River Itchen Flood Alleviation Scheme Preliminary Study€¦ · The work described in this Report...

Submitted to Southampton City Council

Submitted by AECOM Scott House, Alençon Link Basingstoke Hampshire RG21 7PP United Kingdom

Option Report – Consultation Draft

River Itchen Flood Alleviation Scheme Preliminary Study

AECOM River Itchen Flood Alleviation Scheme i

Option Report July 2015

Prepared by: ............................................................. Checked by: ........................................................................

Jon Short Dan Glasson

Senior Coastal Specialist Principal Engineer

Approved by: .............................................................

David Dales

Sector Leader, Water (Government and Public), UK & Ireland

Revision Schedule

Rev No Comments Checked by Approved

by

Date

1 Draft for Comment DG DCD 17/03/15

2 Consultation Draft JