crossings allowing flooding to initiate in the 2%AEP wave-tidal event, with breach potentially occurring in the 1%AEP wave-tidal event. The flood extent would be limited in the 2%AEP due to short duration overtopping, but would become extensive in the 1%AEP event due to the potential breach, and progressively merge with the flood extent caused by the above mechanisms south of the Severn Crossings. The flood extent would become progressively larger and broadly extend across the whole floodplain for events above 1%AEP (as shown in Figure 3-5, Year 2010.

Figure 3-4 and Figure 3-5 show the flood depth and extent that would occur under wave and tidal flooding events, with physically predicted temporary breaching of assets included (only occurring over three high tides). These breach locations were based on analysis of weaker points in the defences, and follows available guidance (EC, 2009). In contrast to this, Figure 3-6 and Figure 3-7 derived from the SFRA shows the flood depth and extent that would occur under various combinations of wave, tidal and fluvial flooding events, with no breaching of tidal defences included. These outputs have been used to inform the fluvial flood risk at each development site.

Figure 3-6 and Figure 3-7 follow SFRA guidelines as regards flood modelling assumptions primarily to inform planning, whereas Figure 3-4 and Figure 3-5 are focussed on determining the socio-economic and environmental impacts of flooding. Whilst the results of these two sets of analyses are markedly different due to the different breach assessments, the sensitivity tests with breaches included (described in Capita Symonds, 2011) do indicate similar flood extents to that applied in this study. This suggests that the sensitivity of flood depths and extents to changes in the location and extent of tidal defence breach is relatively low (i.e. the results of different breach locations are broadly consistent at extreme events).

3.6.1. Comparison with previous studies

Table 3–4 compares the standards of protection stated within the SEFRMS, A-A TDS and the SFRA. Given the different objectives of these studies, and varying level of detail and assumptions we could expect some difference in outputs.

Location SEFRMS 2013 Public Consultation

SEFRMS Technical Analysis

Translated A-A TDS technical analysis

SFRA technical analysis

Between Aust and Avonmouth Docks

“The current probability of tidal flooding is 1 in 200 in any year,...”

0.5-0.1%AEP SoP against wave overtopping, tidal weiring, damage and breach.

1 in 75 yr chance of damage.

1 in 100-1000 yr chance of breach.

Significant flooding of area from 1 in

Significant, if not extensive, flooding in the 1 in 200 yr event.

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Location SEFRMS 2013 Public Consultation

SEFRMS Technical Analysis

Translated A-A TDS technical analysis

SFRA technical analysis

100 yr event onwards.

Avonmouth Docks “...with some localised weaker spots having 1 in 20 chance of flooding in any year.”

10-0.1%AEP SoP against wave overtopping, tidal weiring damage, and breach.

1 in 50-100 yr chance of damage.

1 in 100-1000 yr chance of breach.

Significant flooding of area in 1 in 20 yr event onwards.

Table 3–4: Comparison of recent studies findings

The Severn Estuary FRM Strategy public consultation noted that “The current probability of tidal flooding is 1 in 200 in any year, with some localised weaker spots having 1 in 20 chance of flooding in any year”. This statement refers to:

FRM engineered assets between Aust and Avonmouth Docks. The SEFRMS technical analysis identified a 0.5-0.1%AEP SoP against wave overtopping, tidal weiring, damage and breach at this locality. The translated Atkins (2008) technical analysis identified similar performance with 1.3-0.1%AEP SoP against wave overtopping, tidal weiring, damage and breach at this locality.

Variable assets along Avonmouth Docks. The SEFRMS technical analysis identified a 10-0.1%AEP SoP against wave overtopping, tidal weiring, damage and breach at this locality. The translated Atkins (2008) technical analysis identified similar performance with 5-0.1%AEP SoP against wave overtopping, tidal weiring, damage and breach at this locality.

In overview, the significant impact of potential breaching identifies that this risk needs to be considered in any improvement or investment in FCRM assets, particularly when considering the impacts of climate change. The technical analysis undertaken as part of this study is the most realistic representation of tidal flood risk accounting for both overtopping and breach risk, to develop a resilient scheme and enable development of the sites considering the impacts of climate change over the lifetime of the proposed developments.

3.7. Summary The above consideration of previous studies has identified that:

Extreme water levels from the various studies, whilst varying by up to 0.2m with successive guidance, remain predominantly robust. However, the flooding consequences (probability and extent) are very sensitive to relatively small variations in the EWLs.

Extreme wave climate and fluvial flow definitions remain unchanged. The extreme wave climate is best represented in Atkins (2006, ongoing), with JBA (2012) providing further detail on wave overtopping amounts under particularly extreme conditions. Extreme fluvial flows remain best represented in Capita Symonds (2011).

Successive climate change guidance identifies relative certainty in sea level rise, storminess and flow increases over the next 20 years. However, the successive guidance identifies variable and uncertain changes in sea levels and fluvial flows after this period.

The fluvial and tidal flood risks indicated in Figure 3-4 to Figure 3-7 were used to inform the development and outline design of options in sections 4 and 5.

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Figure 3-4. Predicted flood depths and extents for the approximate 1.3%AEP wave and tidal event, assuming continual maintenance and temporary asset breaches (translated from Atkins, 2006).

Year 2010 Year 2030

Year 2060 Year 2110

Key Depth<0.5m

Depth 0.5-1.0m

Depth 1.0-2.0m

Depth 2.0-3.0m

Depth >3.0m

Proposed development

Predicted breach or overtopping in Avonmouth Docks causes “localised” flooding of both existing development and proposed sites.

Predicted breach or overtopping in Avonmouth Docks, River Avon bank and between the crossings results in widespread inundation of both residential and non-residential assets.

Flood flow route develops across low points in ground levels.

Extents and depths of flooding increase as sea levels continue to rise.

Extents and depths of flooding increase as sea levels continue to rise.

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Figure 3-5. Predicted flood depths and extents for the approximate 0.5%AEP wave and tidal event, assuming continual maintenance and temporary asset breaches (translated from Atkins, 2006).

Year 2010 Year 2030

Year 2060 Year 2110

Key Depth<0.5m

Depth 0.5-1.0m

Depth 1.0-2.0m

Depth 2.0-3.0m

Depth >3.0m


Flood extents and depths greater compared to 1.3% AEP event due to higher tidal levels.

Extents and depths of flooding increase as sea levels rise.

Predicted breach or overtopping in Avonmouth Docks, River Avon bank and between the crossings results in widespread inundation of both residential and non-residential assets.

Extents and depths of flooding increase as sea levels rise.

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Figure 3-6. Predicted flood depths and extents for combined 0.5%AEP wave and tidal event, & 1%AEP fluvial event, assuming continual maintenance and no asset breaches (translated from Capita Symonds, 2011; technical report figures 7.19 & 7.3).

Figure 3-7. Predicted flood depths and extents for combined 5%AEP fluvial and 5%AEP tidal events, assuming continual maintenance, and no asset breaches (translated from Capita Symonds, 2011).

Year 2010 Year 2110 Key Depth<0.5m

Depth 0.5-1.0m

Depth 1.0-2.0m

Depth 2.0-3.0m

Depth >3.0m


Year 2010 Year 2110 Key Depth<0.5m

Depth 0.5-1.0m

Depth 1.0-2.0m

Depth 2.0-3.0m

Depth >3.0m


Combined tidal-fluvial event but no breaching results in more limited flooding linked to existing drainage network as tide locking occurs.

Due to sea level rise even without breaching flood extents and depths are increased significantly from 2010.

Combined tidal-fluvial event but no breaching results in limited flooding linked to existing drainage network as tide locking occurs.

Due to sea level rise even without breaching flood extents and depths are increased significantly from 2010.

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4. Option development

4.1. Introduction and policy context Options to manage flood risks to the Avonmouth and Severnside area have been considered previously, and can be grouped broadly into FCRM and development focused studies.

Notable FCRM focused studies include the Severn Estuary Shoreline Management Plan 2 (SMP2) (Atkins, 2010), the Severn Estuary Flood Risk Management Strategy, and the Avonmouth to Aust Tidal Defence Scheme. The Severn Estuary SMP2 set out preferred policies for managing flood and coastal erosion risks. The relevant policy units to this study are BRIS1, BRIS2 and BRIS3, as shown in Figure 4-1. The preferred policies for these units were Hold the Line throughout 2005-2105.

Figure 4-1. Location of Severn Estuary SMP2 policy units (sourced from Atkins, 2010).

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The strategy and scheme studies were both Environment Agency led, and developed options within the remit of the FCRM GiA funding system. Both these studies predominantly assessed wave and tidal flood risks, and methods of upgrading the existing formal and de facto FCRM asset system, complying with the Severn Estuary SMP2 preferred policies. The draft Severn Estuary FRM Strategy identified in the public consultation during summer 2013 that

“We are working in partnership with Local Authorities and major businesses to improve the defences when required. Contributions from new developments will be necessary to help fund new flood defences. We wish to see the defences improved around Avonmouth Docks so as to keep pace with climate change and ensure the risk of flooding is less than 1 in 200 in any year. The Environment Agency will continue to work with Network Rail to ensure that they are prepared for the potential increase in flood risk. Long term improvements between Severn Beach and Avonmouth Docks are being discussed through the West of England Partnership.”

In the Avonmouth to Aust Tidal Defence Scheme study, potential secondary alignments were limited to considering a minor realignment to the north of the Second Severn Crossing (the rifle ranges), and alternative alignments through and around the Avonmouth Docks area.

Notable development focused studies include the Avonmouth-Severnside Level 2 SFRA, and the various WYG studies. These studies were both Local Authority led (Bristol City Council and South Gloucestershire Council), and considered options to enable the development potential. These studies have assessed tidal and fluvial flood risks, and a wider range of methods to manage flood risk, including wide-scale ground raising of potential development area.

4.2. Range of potential options An initial range of potential options was considered, based on review of the predecessor studies, and an integrated assessment of how to manage wave, tidal and fluvial flood risks to enable development potential. This initial consideration identified the following high level options and their related viability:

Strategic ground raising of all development sites. Without an over-arching management plan, this option could occur piece-meal with the proposed developments. However, if considered strategically, this option could be managed over time rather than ad-hoc. Ground raising could be undertaken to address wave, tidal and fluvial flood risks, and need to consider access and egress. Whilst large-scale land raising could have significant environmental impacts, the required flood storage areas could be combined with habitat compensation, mitigation or enhancement. This option was taken forward for further development.

Tidal barrage between Avonmouth and the Caldicot Levels in Wales. This option could be extremely costly (orders of magnitude greater than any other option), and environmentally unacceptable in the context of solely enabling the Avonmouth-Severnside development potential. Whilst any tidal barrage could be promoted and funded by other parties, successive government reviews (DECC, 2011; and more recently by a government select committee in January 2013) of various proposals have not supported the progression of any scheme. This option was not taken forward for further development.

Storage of tidal inundation volumes. Consideration was made of even unrealistic methods of achieving this option (such as underground storage via historic mining systems) identified that this would be technically extremely difficult, economically prohibitive, and environmentally unacceptable. This option was not taken forward for further development.

Raise perimeter roads and/or secondary embankments. This option could conceptually include for access and egress, and may enable some residual FCRM GiA funding as flood risk to the built environment exists now. Considered individually, this option would address wave and tidal flood risks. Fluvial flood risks from the local drainage network would not be addressed. This option was taken forward for further development and combination with other options.

Improve the existing tidal defences. This option includes an envelope of alignments along the seaward or landward perimeter of Avonmouth Docks, dependent on discussions with the Bristol Port Company. This option could pull in significant FCRM GiA funding as flood risk to the built environment exists now. It would address wave and tidal flood risks. Fluvial flood risks from the local drainage network would not be addressed. This option was taken forward for further development and combination with other options.

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Localised ring banks, ground raising or flood resilience. This option would consist of localised works to address fluvial flood risks, but would not be appropriate for much more significant wave or tidal flood risks. A reduced level of fluvial flood storage compared to the strategic ground raising option would be required, which could be combined with habitat compensation, mitigation or enhancement. This option was taken forward for further development and combination with other options.

Flood warning. This would be integral to any option, but on its own would not achieve the aims of the study. This option was taken forward for further development and combination with other options.

4.3. Further development of combined options The high level options described in section 4.2 were presented and discussed with the Flood Risk Sub-Group on 17

th May 2013. Consideration of the potential combination of high level options to address wave,

tidal and fluvial flood risks identified that a) wave and tidal flood risks would be addressed by either strategic ground raising, or improving the existing tidal defences; and b) fluvial flood risks would be addressed by localised ring banks, ground raising or flood resilience. This resulted in the following combined options (and sub options) for more detailed assessment as alternative ways to manage flood risk along different sections of the study area:

Option 1. Strategic ground raising. Due to the extent of ground raising or embankment height required (up to 4m), ground raising was deemed more resilient to flooding.

Option 2. Improve existing tidal defences, with localised fluvial flood protection. Sub-variations within this option include

a) alignments along the seaward or landward Avonmouth Docks perimeter,

b) whether the de facto railway embankment defence is formally re-engineering as a FCRM structure or not,

c) whether improvements are made between the Severn Crossings, or instead local ground levels and infrastructure embankments (M4 and Severn Tunnel) adjacent to the Second Severn Crossing are relied upon to minimise the flood route south towards Avonmouth and Severnside, and

d) type of engineering improvement. These sub-variations would allow differing flood risks to Avonmouth Docks, the railway line and the communities of Northwick, Redwick and Pilning.

Option 3. Secondary perimeter road raising (A4-A403 defences), with localised fluvial flood protection. Sub-variations within this option include

a) whether the de facto railway embankment defence is formally re-engineered as a FCRM structure and a landward perimeter wall constructed, or the A4-A403 road is raised,

b) whether improvements are made between the Severn Crossings, or instead local ground levels and infrastructure embankments (M4 and Severn Tunnel) adjacent to the Second Severn Crossing are relied upon to minimise the flood route south towards Avonmouth and Severnside. These sub-variations would allow differing flood risks to Avonmouth Docks and village, Chittening Industrial Estate, the railway line and the communities of Northwick, Redwick, and Pilning.

The above options and sub-variations can be considered as alternative ways to manage flood risk along different sections of the study area. This is shown in Figure 4-2 to Figure 4-4. Further consideration of these options would need engagement with third parties (Bristol Port Company, Network Rail and Highways Agency) who could be affected by the options, to understand their views and preferences related to this project.

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Figure 4-2. Extent and location of strategic ground raising (option 1).

35


Figure 4-3. Extent and location of improvements to existing, or construction of new, tidal defences with fluvial flood protection (option 2).

Wave recurve addition Or

Reliance on infrastructure embankments and ground levels

Reliance on de facto defence Or

Re-engineering of the railway embankment

Improvements to seaward defences Or

Landward perimeter wall

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Figure 4-4. Extent and location of secondary defences, with fluvial flood protection (option 3).

Wave recurve addition Or


Re-engineering of the railway embankment Or

A403 road raising

Landward perimeter wall Or

A4-A403 road raising

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5. Option assessment

5.1. Introduction The three combined options were assessed at a high level on engineering, economic and environmental grounds. In overview:

Outline design of the options was undertaken. This took into account flood risks as identified in section 3, and assumed that any level of flood hazard was unacceptable (a conservative assumption). Potential for combined fluvial flood and environmental opportunities was also included. This defined the alignment, type and geometry of works (with sub-variations) required, enabling outline costing and consideration of environmental issues.

Costing of the options, accounting for either the next 20 years of climate change impacts (relatively certain), or next 100 years of SLR (less certain).

Environmental assessment of the options, accounting for the broad range of constraints, opportunities, mitigation and compensation that would need to be considered.

5.2. Engineering design of options

5.2.1. Methodology

The outline design of the options was developed based on:

Existing defence geometry and condition as defined in topographic survey (Atkins, 2010) and NFCDD (received from Environment agency), and existing ground levels as defined in LiDAR (received from Environment Agency).

The consolidated assessment of previous flood risk studies in section 3, defined by the extreme tidal parameters defined in Environment Agency (2010), wave-tidal JPA and wave parameters defined in Atkins (2006), and fluvial flood risk defined in Capita Symonds (2011). Outline design was undertaken to provide at least a 0.5%AEP Standard of Protection.

The envelope of climate change predictions as defined in Environment Agency (2011). For this locality, sea level rise is estimated as 0.1m over the next 20 years (reasonably certain), and 0.7m over the next 100 years (less certain, with a potential range of 0.3m-1.7m).

The methodology used to assess performance and design FCRM asset improvements follows the general approach describe by the Integrated Project FLOODsite (EC, 2009). Design to address wave-tide overtopping was based on guidance given in the EuroTOP manual (EC, 2007). The response to tidal-fluvial overtopping (weiring) was defined using the broad-crested weir equation (which is indirectly supported by guidance in the EuroTOP manual). Structural degradation of assets was based on the R&D project ‘Assessment and measurement of asset deterioration including whole life costing’ (Environment Agency, 2009). Outline design of options included various types of construction (earth embankment, rock armouring, revetment, wave recurve or vertical wall) to the level where geometry is defined (foreshore elevation, slope angles, crest level and roughness).

5.2.2. Option 1. Strategic ground raising

The main forms of works considered in this option were significant ground raising of development sites, and directly related significant flood storage for any functional (fluvial) floodplain impacts. It was assumed that tidal flood risk would be unlikely to be exacerbated at a strategic level. The works could consist of land raising up to 3.3m (c2.3m average) to account for the next 20 years of SLR, or 4m (c3m average) to account for 100 years of SLR, for a development area of 3.1km

2. Flood storage (and directly related environmental

impact) offsetting was assumed to be a similar magnitude.

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5.2.3. Option 2. Improve existing tidal defences, with localised fluvial flood protection

The main forms of works considered in this option are given in Table 5–1. Along the Aust to New Passage reach, engineering improvements assessed were earth embankment raising, revetting of the earth embankment, wave recurve wall addition, or alternatively allowing existing ground levels and infrastructure embankments to minimise tidal flooding through to the development area. The existing earth embankment provides a 1%AEP breach SoP, with flooding through to the development area not occurring until the 0.5%AEP event 2013 (reducing to 2%AEP by 2030, and 5-100%AEP by 2060-2110). To account for the next 20 years of SLR, raising of 0.4m to 0.6m (dependent on engineering type) could be required, and a further raising of 0.6m to 0.9m to account for 100 years of SLR.

Along New Passage to Severn Beach, the existing revetment and wave recurve walls are predicted to continue to provide a breach SoP greater than 0.5%AEP, even with SLR. However, pumping station improvements may well be required to address the increasing amount of wave overtopping that could occur.

Along Chittening Warth, if the railway embankment is assumed to be engineered acceptably for FCRM purposes, it is estimated to provide a greater than 0.5%AEP SoP over the next 20 years. To account for the next 100 years of SLR, raising of 0.3m could be required. If re-engineering of the railway embankment is required, this could consist of a 1.75m height embankment with revetment/rock armouring on the seaward slope.

Along Avonmouth Docks, if the existing seaward alignment is used, to account for the next 20 years of SLR the coastal defences could need raising of 0.3-0.6m and improvements/connection to the lock gate, whilst the River Avon earth embankment would require re-engineering (1.5m height) to be impermeable. Further raising by 0.5m could be needed to account for 100 years of SLR. If a landward alignment is chosen adjacent to the dock perimeter fence, construction of a 0.6m height wall would be required, and raising by 0.7m to account for the next 100 years of SLR.

The above range of improvements would address tidal flood risk. To further address fluvial flood risk, residual ground raising, ring banks and/or flood storage works would be required. An assessment of the potential extent of these works that could be required was made based on the combined 5%AEP tidal and 5%AEP fluvial flood extent and depth (agreed to be the functional floodplain by the Environment Agency) information from the Avonmouth-Severnside Level 2 SFRA study (Capita Symonds, 2011). This analysis accounted for 100 years of SLR as the works would be a one-off opportunity to manage flood risk to development.

Reach Engineering type or

alignment sub-variation Works to provide >0.5%AEP

SoP with 20 years SLR Works to provide >0.5%AEP

SoP with 100 years SLR

Aust to New Passage

No improvement, reliance on ground levels, M4 and Severn

tunnel banks None needed. 0.7m raising.

Earth embankment 0.6m raising Further 0.9m raising

Revetment or rock armour Not appropriate Facing of earth embankment

and 0.6m raising

Wave recurve wall 0.4m addition Further 0.6m addition

New Passage to Severn Beach

No primary defence improvements require, although pumping station improvements may be necessary.

Chittening Warth

Re-engineered railway embankment

1.75m height Not appropriate

Revetment or rock armour Not appropriate Facing of embankment and

0.3m raising

Wave recurve wall Not appropriate 0.3m addition

Avonmouth Docks

Coastal revetment, rock armour or wave recurve wall

0.3m to 0.6m raising 0.5m raising

River Avon re-engineered embankment

1.5m height 0.7m raising

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Landward perimeter wall 0.6m height 0.7m raising

Residual ground raising and flood storage

Ground raising if >30% of development site at risk,

otherwise ring banks

Up to 0.5m ground raising or ring bank height. Development sites seaward of any improvements could require up to 2.5m ground

raising.

Table 5–1. Details of existing tidal defence improvements.

5.2.4. Option 3. Secondary A4-A403 defences, with localised fluvial flood protection

The main forms of works considered in this option are given in Table 5–2. The required works and sub-variations along the Aust to Severn Beach reach are the same as for option 2.

Along Chittening Warth and Avonmouth Docks, as well as the option 2 alignments, an alternative defence alignment was considered, consisting of raising of the A403-A4 road. To account for the next 20 years of SLR, the road would on average need to be raised by 1.5m, and a further 0.7m to account for 100 years of SLR.

Reach Engineering type or

alignment sub-variation Works to provide >0.5%AEP

SoP with 20 years SLR Works to provide >0.5%AEP

SoP with 100 years SLR

Aust to New Passage

No improvement, reliance on ground levels, M4 and Severn

tunnel banks None needed. 0.7m raising.

Earth embankment 0.6m raising Further 0.9m raising

Revetment or rock armour Not appropriate Facing of earth embankment

and 0.6m raising

Wave recurve wall 0.4m addition Further 0.6m addition

New Passage to Severn Beach

No primary defence improvements require, although pumping station improvements may be necessary.

Chittening Warth

Re-engineered railway embankment

1.75m height 0.3m raising

A403 road raising 1.5m raising 0.7m raising

Avonmouth Docks

A4 road raising 1.5m raising 0.7m raising

Landward perimeter wall 0.6m height 0.7m raising

Residual ground raising and flood storage

Ground raising if >30% of development site at risk,

otherwise ring banks

Up to 0.5m ground raising or ring bank height. Development sites seaward of any improvements could require up to 2.5m ground

raising.

Table 5–2. Details of secondary defence works.

5.3. Economic viability and affordability of options

5.3.1. Methodology

Economic damages, benefits and funding (see Appendix C) were assessed in accordance with HM Treasury Guidance, the FCERM Appraisal Guidance (Environment Agency, 2010), the FHRC MCM (2005) and the latest Defra guidance. Assets considered in the assessment included properties (both residential and commercial), agricultural land, transport links, receptors vulnerable to flooding, as well as strategic assets such as power transmission lines, stations and Avonmouth Docks. Potential increases in business rates as a result of the proposed works at Avonmouth and Severnside were based on the analysis given in the latest Amion report (May 2013), which estimates an increase in business rates from £151 million to £206 million by year 20. The projected increase in business rates could provide a pool of money against which the funding for the proposed works could be borrowed.

Costs for options were based on a database drawn from actual construction costs for FCRM projects around the UK, providing unit rates for a wide range of engineering structures and to a level of detail to enable

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costing with variable geometry. Whole life costs (see Appendix B) were developed for options by applying the unit rates and relating these to the extent (width, length, type, number), and applying a range of percentage uplifts to account for local context (25%), optimism bias (60%), as well as business case, design and supervision costs (20%) as appropriate. Items that were not included in the costings were any supporting access/egress, contaminated material processing, services diversions and third party costs (Network Rail, Highways Agency). Fluvial flood storage (and environmental impact) off-setting was conceptually accounted for by the equivalent earthworks required for ground raising.


Details of the strategic ground raising costs are given in Table 5–3. This option would not provide any clear improved flood protection to existing property and infrastructure, and therefore was deemed to provide no benefits in FCRM terms and consequently no potential for FCRM GiA funding. However, the strategic ground raising works are assumed to partially enable the potential growth in business rate income of £151-206M (Amion, 2013) from the development area itself.

Comparison of the likely costs and funding potential clearly identifies that other large-scale funding routes would be required for this option to be affordable. However it is noted that ground raising potentially could incorporate an inert landfill approach. This could have the potential to generate an income. This would require a pro-active approach (five-years) within economic modelling work, and/or agreement of the approach for strategic project and planning purposes.


Details of the option costs are given in Table 5–4. This option would provide improved flood protection to existing property and infrastructure (Avonmouth village, and Northwick, Redwick, Pilning and Avonmouth Docks dependent on sub-variation). Analysis of the FCRM benefit provided by the option indicates more than 3,500 properties better protected, and a PVb of up to £141 million. This indicates that FCRM GiA funding could amount to £13 million. In addition to this, the works are assumed to partially enable the potential growth in business rate income of £151-206 million (Amion, 2013) from the development area itself.

Comparison of the likely costs and funding potential clearly identifies that even with the most expensive option sub-variation, the combined FCRM GiA funding and business rate income growth could enable the works to be affordable. This analysis identified that, as fluvial flood risks could be limited to up to 0.5m depth on proposed development sites, the most economic approach could generally be to construct ring banks. However, it should be noted that fluvial modelling of mitigating flood storage may negate the need for ring banks.


Details of the option costs are given in Table 5–5. This option would provide improved flood protection to existing property and infrastructure, dependent on sub-variations. Analysis of the FCRM benefit provided by the option indicates more than 2,500 properties better protected, and a PVb of up to £47 million. This indicates that FCRM GiA funding could amount to £6 million. In addition to this, the works are assumed to partially enable the potential growth in business rate income of £151-206 million (Amion, 2013) from the development area itself.

Comparison of the likely costs and funding potential clearly identifies that even with the most expensive option sub-variation, the combined FCRM GiA funding and business rate income growth would not enable the works to be affordable. This analysis identified that, as fluvial flood risks would be limited to up to 0.5m depth on proposed development sites, the most economic approach could generally be to construct ring banks. However, it should be noted that fluvial modelling of mitigating flood storage may negate the need for ring banks.

Item Ground raising of up to 3.3m,

accounting for 20 years SLR (0.1m). Ground raising of up to 4m, accounting

for 100 years SLR (0.7m).

Ground raising, flood storage and environmental works (£K)

438,710 539,400

Total funding potential (£K) 151,000-206,000 (business rate income growth)

Table 5–3. Details of strategic ground raising costs and funding potential.

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Item

Option 2a

Aust to Chittening Warth: wave recurve

wall addition

Avonmouth Docks: revetments and embankments

Option 2b

Aust to Chittening Warth: maintain earth embankment, reliance on ground levels, M4

and Severn tunnel banks

Avonmouth Docks: revetments and embankments

Option 2c

Aust to Chittening Warth: maintain earth embankment, reliance on ground levels, M4


Avonmouth Docks: landward perimeter wall

Option 2d

Aust to Chittening Warth: wave recurve

wall addition

Avonmouth Docks: landward perimeter wall

Amount of SLR accounted for

20 years, 0.1m

100 years, 0.7m

20 years, 0.1m

100 years, 0.7m

20 years, 0.1m

100 years, 0.7m

20 years, 0.1m

100 years, 0.7m

Tidal defence works (£K)

13,378 36,308 10,116 26,994 14,578 31,238 17,840 40,551

Fluvial flood defence, storage

and environmental works (£K)

21,138 29,922 21,138 29,922 21,138 29,922 21,138 29,922

Total cost (£K) 34,516 66,230 31,254 56,916 35,716 61,160 38,978 70,473

Total funding potential (£K)

151,000-206,000 (business rate income growth)

13,000 (FCRM GiA)

Table 5–4. Details of improving tidal defence costs and funding potential.

Item

Option 3a

Aust to Severn Beach: wave recurve wall

addition

Chittening Warth: re-engineered railway

embankment

Avonmouth Docks: residual A4-A403 road

raising

Option 3b

Aust to Severn Beach: wave recurve wall

addition

Chittening Warth to Avonmouth Docks: A4-

A403 road raising

Option 3c

Aust to Severn Beach: maintain earth

embankment, reliance on ground levels, M4


Chittening Warth to Avonmouth Docks: A4-

A403 road raising

Option 3d

Aust to Severn Beach: maintain earth

embankment, reliance on ground levels, M4


Chittening Warth: re-engineered railway

embankment

Avonmouth Docks: residual A4-A403 road

raising

Amount of SLR accounted for

20 years, 0.1m

100 years, 0.7m

20 years, 0.1m

100 years, 0.7m

20 years, 0.1m

100 years, 0.7m

20 years, 0.1m

100 years, 0.7m

Tidal defence works (£K)

299,748 435,592 509,085 747,462 505,823 738,148 296,486 426,278

Fluvial flood defence, storage

and environmental works (£K)

35,098 123,113 35,098 123,113 35,098 123,113 35,098 123,113

Total cost (£K) 334,846 558,705 544,183 870,575 540,921 861,261 331,584 549,391

Total funding potential (£K)

151,000-206,000 (business rate income growth)

6,000 (FCRM GiA)

Table 5–5. Details of secondary defence costs and funding potential.

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5.4. Environmental constraints and opportunities of options

5.4.1. Methodology and Baseline

The baseline information collated for the project was based on a desk study, which included review of existing reports (refer to Section 2), consultation through the client group and use of online resources such as MAGIC and the Bristol City Council Explore Bristol local information map (http://www.bristol.gov.uk/explore-bristol). The key environmental features identified within the study area are summarised below and identified in Figure 5-1.

5.4.1.1. Biodiversity and Ecology

Designated Sites

Severn Estuary Special Protection Area (SPA), Special Area of Conservation (SAC), Site of Special Scientific Interest (SSSI) and Ramsar site. These are all located on the boundary of the study area, where it meets the Severn Estuary.

Lawrence Western Moor Local Nature Reserve, which is located just within the eastern boundary of the study.

A number of Sites of Nature Conservation Interest (SNCIs) are located within the study area. Habitats The study area supports a wide variety of habitats. Marine habitats, some of which are designated features of the Severn Estuary European Sites, include mudflats, saltmarsh and coastal grassland. Other, non-marine habitats include; grassland (amenity, improved and semi-improved), scrub, arable farmland, small areas of woodland and standing water (ditches, ponds and wetland areas).

It should be noted that continued maintenance of the tidal defence system could result in coastal squeeze as intertidal habitat is progressively submerged and unable to naturally translate landwards, due to predicted sea level rise. In addition to this, any improvements to the tidal defence system may result in an increase in footprint, further impacting intertidal habitat. Any scheme proposed could require mitigatory or compensatory intertidal habitat to be created. The Severn Estuary FRM Strategy considers both coastal squeeze and footprint impacts at an estuary-wide scale, and estimates that for the Avonmouth and Aust frontage the footprint impact could amount to 3.3Ha for the period 2010-2030.

Protected and Notable Species The study area has the potential to support a number of protected and notable species. Previous studies, which reviewed data provided by the Bristol Regional Environmental Records Centre (BRERC), have identified that the following protected/notable species have been recorded as being present within the study area:

Mammals – badgers, water-voles and bats.

Birds – numerous species of breeding birds (including Schedule 1 species) and wintering waders and wildfowl.

Reptiles – grass snakes and slow worms.

Amphibians – great crested newts.

Invertebrates – the small blue butterfly.

Plants – blue-bell and rough marsh-mallow.

5.4.1.2. Land Contamination

Potentially contaminative activities within the study area which have had or have the potential to cause land contamination have varied significantly since the late 1880s. In general, land to the north of the M4 has had little potentially contaminative activity, whereas such activity to the south has been intensive, variable and continuous since 1880s.

Historical and current potentially contaminative activities within the study area:

Gasworks

Landfill sites

Lime kiln

Road vehicle storage and repair

Dockyards/shipbuilding

Fuel storage depots

Unspecified depots and unknown storage tank

http://www.bristol.gov.uk/explore-bristol

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5.4.1.3. Landscape

In general, the landscape of the study area can be described as being more rural in the north and changing to the large industrial and dock-related industry in the south. There are no national, regional or local landscape designations within the study area. However, the whole area falls within the Severn and Avon Vales Character Area. This character area describes the study area as being a diverse range of flat and gently undulating landscapes, unified by broad river valley character. It is a long-settled area and the assessment recognises that the industrial complexes of Avonmouth and the riverside power stations dominate.

The portion of the study area that lies within South Gloucestershire Council boundary is covered by the Council’s landscape character assessment, with the site falling into two distinct areas; the ‘Pilning Levels’ and the ‘Severn Shoreline and Estuary’. The ‘Pilning Levels’ include the landscape from the M4 at the north of the study area, to the Crooks Marsh area within the Severnside area. It is described as a largely flat, simple, semi-enclosed to open agricultural landscape, divided by rhynes, linear transport routes and punctuated by large-scale industry. The ‘Severn Shoreline and Estuary’ is described as an expansive area, dominated and influenced by the physical and visual presence of the Severn Estuary, tidal pattern and weather conditions. Its character is variable, affected by large-scale infrastructure that crosses over the area and industrial development within the adjacent area. Both areas note the importance of the tidal defences in defining the landscape and views and the fact that defences have been a feature of the area for some time.

The area within Bristol City Council boundary is not covered by a similar assessment.

5.4.1.4. Historic Environment

There are two Scheduled Monuments within the study area:

SM Number 28885 - Heavy Anti-aircraft battery 520m east of Holes Mouth (Grid Ref ST52428083), Bristol City Council.

SM Number 27988 - The Mere Bank and flanking ditches (Grid Ref ST53197936), Bristol City Council (partially within study area).

There are approximately 35 listed buildings within the study area and numerous items noted on the sites and monuments register. As a whole, the Severn Estuary foreshore and in the case of the study area, the adjacent Severn Levels has had a long history of human settlement. This, together with the unique ground conditions that often exist mean there is good preservation of historic organic evidence (pollen, wood and peat) and heritage features. Therefore, there is high potential for as yet undiscovered archaeological remains.

Large parts of the study area remain in agricultural use and as such valuable historic features still exist. For example, historic field boundaries, drainage channels and important hedgerows survive across the study area and the important ridge and furrow earthwork pattern (remains from medieval cultivation) is also visible within a number of fields.

5.4.1.5. Other

In addition to the specific receptors identified above, the following was also considered as part of the option assessment process:

Traffic and transport – potential impacts on roads, Public Rights of Way (PRoW) (including the Severn Way Long Distance Trail) and other infrastructure.

Humans – although integrated within the considerations above, specific consideration was given to noise and pollution impacts.

Opportunities for environmental betterment/enhancement, for example, dual purpose flood storage and habitat creation and integration of green infrastructure.

The above knowledge of the study area and professional judgement were used to inform the option assessment process, and the identification of the constraints and opportunities associated with the three key options. This information is summarised in Table 5–6, along with the engineering and economic factors. More detailed explanation of the environmental constraints and opportunities of each option are described in the sections below.

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Figure 5-1. Key environmental features.

45



This option would involve significant ground raising of the development sites, in some cases up to 4m in height and associated flood storage. This option would therefore require large scale ground works and large quantities of material. Overall, the ground raising would significantly increase the footprint of the works when compared to just the development sites. The following environmental constraints and opportunities have been identified.

5.4.2.1. Constraints

Raising ground levels by up to 4m is likely to result in unacceptable landscape impacts, considering that buildings would still be required to be constructed on top of the raised areas.

The large footprint of construction has a greater potential to impact on archaeology, contaminated land and areas that support protected or notable species. Potential for impact on non-native species such as Japanese Knotweed.

There is potential for permanent loss of habitat, some of which is valuable bird habitat that supports bird features of the Severn Estuary European Sites. The option is also likely to reduce the available space for compensatory habitat creation.

There is potential to impact on the water table and hydrology of the area with implications for water levels within the ditches/rhines that support water voles; also potential impacts on other water bodies that may support great crested newts.

Altered groundwater levels could also affect buried archaeology

A Scheduled Monument (Anti-aircraft battery) would be outside of the protected area and subject to increasing flood risk over time.

Significant carbon footprint of option.

The volumes of material required may not all be available locally and therefore import of significant quantities may be required. This would have more significant impact on natural resources when compared to other options.

If the inert landfill approach was applied, there could be impacts on the traffic, ecology and air pollution.

5.4.2.2. Opportunities

Opportunity to win material from onsite borrow pits, which in turn could be transformed into freshwater habitat. However, due to the footprint of the development that could result from the need for land raising there is expected to be limited space and thus the works may be prohibitive to large areas of habitat creation.

The option also offers an opportunity to undertake large scale contaminated land remediation, which could be an environmental betterment for the area.

When compared to the other options, this option was assessed to be the least environmentally favourable, primarily due to the resultant large footprint and height of the development and volumes of material that could be required.


This option would involve improvements (where necessary) to the existing tidal defences to address tidal flood risk. In addition, works such as ground raising of development sites, ring-banks and/or flood storage would be required to address the fluvial flood risk. The works to address fluvial flood risk and the works to existing tidal defences are expected to be much more limited when compared to Option 1. Much less material would be required and the footprint would be smaller. The following environmental constraints and opportunities have been identified:


Potential impacts on the Severn Estuary European sites, from coastal squeeze and potential for the footprint of the defences and ground raising (or other fluvial protection) to impact the sites. The Severn Estuary Flood Risk Management Strategy (SEFRMS) has identified that improving the tidal defences is the preferred option in this location and as such has addressed the issue of coastal squeeze/footprint impacts on the European Sites, through the identification of potential compensatory habitat areas. Sufficient habitat has been identified to offset losses arising from costal squeeze and footprint of defences over the next 20 years. It is anticipated that compensatory habitat

46


schemes will be delivered by the Environment Agency working in partnership with others, supported by the Regional Habitat Creation Programme. Given any improved/new defences coming forward in the area would in part be to protecting new development it is likely that new development may therefore be required to provide an appropriate contribution towards compensatory habitat.

Tidal embankment works and ground raising or ring-banks to address fluvial flooding would require material, which has an associated carbon impact and impact on natural resources

Improving existing tidal defences is expected to have less risk of potential impact on as yet unknown heritage features. However, excavation associated with ground raising or ring-banks for development sites would still carry a risk.

Improving existing tidal defences is expected to have less risk of disturbing contaminated ground. However, excavation for development sites and associated fluvial flood protection works would still carry a risk.

Potential impacts on protected species and risk of encountering non-native species such as Japanese Knotweed. However, due to reduced scale of works, the risk is considered less compared to Option 1.

Potential for fluvial flood protection works (ground raising/ring-banks) to impact on the local water table and hydrology, which in turn could impact on water bodies that support water voles and great crested newts. However, it should be noted that the risk and scale of potential impact is expected to be less than Option 1.


Smaller volumes of material required for ground raising and for embankment improvements when compared to Option 1. Reduced impact on natural resources.

Requirement to source material and provide fluvial flood risk management could provide opportunity to create a mosaic of wetland habitats (scrapes, pools and wet grassland) with green corridors integrated into the design.

Opportunity to provide green infrastructure such as cycle ways/footpaths as part of improvement works to existing tidal defences.

Opportunity to improve visual amenity by improving foreshore frontages that have been subject to mixed methods of repair over time.

Scheduled Monument (anti-aircraft battery) would remain within flood protection area. Overall this option is considered to be the most favourable environmental option as it offers good opportunities for environmental betterment and is considered to have the least construction and operational impacts of the options.


This option could involve a combination of improving existing tidal defences, building secondary tidal defences by raising roads (A4 and A403) and measures to address fluvial flooding (ground raising and/or ring-banks and/or flood storage). The scale of works associated with this option is expected to be less than that for Option 1 but due to the secondary defences, greater than Option 2. The following environmental constraints and opportunities have been identified:


Potential impacts on the Severn Estuary European sites, from coastal squeeze and potential for the footprint of the defences and ground raising (or other fluvial protection) to impact the sites. The Severn Estuary Flood Risk Management Strategy (SEFRMS) has identified that improving the tidal defences is the preferred option in this location and as such has addressed the issue of coastal squeeze/footprint impacts on the European Sites, through the identification of potential compensatory habitat areas. Sufficient habitat has been identified to offset losses arising from costal squeeze and footprint of defences over the next 20 years. It is anticipated that compensatory habitat schemes will be delivered by the Environment Agency working in partnership with others, supported by the Regional Habitat Creation Programme. Given any improved/new defences coming forward in the area would in part be to protecting new development it is likely that new development may therefore be required to provide an appropriate contribution towards compensatory habitat.

Road raising, tidal embankment works and ground raising would require material, which has an associated carbon impact and impact on natural resources.

Working on existing tidal defences and raising roads is expected to have less risk of potential impact on as yet unknown heritage features. However, excavation for development sites and associated fluvial flood protection works would still carry a risk.

47


Scheduled Monument (anti-aircraft battery) would be outside of the protected area and subject to increasing flood risk over time.

Working on existing tidal defences and raising roads is expected to have less risk of potential impact on contaminated ground. However, excavation associated with the development sites and fluvial flood protection would still carry a risk.

Potential for impact on protected species and risk of encountering non-native species such as Japanese Knotweed. Considered less of an impact compared to Option 1.

The A403 and A4 are busy roads and therefore there is likely to be significant traffic disruption and impact during construction.

Raising roads would mean traffic is raised and therefore noise, air pollution and visual impacts are likely to be increased.

Potential for fluvial flood protection works (ground raising/ring-banks) to impact on the local water table and hydrology, which in turn could impact on water bodies that support water voles and great crested newts. However, it should be noted that the risk and scale of potential impact is expected to be less than Option 1.


Smaller volumes of material required for ground raising and for embankment improvements when compared to Option 1. Reduced impact on natural resources.

Requirement to source material and provide fluvial flood risk management could provide opportunity to create a mosaic of wetland habitats (scrapes, pools and wet grassland) with green corridors integrated into the design.

Opportunity to provide green infrastructure such as cycle ways/footpaths as part of road raising and improvement works to existing tidal defences.

Opportunity to improve visual amenity by improving foreshore frontages that have been subject to mixed methods of repair over time. Locations for improvement reduced in comparison to Option 2.

Like option 2, this option offers opportunity for environmental betterments but the associated impacts on major transport routes (A403/A4) and the expected larger footprint of works means that this option is less environmentally favourable than Option 2.

5.4.5. Summary

A combined assessment on technical, economic and environmental grounds was undertaken for the three main options. This is given in Table 5–6. Overall, it is apparent that Option 2 would be preferable, across each of the assessment streams. Further consideration of which sub-variation of option 2 would be preferred does not produce clear evidence for decision-making. This is due to the need to engage with third parties (Bristol Port Company, Network Rail and Highways Agency) who could be affected by the options, to understand their views and preferences related to this project.

48


Engineering Economics Environment

Common issues, constraints,

opportunities and assumptions

Development as per Growth Potential Model outputs.

EA (2011) EWLs used, SEFRMS wave climate used, SFRA fluvial flooding used.

EA (2011) climate change guidance used, medium 95%ile emissions scenario (0.7m sea level rise by 2110).

Earth embankment, ground/road raising, rock armouring, revetment and vertical or wave recurve options assessed, most effective chosen.

EC, CIRA and BS codes used for outline design.

Tidal flood storage off-setting not required.

Fluvial flood storage off-setting or relocation required (un-quantified).

Potential developer contributions of up to £160M.

Fluvial flood storage cost currently excluded.

Environmental mitigation cost currently excluded.

Present Value benefits based on NPD3 and 2010-2030 period.

Latest Partnership Funding Calculator for FCRM GiA used.

3rd

party (NR, HA), services works, land remediation costs currently excluded.

Cost uplifts of up to 40% for appraisal/design/construction, optimism bias of 60%.

Hallen Marsh mitigation site impacted by A06 site (after 2030).

SM (anti-aircraft battery) impacted by A07 site (2012-2016).

SNCIs potentially impacted upon.

SM (drainage network) partially impacted by A23 (after 2030).

Enhancement/mitigation opportunities (scrapes, wet grassland) associated with fluvial flood storage off-setting.

Development impact on natural areas and therefore protected and/or non native species.

Potential for all ground raising works to impact on the water table/local hydrology and therefore impact on water bodies that may support water voles or great crested newts.

Potential impacts on Severn Estuary European Sites.

Option 1. Strategic ground raising.

No further engineering of existing tidal defences.

Land raising of up to 3.3m (c2.3m average) for a development area of 310Ha, and a further 0.7m if accounting for 100 year sea level rise.

Greater fluvial flood storage off-setting or relocation required due to height of raising and side-slopes.

Cost of strategic land raising of £440M (up to £540M including 100 year sea level rise).

No Present Value benefits.

No FCRM GiA contribution.

Increased opportunity to phase developer contributions.

Potential growth in business rates of £151-£206M would only support a small portion of the overall £439M-£539M cost.

No improved protection to existing residential properties, access/egress not enabled.

Large material volumes requiring large/deep freshwater pools, and increased footprint encroachment on adjacent habitat mitigation sites.

Significant risks and opportunities for land remediation and contamination. Significant UXO risk.

Likely unacceptable landscape, and increased footprint, impacts.

Significant carbon and sustainability issues due large quantities of material required.

Greater potential for impact on undiscovered heritage remains.

Road traffic disruption, noise and air pollution.

Limited opportunity to provide green infrastructure (cycle ways/footpaths).

Opportunity to undertake large scale contaminated land remediation.

Option 2. Improve existing tidal defences with localised fluvial

flood protection.

Earth embankment raising of 0.6m between the Severn Crossings, and a further 0.9m if accounting for 100 year sea level rise.

Addition of 0.3m wave recurve wall to railway embankment, if accounting for 100 year sea level rise.

Wave recurve wall and revetment raising of 0.3-0.6m along docks estuary frontage, if accounting for 100 year sea level rise. The dock expansion may negate this need.

Earth embankment reconstruction 1.5m height along docks river frontage, and a further 0.5m if accounting for 100 year sea level rise.

If a landward dock perimeter is chosen, construction of a 0.6m height wall, and a further 0.8m if accounting for 100 year sea level rise.

Residual fluvial flood risks would be addressed via c300m of ring banks, and c800,000m3 of land raising (between 0.1 to 0.5m for a development area of 220Ha), accounting for 100 year flow increase.

Cost of tidal defence improvements between Aust and Avonmouth Docks of £4M (up to £14M including 100 year sea level rise).

Costs of defence improvements around Avonmouth Docks dependent on alignment, between £9M to £12M (up to £15M to £26M including 100 year sea level rise).

Cost of residual ring banks/ground raising of £21M (up to £30M including 100 year sea level rise).

Present Value benefits of £141M compared to ‘do nothing’.

Potential FCRM GiA contribution of £13M.

Reduced opportunity to phase developer contributions.

Potential growth in business rates of £151-£206M, and FCRM GiA contributions of up to £13M, could support the overall £34M-£61M cost.

Improved protection to >3500 existing residential properties (Avonmouth village, Severn Beach, Pilning and distributed in floodplain), access/egress enabled but not resilient.

Reduced material volumes enabling mosaic of habitats.

Reduced risks and opportunities for land remediation and contamination. Localised UXO risk dependent on alignment.

Localised detrimental landscape impacts. Opportunity to improve foreshore frontages.

Reduced carbon and sustainability issues if primary source material used.

Lowest impact on undiscovered heritage features.

Opportunity to provide green infrastructure (cycle ways/footpaths).

Option 3. Secondary defences with

localised fluvial flood protection.

Earth embankment raising of 0.6m between the Severn Crossings, and a further 0.9m if accounting for 100 year sea level rise.

Addition of 0.3m wave recurve wall to railway embankment, if accounting for 100 year sea level rise.

A403/A4 road raising for 7.8km of 1.2m, and a further 0.8m if accounting for 100 year sea level rise.

Ground raising for development sites A07, A14, A22 of 1-2.5m (350,000m3) on seaward side of A403/A4.

Residual fluvial flood risks would be addressed via c300m of ring banks, and c800,000m3 of land raising (between 0.1 to 0.5m for a development area of 220Ha), accounting for 100 year flow increase.

Cost of tidal defence improvements between Aust and Avonmouth Docks of £4M (up to £14M including 100 year sea level rise).

Costs of A4/A403 road raising of £509M (up to £747M including 100 year sea level rise).

Alternative cost of combined re-engineering of the railway embankment and remaining A4-A403 road raising of £300M (up to £436M including 100 year sea level rise).

Cost of residual ring banks/ground raising of £60M-£119M (up to £69M-£123M including 100 year sea level rise).

Present Value benefits of £47M compared to ‘do nothing’.

Reduced opportunity to phase developer contributions.

Potential FCRM GiA contribution of £6M.

Potential growth in business rates of £151-£206M, and FCRM GiA contributions of up to £6M, would only support a small portion of the overall £335M-£871M cost.

Improved protection to >2500 existing residential properties (Severn Beach, Pilning and distributed through floodplain), access/egress enabled in a resilient manner.

Reduced material volumes enabling mosaic of habitats.

Reduced risks and opportunities for land remediation and contamination. Localised UXO risk.

Road raising landscape impacts. Reduced opportunity to improve foreshore frontages.

Carbon and sustainability issues if primary source material used.

Potentially reduced impact on undiscovered heritage features.

Increased road traffic disruption, noise and air pollution.

Opportunity to provide green infrastructure (cycle ways/footpaths).

Table 5–6. Combined assessment of options.

Key: Colouring relates to relative impact compared to other options. Green = Relatively low/positive. Amber = Relatively similar. Red = Relatively high/negative.

49


6. Flood hazard and option phasing

6.1. Introduction Further consideration was made of detailed flood hazard, to provide information on potential phasing of the construction works into the future.

6.2. Acceptable flood hazard and development The matrix given in Defra/EA (2006) was used to develop the potentially acceptable hazards for development types as shown in Table 6-1. It was assumed that:

No development would be deemed acceptable under flood hazard categorised as danger for most or all. At this level of flood hazard, structural solutions would be required to reduce the hazard.

Type B1, B2 and B8 development could potentially be deemed acceptable, with flood warning, under flood hazard categorised as danger for some (note assumptions in Table 6-1).

Any Type A and B development could potentially be deemed acceptable, with flood warning, where flood hazard is less than danger for some (note assumptions in Table 6-1).

Assumptions:

Site specific FRAs would need to demonstrate that the rate of onset of flooding is acceptable and can be appropriately mitigated for by onsite measures.

Any potentially sensitive chemical/processing/hazardous waste facilities (i.e. more and highly vulnerable) would not be permitted in the flood risk area.

Site specific works include provision of safe refuge areas, dry access and egress routes, adequate Flood Warning is available, and viable Flood Management Plans documenting the procedures are in place to enable safe egress in the event of a flood.

Table 6–1. Development types and flood hazard.

0.05 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1 1.5 2 2.5

0 A-B A-B A-B B1,B2,B8 B1,B2,B8 B1,B2,B8

0.1 A-B A-B A-B B1,B2,B8 B1,B2,B8

0.25 A-B A-B A-B B1,B2,B8

0.5 A-B A-B A-B

1 A-B A-B B1,B2,B8

1.5 A-B A-B B1,B2,B8

2 A-B B1,B2,B8 B1,B2,B8

2.5 A-B B1,B2,B8 B1,B2,B8

3 A-B B1,B2,B8 B1,B2,B8

3.5 A-B B1,B2,B8

4 A-B B1,B2,B8

4.5 B1,B2,B8 B1,B2,B8

5 B1,B2,B8 B1,B2,B8

Depth of flooding (m)

Velo

cit

y (

m/s

)

Key

Danger for some

Danger for most

Danger for all

Development types: A1 shops, A2 financial and professional services, A3 restaurants and cafes, A4 drinking establishments, A5 hot food takeaways.

Development types: B1 business (offices other than those that fall within A2), B2 general industrial, B8 storage or distribution.

50


6.3. Option phasing response to flood hazard The flooding information given in Figure 3-4 to Figure 3-7 was used to provide flood depths (in the 0.5%AEP event to be compliant with NPPF/PPS25) at the proposed development sites between 2010 and 2030 (as per development phases), and then 2060 and 2110. As flood velocity information was not readily available, a bounding assumption that all velocities are less than 2m/s was applied.

6.3.1. Description of flood hazards

The flood hazards under the do nothing and maintenance (for baseline reference and understanding of hazards without intervention) options, are given in Figure 6-1. This indicates that up to and shortly after 2030, with continued maintenance of the existing FCRM system:

In development phase 2012-2016 at Avonmouth, sites A4, A7, A11, A13, A15 and A20 would be at tidal flood hazard of danger for most or all. Fluvial flood hazard would be at danger for some or none.

In development phase 2017-2021 at Avonmouth, sites A5, A15a, A16 and A17 would be at tidal flood hazard of danger for most. Fluvial flood hazard would be at danger for some.

In development phase 2022-2026 at Avonmouth, sites A14 would be at tidal flood hazard of danger for most. Fluvial flood hazard would be at danger for some or none.

In development phase 2026-2030 at Avonmouth, sites A12 and A19 would be at tidal flood hazard of danger for most. Fluvial flood hazard would be at danger for some or none.

In development phases after 2030 at Avonmouth, sites A6 and A23 would be at tidal flood hazard of danger for all. Fluvial flood hazard for site A23 would be at danger for most.

At Severnside, sites S11-14 within the development pipeline (assumed to occur prior to 2030) would be at tidal flood hazard of danger for most or all. Fluvial flood hazard would be at danger for some or none.

Whilst this is based on the 0.5%AEP events to be compliant with PPS25/NPPF, it is notable that in the 1.33%AEP events tidal flood hazards for some proposed development sites would still likely be categorised as danger for most or all.

6.3.2. Option phasing response

The above summary suggests that structural interventions, particularly to reduce tidal flood hazard, would likely be required in the initial development phases. Alternatively, the currently proposed development phasing or type could be amended to avoid higher (tidal) hazard locations.

However, fluvial flood hazard to the proposed development sites could predominantly be managed with non-structural solutions until between 2030-2060. The exception to this would be at sites A7, A13-15a, A23, S6 and S7. This indicates that loss of fluvial flood storage could be minimal with the proposed developments if ground levels are not altered and any increase run-off is managed at source. This would significantly reduce the engineering, economic and environmental impacts of ground raising or ring banks, although related opportunities for additional environmental enhancement would also similarly be reduced.

It should be noted that the validity of the above statement and the cost profile is dependent on the extent of climate change that is accounted for in any structural interventions, and the level of flood hazard that is deemed acceptable for development. It should be noted that applying the assumption that danger for some or none could be addressed by non-structural measures, potentially results in a significant reduction in intervention and cost for fluvial flood hazard intervention, compared to the high level assumptions in section 5. The potential phasing of activities is provided in Figure 6-2, including flood hazard to be addressed, engineering/environmental works, and an initial cost profile. Figure 6-3 further shows the activity phasing graphically. This indicates that potential phasing would be:

2012-2016. Construction of the landward perimeter wall, or seaward revetments and embankments, along Avonmouth Docks. This will address the source of greatest tidal flood risk to National Planning Policy Framework (NPPF) requirements, particularly to the general Avonmouth development area and Avonmouth village. Non-structural measures could be acceptable to address fluvial flood risks.

51


2017-2030. Wave recurve wall addition to the Aust to New Passage earth embankment if required. This will address the next greatest source of tidal flood risk to NPPF requirements, particularly to the Severnside development area and the communities of Northwick, Redwick and Pilning. Residual ring banks or ground raising for development sites at unacceptable fluvial flood hazard during the successive epochs 2012-2016, 2017-2021, 2022-2026, and 2027-2030, as they occur. This should address the primary fluvial flood risks to NPPF requirements, flood storage and environmental compensation/mitigation requirements. In addition to this, re-engineering of the railway embankment along Chittening Warth could be considered.

Potential variations in the above phasing of works could include or be influenced by:

The extent to which predicted SLR is included in the proposed works.

The extent to which flood hazard with danger for some or none is deemed able to be addressed by non-structural FRM measures.

Timing and form of the Avonmouth Docks deep water expansion. Any proposals would need to be discussed and agreed with Bristol Port Company. There is an opportunity that the deep water expansion could incorporate design for tidal flood risks.

Timing and form of re-engineering the railway embankment. Any proposals would need to be discussed and agreed with Network Rail. There is an opportunity to improve the railway embankment to comply with Network Rail’s internal policies on infrastructure resilience (CIRIA, 2010).

Timing and type of development within identified sites.

52


Figure 6-1. Flood hazard summary.

A3,

A8-

A10

A4,

A13,

A15

A7 A11 A18 A20 A22 A5 A15aA16-

A17A14 A21

A1-

A2A12 A19

A1-

A2

A3,

A8-

A10,

A12,

A19,

A22

A4,

A7,

A13-

A14,

A15a

A5,

A6,

A11,

A20

A13-

A15,

A16-

A18

A21 A23

A1-

A2,

A16-

A17

A3-

A4,

A18

A5-

A6,

A8-

A11,

A20

A7,

A13-

A15a,

A23

A12,

A19,

A21-

A22

A1-

A4,

A16-

A18

A5-

A6,

A8-

A12,

A19-

A22

A7,

A13-

A15a,

A23

Tidal

Fluvial

Combined

Tidal

Fluvial

Combined

Do Nothing

Maintenance (existing)

21102017-2021 2022-2026

Scenario Flood Risk Source

2012-2016 After 2030 20602027-2030

24 25 26 27 24 25 26 27 24 25 26 27 24 25 26 27

Tidal

Fluvial

Combined

Tidal

Fluvial

Combined

2110

Do Nothing

Maintenance (existing)

Scenario Flood Risk Source2010 2030 2060

Flood Hazard Key

Danger for some

Danger for most

Danger for all

53


Note: For the fluvial flood hazard intervention works assuming structural measures for all hazard levels, flood storage costs conceptually also represent environmental mitigation cost. For the fluvial flood hazard intervention works assuming non-structural measures only for danger to most or all, the environmental mitigation works are costed based on representative values from Defra, being £15K/Ha for terrestrial, water-dependent habitat (amounting to £3,600K over the proposed development profile).

Figure 6-2. Indicative phasing of works.

A3,

A8-

A10

A4,

A13,

A15

A7 A11 A18 A20 A22 A5 A15aA16-

A17A14 A21 A1-A2 A12 A19 A1-A2

A3,

A8-

A10,

A12,

A19,

A22

A4,

A7,

A13-

A14,

A15a,

26

A5,

A6,

A11,

A20,

27

A13-

A15,

A16-

A18,

24-25

A21 A23

Tidal source

Fluvial source

Flood hazard intervention (tidal)

Environmental compensation/mitigation (tidal)

Flood hazard intervention (fluvial), structural measures only for

danger to most or all

Environmental compensation/mitigation (fluvial), structural

measures only for danger to most or all

Flood hazard intervention (fluvial), structural measures for all

hazard levels

Environmental compensation/mitigation (fluvial), structural

measures for all hazard levels

Economic growth potential in Avonmouth-Severnside, £K

Cumulative potential FCRM GiA funding due to existing

properties being at reduced flood risk, £K

Cumulative intervention cost profile, £K (tidal)

Cumulative intervention cost profile, £K (fluvial), structural

measures only for danger to most or all

Cumulative intervention cost profile, £K (fluvial), structural

measures for all hazard levels

Total cumulative intervention cost profile, £K (fluvial and tidal) 11,000 to 22,100 12,300 to 25,300 12,900 to 28,100 13,400 to 32,100 15,500 to 35,700

10,000 to 14,500 11,900 to 14,60011,000 to 14,600 11,400 to 14,600 11,600 to 14,600

Residual ring banks or ground raising for sites A1, A2, A4, A7, A13, A14, A15, A15a, A16, A17, A18, A23, 24, 25, 26.

Site specific environmental mitigation, prior to development.

7,600 10,800 13,500 17,500 21,100

900 1,400 1,500 1,800 3,600

13,070 13,070 13,070 13,070

To enable Avonmouth development sites, raised

revetments and re-engineered embankments along

seaward/landward perimeter of Avonmouth Docks.

Habitat compensation for coastal squeeze and increased footprint provided for by Steart, Congresbury Yeo and Alvington realignment sites.

Site specific environmental mitigation, prior to development.

Flood warning for all sites, except A23 requiring ring banks and flood storage.

To enable Severnside development, wave recurve addition to embankment along Aust to New Passage.

Alternatively, reliance on ground levels, M4 and Severn tunnel banks; however this would allow increasing flood risk

to the Severnside area.

After 20302012-2016 2017-2021 2022-2026 2027-2030

Flood hazard to be addressed by FCRM interventions

0 10,300 26,400 42,400 78,200

13,070

54


Figure 6-3. Indicative phasing of works in graphical form.

2017-2030: Wave recurve addition Or


2017-2030: Reliance on de facto defence

Or Re-engineering of the railway

embankment

2013-2017: Improvements to seaward defences

Or Landward perimeter wall

55


7. Recommended option and scope for next stage

7.1. Recommendations The assessments described in this report have identified a broad recommended option, phasing of works, and potential for funding, described as:

2012-2016. Construction of the landward perimeter wall, or seaward revetments and embankments, along Avonmouth Docks. This will address the source of greatest tidal flood risk to National Planning Policy Framework (NPPF) requirements, particularly to the general Avonmouth development area and Avonmouth village. Non-structural measures could be acceptable to address fluvial flood risks.

2017-2030. Wave recurve wall addition to the Aust to New Passage earth embankment if required. This will address the next greatest source of tidal flood risk to NPPF requirements, particularly to the Severnside development area and the communities of Northwick, Redwick and Pilning. Residual ring banks or ground raising for development sites at unacceptable fluvial flood hazard during the successive epochs 2012-2016, 2017-2021, 2022-2026, and 2027-2030, as they occur. This should address the primary fluvial flood risks to NPPF requirements, flood storage and environmental compensation/mitigation requirements. In addition to this, re-engineering of the railway embankment along Chittening Warth could be considered.

Subsequent to this, the next stages to promote the project would be a) development of the business case. Most of the supporting studies have now been undertaken, and a business case (format dependent on funding route) is now required to formally provide the case for funding the project; and b) development of the detailed design. The required tasks to progress the recommended option 2 for these stages are described below.

7.2. Business case It is understood that the business case will be progressed by the relevant authorities. As it is likely that approximately 10-30% of the scheme funding could be sourced from FCRM GiA, this may well require the business case to meet standard Project Appraisal Report requirements.

To support the business case, it is suggested that further engagement with stakeholders is required, particularly those directly affected by the proposals. This would particularly include Bristol Port Company, Network Rail and the Highways Agency. As the proposals progress to detailed design, further engagement with wider stakeholders and the public will be required.

Further to this, whilst outline design and costing of works to address wave and tidal flood risks are deemed well developed to support the business case, fluvial flood design is not. Outline modelling and design of fluvial flood storage to prove viability and quantify outline costs is required. There is a potentially considerable benefit that could be identified, in that the fluvial flood storage (and environmental mitigation) works may result in no need to construct localised ring banks or ground raising. The findings of this work will necessarily influence the subsequent tasks required to develop the detailed design and associated assessments.

7.3. Site investigations

7.3.1. Topographic survey

Topographic survey of the existing tidal defences (excluding the railway line), and possible landward perimeter wall area, already exists from Atkins (2006) and Atkins (2011). This supports the general viability of improvements in these areas. However, the areas affected by the fluvial flood protection works (ground raising, ring banks and/or flood storage) are not known to have any topographic survey. This would be required to inform the detailed design of any fluvial flood protection works. Considering the extent of the area required to be surveyed, a potential alternative to conventional ground-based survey could be high resolution LiDAR methods.

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7.3.2. Services searches

It is apparent that the recommended option works would at least partly be in heavily urbanised or industrial areas. A detailed services search would need to be undertaken to provide information on existing underground, surface and overground services.

7.3.3. Geotechnical and contamination investigations

Geotechnical investigations were undertaken in Atkins (2006), along the existing tidal defences (excluding the railway line) and possible landward perimeter wall area. These investigations highlighted that improvements in these areas would be viable, but would need to particularly account for settlement over time. However, similar to the topographic survey requirements, the areas affected by the fluvial flood protection works are not known to have any geotechnical investigations. This would be required to inform the detailed design of any fluvial flood protection works. It would be efficient to integrate this with contaminant sampling and testing at the same time.

7.3.4. Unexploded ordnance and bombs

Previous studies (Atkins, 2006; ongoing) have clearly identified that the Avonmouth/Severnside area, particularly around Avonmouth Docks, could be affected by unexploded ordnance or bombs (UXO/UXB). In light of this, an initial desk-study and then detailed UXO/UXB on the ground survey, would be required.

7.4. Engineering design Detailed design activities should be undertaken in compliance with the Construction, Design and Management Regulations (2007). Detailed design of the proposed tidal defence works could proceed once the above investigation findings are known. In addition to this, hydraulic and structural detailed design of the proposed tidal defence works would require application of the latest extreme wave, tide and JPA guidance. To confirm the detailed design it is recommended that physical modelling be undertaken, particularly relating to the possibly more complex designs at Aust to New Passage, and along Avonmouth Docks if the seaward alignment is chosen.

Detailed design of the fluvial flood protection works could proceed once the outline modelling and design is confirmed. The design process would not require physical modelling, but may require focussed, supporting numerical modelling as the design proceeds.

Supporting management plans could be developed at this stage as well, addressing transport and materials management. To prove compliance with NPPF requirements, a scheme FRA should be undertaken also.

In addition to this, early contractor involvement (ECI) could be considered to inform the detailed design process, improving the design quality in relation to appropriateness for construction, and costing. At the conclusion of the detailed design process, a tender contract pack, and the tender process, will need to be undertaken.

7.5. Environmental assessment The following potential impacts have been identified and would need further consideration if the project progressed. In some instances there may be different impacts associated with the works to existing tidal defence improvements compared to those that may arise due to the fluvial flood protection works and therefore, where appropriate, these have been split under the key receptor headings.

7.5.1. Human Beings, Traffic and Transport

Potential Adverse Impacts

Tidal Defence Improvements Fluvial Flood Protection

Potential for temporary noise disturbance during construction. The nature of the immediately affected area will need consideration (i.e. industrial or residential) but for the most part, it is not expected that a noise impact assessment will be required.

Potential for temporary impacts on traffic and transport and therefore careful consideration of construction access routes and compounds will be required.

Temporary impact is expected on the Severn Way Long Distance Trail. Consent to close or divert the trail during construction is likely to be required.

Public rights of way (PRoW) cross a number of the development sites and therefore options to maintain these routes open will need to be investigated. If not, consent to change the route will be required.

Potential Opportunities/Betterments

57


Opportunity to integrate enhanced green infrastructure into the works. This could take the form of improvements to existing infrastructure such as the Severn Way or other PRoW or new amenities.

Improved flood protection will be provided to more than 3,500 residential properties and businesses.

7.5.2. Flora, Fauna and Biodiversity



The study area is known to have potential to support a number of protected species and possibly non-native species (eg Japanese Knotweed) and therefore appropriate ecological surveys will be required. As a start, an extended phase 1 ecology survey should be undertaken of the footprint of the works. Any impacts on protected species and/or valuable habitat as a result of the works would need appropriate mitigation and/or compensation.

Potential for all aspects of works to impact on a number of SNCIs within the BCC boundary. The sites include; Lamplighters Marsh, Gloucester Road Railway Sidings, St Andrews Road Rhine, Salt Rhine and Moorhouse Rhine, Kings Weston Lane Rhine and Hallen Marsh Junction. The details of these sites are required to identify potential impacts and mitigation. Compensation habitat for any losses may be required. It should be noted that SNCIs within SGC boundary were not available at the time of writing and therefore have not been specifically identified within this report.

Potential for temporary construction impacts on the Severn Estuary European Sites and therefore a Habitat Regulations Assessment (HRA) would be expected to be required. As stated within Section 5.4.3, permanent impacts are being compensated for under the SEFRMS and therefore have not been considered a potential impact of this project.

Potential for construction and operation (through loss of habitat/disturbance) impacts on supporting habitat for bird species that are features of the European Sites. A Habitat Regulations Assessment (HRA) would be expected to be required. The HRA for the flood risk management works will need to be taken into account in the preparation of the HRA for the Development Strategy, during consideration of in combination effects.

Potential impacts on the water table and local hydrology and the associated ditch/water body network. As a result there could be impacts on habitats suitable for protected species such as water vole and great crested newt. Further investigation would be required as the project progresses.


Opportunities to create habitat and potentially offset impacts on European Sites arising from the delivery of the Strategy, through integration with flood storage areas, should be investigated further as the project progresses.

The need for material to complete the works means that onsite borrow pits could be sourced, these could be developed into fluvial flood compensation areas that also provide wetland habitat.

7.5.3. Land Contamination



Due to the nature of the past and present land use of the study area, there is a risk that any excavation works could disturb contaminated land. Minimal excavation is expected to be associated with working on the existing tidal defences but extensive ground works and excavation are expected to be required for the development sites and fluvial flood protection. Therefore, further contaminated land assessment would be required as the project progresses and more detail becomes available.


Winning material from onsite borrow pits provides an opportunity to implement ground contamination remediation works, which would be an environmental betterment for the area.

Holding the line of existing tidal defences protects land that may be contaminated and therefore reduces the risk of associated pollution through flooding.

7.5.4. Landscape



Improving the tidal defences within and around the Avonmouth docks and industrial area is not expected to have significant landscape impact as much of the area is of low landscape value and is not easily accessible or visible by the general public or residential dwellings. Elsewhere further consideration would need to be given to impacts on views from residential areas. Overall, any

Ground raising or ring-banks are expected to be minimal in height (up to 0.5m) and therefore, are not expected to have significant landscape impacts. However, as with the tidal defence works, further consideration to specific landscape impacts will need to be completed during detailed design.

58


increases in the height of tidal defences (up to 1.75m) are expected to be imperceptible over the wider landscape area. Further consideration of specific landscape impacts, including finishes to any raised defences, will need to be completed during detailed design.

Potential impact on historic landscape features such as hedgerows and rhines and possibly on fields with the historic ridge and furrow features. Options to minimise impacts would need exploring during detailed design.


Opportunities to improve the appearance of existing tidal defences and associated amenities (e.g. accessible areas of the tidal defences) should be considered as the project progresses.

7.5.5. Historic Environment



The route of the existing tidal defence passes in close proximity to a SM (historic anti-aircraft battery site) and therefore there is potential to impact on the site. No direct impacts are expected but access requirements for the works may require SM consent from English Heritage. It should be noted that current development within the study area is already being undertaken in close proximity to this SM.

Ground raising or ring-banks have potential to impact on the other SM within the study area (the Mere Bank and flanking ditches). Once more detail is known about this work further consideration will need to be given to ensuring no impact.

Working on the existing tidal defences is expected to have little risk of impacting on known or as yet undiscovered archaeological assets. However, it should be noted that the defence embankments themselves may have historic value and therefore consultation is required with the local authority archaeologist once more detail of the works is known. Archaeological site investigation may be required.

Excavation associated with the development sites and ground raising or flood storage has a greater potential to impact on archaeological features. An archaeological evaluation of the works footprint should be undertaken and in some cases, archaeological site investigation may be required.


Tidal defence improvements provide protection to historic features that otherwise may be lost or impacted upon by flooding.

7.5.6. Overview

It is likely that the flood risk management works under Option 2 will be undertaken in phases as and when the requirements arise (i.e. due to sea level rise or when development sites come on line). It is therefore likely that individual environmental assessments may need to be undertaken but with an overarching view on potential in combination effects from all the works.

The works are expected to require planning permission and will therefore fall under the Town and Country Planning (Environmental Impact Assessment) Regulations. A screening decision would need to be sought from the local planning authorities (BCC and SGC) as to the need for a statutory EIA under these regulations. In addition to this a Water Framework Directive assessment would be likely to be needed.

Regardless of whether statutory EIA is required, some form of more formal environmental assessment will be required to support the business case and ultimately the detailed design and any planning applications. As part of this process, the following environmental tasks are expected to be required:

An Extended Phase 1 Ecology Survey and any further Phase 2 surveys that may be identified.

A Habitat Regulations Assessment taking into account potential construction impacts associated with all works and potential permanent impacts that may arise from loss of supporting bird habitat due the fluvial flood protection works.

A contaminated land desk study that focuses on the development areas which are likely to be subject to ground raising/ring banks and areas identified for borrow pits/flood storage/habitat creation.

A historic environment desk study that focuses on the development areas which are likely to be subject to ground raising/ring banks and areas identified for borrow pits/flood storage/habitat creation.

Consultation with environmental consultees, for example; local planning authorities, English Heritage, Natural England and the Environment Agency.

In addition to planning, the following consents are likely to be required:

For works below mean high water, a marine licence is expected to be required under the Marine and Coastal Access Act (2009) (MCAA) from the Marine Management Organisation (MMO).

Permanent Flood Defence Consent is expected to be required from the Environment Agency.

59


8. References

Amion and WYG, 2012. Avonmouth/Severnside Outline Development Strategy.

Amion, 2013. Enterprise Zone and Enterprise Areas – Growth Model – Technical Report.

Atkins, 2006. Avonmouth to Aust Tidal Defence Scheme.

Atkins, 2010. Severn Estuary Shoreline Management Plan 2.

Atkins, ongoing. Severn Estuary Flood Risk Management Strategy.

Bristol City Council, 2011. BCC Adopted Core Strategy.

Bristol City Council, 2011. Bristol Local Economic Assessment.

Bristol City Council, 2012. Bristol Development Framework – Central Area Action Plan. Options Consultation.

Bristol City Council, 2013. Growth Potential model outputs matrix.

Bristol City Council, 2013. Site Allocations and Development Management Policies, Bristol Local Plan.

Bristol City Council. Options for Site Allocations and Proposed Designations – Avonmouth and Kingsweston, Bristol Development Framework.

Bristol City Council, 2012. Central Area Action Plan, Bristol Development Framework. Options Consultations.

Bristol City Council. The Bristol 20:20 Plan; Bristol’s Sustainable City Strategy.

Bristol City Council, 2011. Bristol Development Framework Core Strategy (Adopted June 2011).

Bristol Parks and Green Space Strategy.

Buro Happold, 2011. Avonmouth / Severnside Integrated Development Strategy – Flood Risk. Flood Risk Management Study.

Capita Symonds, 2011. Avonmouth/Severnside Strategic Flood Risk Assessment Level 2.

Cresswell Stage 1: Distribution of Wetland Birds within the Study Area.

Cresswell Stage 2: Review of Consent at Severnside and Avonmouth Impact Assessment.

Environment Agency, 2007. Bristol Avon Catchment Flood Management Plan.

Environment Agency, 2007. Severn Tidal Tributaries Catchment Flood Management Plan.

Halcrow, 2009. BCC Strategic Flood Risk Assessment Level 1.

JBA, 2011. BCC Preliminary Flood Risk Assessment.

JBA, 2012. Somerset North Coast Flood Warning Improvements.

Scott Wilson, 2009. SGC Strategic Flood Risk Assessment Level 1.

Severn Estuary Coastal Group, 2010. Severn Estuary Shoreline Management Plan 2.

Severn Estuary Rapid Coastal Zone Assessment Survey: National Mapping Programme South Gloucestershire Council Economic Development Strategy 2012-2016.

60


South Gloucestershire Council Economic Development Strategy 2008-2015.

South Gloucestershire Economic Review (2007).

South Gloucestershire Council, 2011. Local Economic Assessment.

South Gloucestershire Council, 2012. Strategy 2012-2016.

South Gloucestershire Council, 2010. Core Strategy; Planning for Future Development in South Gloucestershire.

WYG, 2012. Avonmouth/Severnside Integrated Development, Infrastructure, and Flood Risk Management Study.

61


Appendix A: Flood Risk Interpretation

62


A.1. Consideration of ‘Somerset North Coast Flood Warning Improvements’, JBA (2012).

Common comments

General climate change: Not relevant as solely present day modelling.

Wave-tide JPA: JPA not done. The wave-tide combinations are effectively very extreme events.

Fluvial-tidal JPA: Not relevant.

Wave extremes: Wave climate does not need to be translated.

Wave overtopping: Assessment applies current EuroTOP guidance.

Fluvial extremes: Not relevant.

Tidal extremes: New EA (2011) GIS dataset used; still valid.

Scenario reference Physical parameters

Extreme wave and JPA

translation discussion

Other translation

discussion

sev24010_10-100d_s

Force 6, 240degN, 10-

100yrEWL, c1.14mHs

10yr EWL results translate

to c0.1%AEP JPA. Greater

than 25yr EWL is

>0.1%AEP.

With 0.7m of SLR, the 10yr

EWL results translate to

c1%AEP JPA.

sev24010_10-100d_s


100yrEWL, c1.75mHs

All EWL results translate to

>0.1%AEP JPA.



c0.1%AEP JPA.

sev24010_10-100d_s


100yrEWL, c2.43mHs


>0.01%AEP JPA.



>0.1%AEP JPA.

sev27010_10-100d_s


100yrEWL, c1.16mHs



than 25yr EWL is

>0.1%AEP.



c1%AEP JPA.

sev27010_10-100d_s


100yrEWL, c1.81mHs


>0.1%AEP JPA.



c0.1%AEP JPA.

sev27010_10-100d_s


100yrEWL, c2.52mHs


>0.01%AEP JPA.



>0.1%AEP JPA.

sev30010_10-100d_s


100yrEWL, c1.08mHs



than 25yr EWL is

>0.1%AEP.



c1%AEP JPA.

sev30010_10-100d_s


100yrEWL, c1.72mHs


>0.1%AEP JPA.



c0.1%AEP JPA.

sev30010_10-100d_s


100yrEWL, c2.43mHs


>0.01%AEP JPA.



>0.1%AEP JPA.

63


A.2. Consideration of ‘Strategic Flood Risk Assessment - Level 2, Avonmouth/Severnside’, Capita Symonds (2011).

Figure

referenceBase definition Physical parameters Extreme tide data Discussion on extreme tides and breaches

7.1AVM_05m_2T_100yF12h_2010_WD1_347_d

(maxmax)_g002.502yr tidal, 100yr fluvial (12hr) - 2010

7.1AVM_05m_2T_1000yF12h_2010_WD1_348_

d(maxmax)_g002.502yr tidal, 1000yr fluvial (12hr) - 2010

7.1AVM_05m_20T_20yF12h_2010_WD1_347_d

(maxmax)_g002.5020yr tidal, 20yr fluvial (12hr) - 2010

20yr tidal EWL used (2007) was 8.78mAOD.

New EA (2011) GIS dataset (ch380) gives

T20=8.67mAOD (no CI).

Extreme tide return period translated to 25-

50yr.

7.1AVM_20m_200T_2yF12h_2010_UD1_445_d(

maxmax)_g010200yr tidal, 2yr fluvial (12hr) - 2010

7.19Actual Risk: Peak Flood Depths , Current

Case200yr tidal, 2yr fluvial - 2010

7.20Actual Risk: Peak Flood Hazard (DEFRA),

Current Case200yr tidal, 2yr fluvial - 2010

7.1AVM_20m_1000T_2yF12h_2010_MaxVol_UD

1_445_d(maxmax)_g0101000yr tidal, 2yr fluvial (12hr) - 2010

7.1AVM_20m_1000T_2yF12h_2010_UD1_445_d

_g010_Max1000yr tidal, 2yr fluvial (12hr) - 2010

7.1AVM_20m_1000T_2yF24h_2010_UD1_445_d

_g010_Max1000yr tidal, 2yr fluvial (24hr) - 2010

7.21Residual Risk: Peak Flood Depths, Current


7.22Residual Risk: Flood Hazard (DEFRA),

Current Case1000yr tidal, 2yr fluvial - 2010

7.14M49 Culvert Blockage Impacts (200yr Fluvial

and 2 yr Tidal Future Case2yr tidal, 200yr fluvial - 2110

7.13M49 Culvert Blockage Impacts (1000yr Fluvial

and 2 yr Tidal Future Case2yr tidal, 1000yr fluvial - 2110

7.15Residual Risk: Peak Flood Depths, Future


7.16 Residual Risk: Flood Hazard, Future Case 2yr tidal, 1000yr fluvial - 2110

7.3 Actual Risk: Peak Flood Depths Future Case 200yr tidal, 2yr fluvial - 2110

7.4Actual Risk: Flood Hazard (DEFRA) Future


7.5Actual Risk Inundation over time 200 year

tide and 2year fluvial future case200yr tidal, 2yr fluvial - 2110

7.7Breach 2 - 200yr Tide and 2yr Fluvial Future


As above. Breach location for the

embankment south of Severn Beach. Breach

width similar to various more recent

R&D/guidance. Still valid.



As above. Breach location for the

embankment along Northwick Oaze. Breach

width similar to various more recent


7.6Breach 1 - 1000 yr Tide and 2yr Fluvial

Future Case1000yr tidal, 2yr fluvial - 2110

Extreme tide return period translated to 500yr

(2010). Application of 1m to 500yr RP (2010)

gives 10.33mAOD. This is equivalent to

>1000yr (M95% at 2110). Breach location for

the embankment south of Severn Beach.

Breach width similar to various more recent


7.8Breach 3 - 1000yr Tide and 2 yr Fluvial

Future Case1000yr tidal, 2yr fluvial - 2110

As above. Breach location for the wave

recurve wall and revetment at Severn Beach.

Breach width similar to various more recent

R&D/guidance, although unlikely to breach.

7.9Breach 4 - 1000yr Tide 2yr Fluvial Future


As above. Breach location for the wave

recurve wall and revetment north of Severn

Beach. Breach width similar to various more

recent R&D/guidance, although unlikely to

breach.



As above. Breach location for the revetment

north of Severn Beach. Breach width similar

to various more recent R&D/guidance,

although unlikely to breach.

Fluvial extremes: BTP model used for fluvial modelling, with FEH. Noted that LSDB may have updated hydrology. Extreme flows (2010) do not need to be translated, although will need updating in Phase 2;

application of +30% is equivalent to beyond 2110 (M95%).

Tidal extremes: discussed within the table.

Common comments

General climate change: Use of Defra 2006 CC guidance gives 1m SLR, +30% flow, +10% waves. Future scenarios are now superseded by EA (2011).

Wave-tide JPA: ABPmer (2005) JOIN-SEA JPA used: still valid.

Fluvial-tidal JPA: No fluvial-tidal JPA undertaken. Simplified Method JPA in SEFRMS notes moderate dependence between fluvial and tidal events. All modelled events effectively represent varying fluvial-

tidal pairs on the 0.5% to 0.1% JP AEP curve.

Wave extremes: Wave climate does not need to be translated, although wave overtopping is superseded. Climate change guidance is superseded, although impact likely to be minimal (now no clear view,

rather than +10% in Defra, 2006).

Wave overtopping: Owen (W178) method used: multipley superseded by Besley (1999) and EuroTOP (2005).

200yr tidal EWL used (2007) was

9.16mAOD. New EA (2011) GIS dataset

(ch380) gives T200=9.11mAOD,


Extreme tide return period translated to

250yr.




Extreme tide return period relatively

unchanged at 2yr.



(ch380) gives T1000=9.43mAOD (no CI).

Extreme tide is equivalent to 250yr RP



1000yr (M95% at 2110).



(ch380) gives T200=9.11mAOD,





Extreme tide is relatively unchanged at 2yr


gives 9.29mAOD. This is equivalent to 10-

20yr (M95% at 2110).



(ch380) gives T1000=9.43mAOD (no CI).

Extreme tide return period translated to

500yr.

64


A.3. Consideration of ‘Avonmouth to Aust Tidal Defence Scheme’, Atkins (2008).

Scenario reference Extreme tide dataTranslation of extreme tides (2010) Translation of extreme tides (2110)

1yr wave-tidal event


8.29mAOD. New EA (2011) GIS

dataset (ch380) gives T1=8.16mAOD

(no CI).

Extreme tide return period translated

to 2-5yr.

Application of 1m to 1yr RP (2010)


10-20yr (M95% at 2110).




dataset (ch380) gives T5=8.43mAOD

(no CI).


to 10yr.



50yr (M95% at 2110).




dataset (ch380) gives



to 20yr.



100yr (M95% at 2110).







to 25-50yr.



150yr (M95% at 2110).







to 75yr.



300yr (M95% at 2110).






No change to extreme tide return

period.



500yr (M95% at 2110).







to 150yr.


gives 10.06mAOD. This is equivalent

to 500-1000yr (M95% at 2110).

1000yr wave-tidal

event




T1000=9.43mAOD (no CI).


to 300-500yr.

Application of 1m to 1000yr RP

(2010) gives 10.22mAOD. This is

equivalent to 1000-10000yr (M95% at

2110).

Fluvial extremes: Not relevant.

Tidal extremes: discussed within the table.

Wave extremes: Wave climate does not need to be translated. Climate change guidance is superseded, although impact likely to be

General climate change: Use of Defra 2006 CC guidance gives 1m SLR, +30% flow, +10% waves. Future scenarios are now

Common comments

Wave-tide JPA: ABPmer (2005) JOIN-SEA JPA used: still valid.

Fluvial-tidal JPA: Not relevant.

Wave overtopping: Assessment applies current EuroTOP guidance.

65


Appendix B: Cost estimates

66


B.1. Unit Cost Estimate Matrix.

Height (m) 0.25 299

0.5 599

0.75 898

1 1198

1.25 1474

1.5 1750

1.75 2026

2 2302

2.25 2302

2.5 2302

3 2302

3.25 2667

3.5 3031

3.75 3395

4 3759

4.5 4662

5 5566

5.5 6641

6 7717

Height increase (m) 0.25 152

0.5 305

0.75 635

1 965

1.25 1071

1.5 1177

2 1349

2.5 1784

2.75 2149

3 2515

Height (m) 0.25 243

0.5 486

0.75 730

1 973

1.25 1216

1.5 1459

1.75 1702

2 1946

2.25 2189

2.5 2432

2.75 2675

3 2919

3.25 3162

3.5 3405

4 3891

4.5 4378

5 4864

5.25 5107

5.5 5351

6 5837

Height (m) 0.25 299

0.5 598

0.75 897

1 1197

1.25 1666

1.5 2136

2 3075

2.5 3747

3 4420

Height (m) 0.25 518

0.5 1036

0.75 1554

1 2072

1.25 2121

1.5 2169

2 2267

2.5 2403

3 2540

3.5 2783

4 3027

4.5 3270

5 3513

5.5 3702

6 3891

Height (m) 2 647

2.5 817

3 986

Depth (m) 0.25 340

0.5 681

0.75 1021

1 1362

1.25 1702

1.5 2043

2 2724

2.25 3064

2.5 3405

2.75 3745

3 4086

Height (m) 0.25 155

0.5 309

0.75 464

1 618

1.5 927

Cost per m 52

No. of fields (100mx250m) 1 31347

2 62694

3 94041

4 125388

Sediment type (per m3) Mud N/A

Sand 22

Gravel 22

Flood resilience

Cost per individual property 10809

Type (cost per km) Earth Embankment 1081

Wall 5405

Rock armour 5405

Concrete Revetment 5405

Type (cost per polder) Polder fields 811

Small (<400) 37500

Medium (400-900) 80000

Large (>900mm) 225000

Small (<400) 75000

Medium (400-900) 160000

Large (>900mm) 450000

Monitoring

Type (cost per km per year) Visual monitoring 1081

Managed realignment works

Cost per structure RTE 300000

Cost per 500m Embankment breach 150000

Flood warning and forecasting

Cost per system 1 5405

Replacement of Outfalls (£ per item)

Replacement costs of outfalls/sluices taken from the Parrett Strategy (January 2009)

Annual visual maintenance estimated as £1000/km, based on PEFRMS and RGW PAR. Cost is at March 2009 base date.

Generation of flood warning system for Gwent Levels, Lympstone and Shaldon indicate a small cost of £5000 per system. This only includes for

design and implementation of a system, not notification of residents etc. Cost is at March 2009 base date.

Vehicle access EA cost for vehicle access was based on tarmac/concrete access track. Atkins review cost of £48/m is much less and based on a stone track. It is

likely the majority of the access tracks would be stone rather than tarmaced, hence the Atkins review rate has been used. Cost is at March 2009

base date.

Polder fields (wet construction)

Polders assumed to be timber stake/willow spiling construction with 1.5m height. Rumney Great Wharf is the only example available. Cost is at 2009

base date.

RTE estimated as £300,000 and breach estimated as £150,000 per 500m, all sourced from Slimbridge MR FS (Atkins, 2009).

Foreshore recharge

Refurbishment of Outfalls (£ per item)

Refurbishment costs of outfalls/sluices taken from the Parrett Strategy (January 2009).

Wave recurve wall added to existing structure (dry construction)

Embankment maintenance cost sourced from PEFRMS. Hard engineered structures maintenance cost sourced from Clevedon to St. Thomas's

Head Strategic Overview. Polder field maintenance from Rumney Great Wharf PAR. Costs are at March 2009 base date. Check with EA Midlands

SAMPs data indicates embankment maintenance cost of ~£13.7K/10km=£1.4K/km (Longney to Elmore), ~£1.8K/5km=£0.4K/km (Sharpness to New

Grounds). This broadly agrees with used embankment maintenance costs of £1.08/km.

Single height of wave recurve wall designed for: indicative of similar type to those used in Caldicot Levels SDI. Cost is at March 2009 base date.

GQ sourcing this information. Previous estimates include £20000 per property (Atkins estimate) and £2500 per property (for 46 properties in

Appleby, Cumbria, quoted at CIWEM10). Obviously a wide potential range; £10000 taken as representative.

Maintenance of existing structures

Final sand/gravel cost taken from Defra R&D project FD1923 looking at costs from a wide range of recharge techniques and beach locations (a

range of £18-23/m3 was found, with an average of £21/m3 being used). Mud recharge is difficult and rare: Rumney Great Wharf feasibility study

on mud recharge found costs and practicalities (at a very viable site) were significant, making mud recharge impractical. Cost is at 2009 base date.

Cut-off structure

Costs based on EA data. Atkins review using Spons developed costs within 20% of EA data.

Use of ground anchors for concrete revetments could reduce EA cost by 50%, but cannot be assumed to occur everywhere: higher EA cost used,

with further detailed costs consideration to be carried out at scheme level. Cost is at March 2009 base date.

Retaining wall (wet construction)

EA cost and Atkins review cost agree. Cost is at March 2009 base date.

Rock armour of front face/toe protection (wet construction)

EA cost for the rock armouring of the front face add-on used in lieu of any further data. Cost is at March 2009 base date.

FRM Response Type Base price (£) (for rural, fluvial location) ** NOTE ALL PRICES ARE AT MAR 2009 BASEDATE **

New earth embankment (dry construction)

Geometry assumed to be 1/3 front slope, 5m crest width, 1/4 back slope as agreed with GQ. Atkins review using Spons developed a cost of

£2130/m for 2m high embankment; this will be used as it is for where material cannot be sourced from a local borrow pit, which from SW

experience may not always be possible. In addition to this, the use of non-local material would avoid impact on heritage (noted as an important point

by heritage stakeholders). The ratio difference has been applied to all embankment heights. Cost is at March 2009 base date.

Freestanding masonry wall (dry construction)

EA cost indicated £467/m for 1m high wall. This is based on masonry walls with no internal reinforcement (i.e. concrete). Atkins review using Spons

developed a cost of £1107/m for 1m high wall; this will be used as it is for reinforced, FRM engineered walls. This ratio difference has been applied

to all wall heights. Cost is at March 2009 base date.

Improved earth embankment (dry construction)

EA cost and Atkins review cost agree. Cost is at March 2009 base date.

Concrete revetment to existing embankment (dry construction)

67


B.2. Strategic ground raising, accounting for SLR to 2030 and 2110.

Ground raising - wider area

QuantityUnit Rate

(£/Unit)Cost (£)

Land Raising (titles ref. individual sites)Volume

(total)

Total Area 7,130,000

7,130,000

Excavation of material to provide key for raising (subsoil) 310,000 5.00£ 1,550,000£

Removal of excavated material from site 310,000 10.00£ 3,100,000£

Geotextile to base of raised area 3,100,000 5.00£ 15,500,000£

Imported fill material (assume clay to provide cut-off) 6,665,000 20.00£ 133,300,000£

For areas where raising = 0.1m

Preparation of upper surface of fill 3,100,000 1.00£ 3,100,000£

For areas where raising > 0.1m

Geofabric (Terram 1000) 3,100,000 1.00£ 3,100,000£

Surfacing

Sub-base (Type 1 material, 150mm thk) 3,100,000 4.50£ 13,950,000£

Blinding Concrete (100mm thk) 3,100,000 5.00£ 15,500,000£

Sub-total 189,100,000£

Design Fee 9,455,000£

Surveys & Ground Investigation (inc. utilities, UXO, contam land classification) 18,910,000£

Supervision 9,455,000£

Contractors Prelims, site set-up and supervision, traffic management 47,275,000£

274,195,000£

Optimism Bias 164,517,000£

438,712,000£

Land raising between 0 to 3.3m (c2.3m average) for a development area of 3,100,000m2, accounting for 20 year sea level rise.

Flood storage offsetting assumed to be similar magnitude.

Ground raising - wider area

QuantityUnit Rate

(£/Unit)Cost (£)

Land Raising (titles ref. individual sites)Volume

(total)

Total Area 9,300,000

9,300,000

Excavation of material to provide key for raising (subsoil) 310,000 5.00£ 1,550,000£

Removal of excavated material from site 310,000 10.00£ 3,100,000£

Geotextile to base of raised area 3,100,000 5.00£ 15,500,000£

Imported fill material (assume clay to provide cut-off) 8,835,000 20.00£ 176,700,000£

For areas where raising = 0.1m

Preparation of upper surface of fill 3,100,000 1.00£ 3,100,000£

For areas where raising > 0.1m

Geofabric (Terram 1000) 3,100,000 1.00£ 3,100,000£

Surfacing

Sub-base (Type 1 material, 150mm thk) 3,100,000 4.50£ 13,950,000£

Blinding Concrete (100mm thk) 3,100,000 5.00£ 15,500,000£

Sub-total 232,500,000£

Design Fee 11,625,000£

Surveys & Ground Investigation (inc. utilities, UXO, contam land classification) 23,250,000£

Supervision 11,625,000£

Contractors Prelims, site set-up and supervision, traffic management 58,125,000£

337,125,000£

Optimism Bias 202,275,000£

539,400,000£

Land raising between 0 to 4m (c3m average) for a development area of 3,100,000m2, accounting for 100 year sea level rise. Flood storage

offsetting assumed to be similar magnitude.

68


B.3. Residual ring banks, accounting for SLR to 2030 and 2110.

Ring banks - localised

Development phasesPerimeter of

site (m)Rate (£/m) Cost (£)

2013-2016: A4, A7, A13, A15, A18 5,455 599 3,266,895

2017-2021: A15a, A16, A17 2,289 599 1,370,446

2022-2026: A14 1,992 599 1,192,728

2027-2030: A1, A2 3,049 599 1,708,105

After 2030: A23, 24, 25, 26 2,627 599 1,573,120

Sub-total 9,111,295£

Design Fee 5% 455,565£

Surveys & Ground Investigation (inc. utilities, UXO, contam land classification) 10% 911,130£

Supervision 5% 455,565£

Contractors Prelims, site set-up and supervision, traffic management 25% 2,277,824£

Total 13,211,378£

Optimism Bias 60% 7,926,827£

21,138,204£

Residual fluvial flood risks would be addressed via ring banks of up to 0.5m height, accounting for 20 year flow increase. Required

flood storage offsetting would be less than this.

Ring banks - localised

Development phasesPerimeter of

site (m)Rate (£/m) Cost (£)

2013-2016: A4, A7, A13, A15, A18 5,455 599 3,266,895

2017-2021: A15a, A16, A17 2,289 599 1,370,446

2022-2026: A14 1,992 599 1,192,728

2027-2030: A1, A2 3,049 599 1,708,105

After 2030: A23, 24, 25, 26 2,627 599 1,573,120

Up to 2110 16,256 Variable 3,785,764

Sub-total 12,897,059£


Surveys & Ground Investigation (inc. utilities, UXO, contam land classification) 10% 1,289,706£



Total 18,700,736£


29,921,177£

Residual fluvial flood risks would be addressed via ring banks of up to 0.5m height, accounting for 100 year flow increase. Required

flood storage offsetting would be less than this.

69


B.4. Tidal defence improvements, accounting for SLR to 2030.

Relevant

placenameImprovement 2

%

AE

P

So

P

1%

AE

P

So

P

0.5

%

AE

P

So

P

0.1

%

AE

P

So

P

Earth embankment

with wave recurve380 380 1,712 3,334

Severn Beach

wave wall

108 108 108 108

Rock armoured

embankment

6 6 6 6

Wave wall 89 89 89 89

Railway

embankment392 392 392 392

BPC defences, 346 346 346 1,054

BPC defences, 77 77 77 77

BPC defences, 147 147 147 147

Grass embankment

(clay core)4,606 4,606 6,389 8,172

Earth embankment 73 73 73 73

Severn Beach

wave wall

108 108 108 108

Rock armoured

embankment

6 6 6 6

Wave wall 89 89 89 89

Railway


BPC defences, 346 346 346 1,054

BPC defences, 77 77 77 77

BPC defences, 147 147 147 147

Grass embankment

(clay core)4,606 4,606 6,389 8,172

0

Earth embankment 73 73 73 73

Severn Beach

wave wall

108 108 108 108

Rock armoured

embankment

6 6 6 6

Wave wall 89 89 89 89

Railway


Perimeter wall 1,048 1,048 2,598 2,598

Perimeter wall 1,201 1,201 3,370 3,370

Perimeter wall 1,030 1,030 2,889 2,889

Perimeter wall 1,802 1,802 5,055 5,055

0

Earth embankment 380 380 1,712 3,334

Severn Beach

wave wall

108 108 108 108

Rock armoured

embankment

6 6 6 6

Wave wall 89 89 89 89

Railway


Perimeter wall 1,048 1,048 2,598 2,598

Perimeter wall 1,201 1,201 3,370 3,370

Perimeter wall 1,030 1,030 2,889 2,889

Perimeter wall 1,802 1,802 5,055 5,055

Option 2d

Option 2c

Option 2b

Option 2a

Description Cost for best value defence hardening when and where necessary (£K)

70


B.5. Tidal defence improvements, accounting for SLR to 2110.

Relevant

placename

FRM asset

description 2%

AE

P

So

P

1%

AE

P

So

P

0.5

%

AE

P

So

P

0.1

%

AE

P

So

P

Earth embankment

with wave recurve5,289 6,287 8,054 9,677

Severn Beach

wave wall

540 540 540 540

Rock armoured

embankment

28 28 28 28

Wave wall 443 443 443 443

Railway

embankment2,350 2,350 2,350 3,514

BPC defences, 2,438 3,146 3,146 3,854

BPC defences, 384 384 488 979

BPC defences, 735 735 735 1,875

Grass embankment

(clay core)11,831 11,831 13,614 15,397

Grass embankment

(clay core)363 363 363 363

Severn Beach

wave wall

540 540 540 540

Rock armoured

embankment

28 28 28 28

Wave wall 443 443 443 443

Railway

embankment2,350 2,350 2,350 3,514

BPC defences, 2,438 3,146 3,146 3,854

BPC defences, 384 384 488 979

BPC defences, 735 735 735 1,875

Grass embankment

(clay core)11,831 11,831 13,614 15,397

Grass embankment

(clay core)363 363 363 363

Severn Beach

wave wall

540 540 540 540

Rock armoured

embankment

28 28 28 28

Wave wall 443 443 443 443

Railway

embankment2,350 2,350 2,350 3,514

Landward 3,570 3,570 5,119 4,995

Landward 3,664 3,664 5,833 5,660

Landward 3,996 3,996 5,855 5,707

Landward

perimeter wall6,993 6,993 10,246 9,987

Earth embankment

with wave recurve5,289 6,287 8,054 9,677

Severn Beach

wave wall

540 540 540 540

Rock armoured

embankment

28 28 28 28

Wave wall 443 443 443 443

Railway

embankment2,350 2,350 2,350 3,514

Landward 3,570 3,570 5,119 4,995

Landward 3,664 3,664 5,833 5,660

Landward 3,996 3,996 5,855 5,707

Landward

perimeter wall6,993 6,993 10,246 9,987

Option 2d

Option 2c

Option 2b

Option 2a

Description Cost for best value defence hardening when and where necessary (£K)

71


B.6. Road raising, accounting for SLR to 2030 and 2110.

Road raising

Unit QuantityUnit Rate

(£/Unit)Cost (£)

Excavation of material to provide key for road m3

16,053 5.00£ 80,263£

Removal of excavated material from site m3

16,053 10.00£ 160,526£

Geotextile to base of raised area m2

160,526 5.00£ 802,630£

Fill material (assume clay to provide cut-off) m3

####### 15.00£ 213,812,057£

Preparation of slopes m2

107,847 1.00£ 107,847£

Topsoil m3

21,569 5.00£ 107,847£

Seeding of slopes m2

107,847 0.65£ 70,101£

Surfacing

Geofabric (Terram 1000) m2

58,215 1.00£ 58,215£

Road surfacing (Rolled Asphalt 40mm surface course) m2

58,215 8.00£ 465,720£

Road surfacing (Rolled Asphalt 60mm binder course) m2

58,215 10.00£ 582,150£

Road surfacing (Rolled Asphalt 200mm sub base) m2

58,215 22.00£ 1,280,730£

Line Painting m 31,048 1.00£ 31,048£

Kerbs (125 x 255) m 15,524 20.00£ 310,480£

Kerbs foundation (200 x 100) m 15,524 2.50£ 38,810£

Sub-total 217,908,424£

Design Fee 5% 10,895,421£


Supervision 5% 10,895,421£


Total 315,967,214£


505,547,543£

Road raising for 7762m, of an average height of 1.5m, accounting for 20 year sea level rise

Road raising


(£/Unit)Cost (£)

Excavation of material to provide key for road m3

16,053 5.00£ 80,263£


16,053 10.00£ 160,526£


160,526 5.00£ 802,630£


######## 15.00£ 313,478,649£


107,847 1.00£ 107,847£

Topsoil m3

21,569 5.00£ 107,847£


107,847 0.65£ 70,101£

Surfacing


58,215 1.00£ 58,215£

Road surfacing (Rolled Asphalt 40mm surface course) m2

58,215 8.00£ 465,720£

Road surfacing (Rolled Asphalt 60mm binder course) m2

58,215 10.00£ 582,150£

Road surfacing (Rolled Asphalt 200mm sub base) m2

58,215 22.00£ 1,280,730£

Line Painting m 31,048 1.00£ 31,048£

Kerbs (125 x 255) m 15,524 20.00£ 310,480£

Kerbs foundation (200 x 100) m 15,524 2.50£ 38,810£

Sub-total 317,575,016£

Design Fee 5% 15,878,751£


Supervision 5% 15,878,751£


Total 460,483,773£


736,774,036£

Road raising for 7762m, of an average height of 2.2m, accounting for 100 year sea level rise

72


B.7. Railway embankment engineering, accounting for SLR to 2030 and 2110.

Road raising


(£/Unit)Cost (£)

Excavation of material to provide key for railway m3

4,773 5.00£ 23,864£


4,773 10.00£ 47,728£


47,728 5.00£ 238,640£


110,580 15.00£ 1,658,697£


25,769 1.00£ 25,769£

Topsoil m3

5,154 5.00£ 25,769£


25,769 0.65£ 16,750£

Surfacing


23,500 1.00£ 23,500£

Surfacing

Sub-base (Type 1 material, 150mm thk) m2

23,500 4.50£ 105,750£

Rail Infrastructure (assume new) m 2,350 1,500.00£ 3,525,000£

Sub-total 5,691,467£


Surveys & Ground Investigation (inc. utilities, UXO, contam land classification) 10% 569,147£



Network Rail Fees

Design 8% 455,317£

Management 10% 569,147£

Total 9,277,091£


14,843,346£

Re-engineering the railway embankment for FCRM. The outline dimensions for design are 2.35km length, and an average embankment height increase of

1.75m (seaward toe level of 7.75mAOD, existing crest level of 9.5mAOD)

73


Appendix C: FCRM GiA estimate

74


C.1. FCRM GiA calculator.

FCRM Partnership Funding Calculator for Flood and Coastal Erosion Risk Management Grant in Aid (FCRM GiA)

Version 7 April 2013

Avonmouth-Severnside, 0.1% AEP 2010 to 2030

Unique Project Reference

Key

All figures are in 'thousands of pounds' (£k) \z\z

Figures in Blue to be entered onto MTP

SUMMARY: prospect of FCRM GiA funding

Scheme Benefit to Cost Ratio: 11.22 to 1

Effective return to taxpayer: 11.22 to 1

Raw Partnership Funding Score 98% (1) Effective return to area: n/a to 1

External Contribution or saving required to achieve an Adjusted Score of 100% 301 (2)

Adjusted Partnership Funding Score (PF) 98% (3)

PV FCRM GiA towards the up-front costs of this scheme (Cost for Approval) - (4)

1. Scheme details

Risk Management Authority type of asset maintainer EA (5) Yes (6)

Duration of Benefits (years) 20 (7)

PV Whole-Life Benefits: 150,070 (8)

PV Costs

PV Appraisal Costs (9)

PV design & Construction Costs 13,378 (10)

Sub Total - PV Cost for Approval (appraisal,design,construction) 13,378 (11)

PV Post-Construction Costs (12)

PV Total Whole-Life Costs: 13,378 (13)

PV Contributions secured to date

PV Local Levy secured to date (14)

PV Public Contributions secured to date (15)

PV Private Contributions secured to date (16)

PV Funding form other Environment Agency functions/sources secured to date (17)

PV Total Contributions secured to date 0 (18)

WARNING: Contributions less than minimum required in cell (2)

2. Qualifying benefits under Outcome Measure 2: households better protected against flood risk

Number of households in: Before After

20% most deprived areas 0 0 0

21-40% most deprived areas 0 0 0

60% least deprived areas 3,595 0 -3,595 0

At: Moderate Significant Very Moderate Significant Very Moderate Significant Very

risk risk significant risk risk significant risk risk significant

risk risk risk

Annual damages avoided, compared with a household at low risk 0.150 0.600 1.350

Change in household damages, in: Per year Over lifetime of scheme Qual. benefits (discounted)

20% most deprived areas OM2 (20%)

21-40% most deprived areas OM2 (21-40%)

60% least deprived areas OM2 (60%)

3. Qualifying benefits under Outcome Measure 3: households better protected against coastal erosion

Number of households in: Damages per household avoided:

20% most deprived areas - - Annual damages avoided 6£ 6£

21-40% most deprived areas - - Loss expected in 50 20 years

60% least deprived areas - - 1£ 3£

Long-term

loss

Medium-term

loss

Long-term

loss

Medium-term

loss

Change in household damages, in: Year 1 loss avoided: Over lifetime of scheme: Qual. benefits (discounted):

20% most deprived areas OM3 (20%)

21-40% most deprived areas OM3 (21-40%)

60% least deprived areas OM3 (60%)

4. Qualifying benefits under Outcome Measure 4: statutory environmental obligations met

Payments under: Assumed benefits per unit: Qual. benefits (discounted):

OM4a Hectares of net water-dependent habitat created OM4a

OM4b Hectares of net intertidal habitat created OM4b

OM4c Kilometres of protected river improved OM4c

OM4

5. Qualifying benefits arising from the overall scheme, for entry into the Medium-Term Plan

OM, deprivation: Qual. benefits: Payment rate: FCRM GiA contribution:

OM1 5.56 p in the £1

OM2 20% most 45.0

21-40% 30.0

Least 60% 20.0

OM3 20% most 45.0

21-40% 30.0

Least 60% 20.0

OM4 100.0

Total

Raw Score Contribution

for 100%

Score

(£k)As scenario above 98% 301

Sensitivity 1 - Change in PV Whole Life Cost (25% increase) 78% 3,646

Sensitivity 2 - Change in OM2 - 50% of households in Very Significant (Before) risk may already be in Significant Risk band 98% 301

Sensitivity 3 - Change in OM3 - 50% of households in Medium Term loss (Before) may already be in Long Term loss 98% 301

Sensitivity 4 - Increase Duration of Benefits by 25% 69% 4,094

Sensitivity 5 - Reduce Duration of Benefits by 25% 68% 4,285

END OF WORKSHEET

-£

This scheme is to be maintained by the EA (ref cell 5). Any contributions

needed (ref cell 2) are to help fund both the ongoing and up-front costs (Cost

for Approval), and should be entered into cells(14-17).

The total value of any necessary contributions will depend on whether

maintenance (ongoing costs) is funded through revenue FCRM GiA, or by

other means.

Cell (2) shows the minimum amount of contributions and/or reductions in

scheme cost that are required to raise the Adjusted PF Score to at least

100%. Further increases on this will improve this scheme's chances of an

FCRM GiA allocation in the desired year. Planned savings and contributions

should be entered into cells(9,10,12) and cells(14-17).

All costs and benefits must be on a Present Value (PV)

Whole-Life basis over the Duration of Benefits period. Where

Contributions are identified these should also be on a

Present Value basis.

Is evidence available that a Strategic Approach has been taken,

and that double counting of benefits has been avoided ?

-£

-£

-£

-£

Project Name

Input cells

Calculated cells

-£

Before

-£

-£

Change due to scheme

-£

-£

32,813£

-£

-£

-£

15£

-£ -£ -£

-£ -£

-£

150,070£ 13,077£

-£

-£

-£

-£

-£

-£

-£

Sensitivity Testing. It is important that users of this calculator appreciate the implications on funding from changes to input data which may become necessary as the project develops and better information is available.

Five typical tests are provided below. Users should consider how appropriate these are to their project, what other tests may be appropriate and how best to use the information with all those that may be involved in the project.

-£

2,157-£ 43,140-£

Present value of Year 1 loss (i.e. first year damages,

discounted based on when loss is expected)

50£

80£

-£

6,514£

-£

117,257£

32,813£

-£ -£

6,563£

Maximum for Outcomes delivered

75


Dr Paul Canning Senior Coastal Engineer

Atkins The Hub 500 Park Avenue Aztec West Almondsbury Bristol BS32 4RZ Email: [email protected] Telephone: 01454 662290

© Atkins Ltd except where stated otherwise. The Atkins logo, ‘Carbon Critical Design’ and the strapline ‘Plan Design Enable’ are trademarks of Atkins Ltd.

mailto:[email protected]

flood risk sub group - south gloucestershire · d1 report structure p canning 03/06/13 i1 draft...

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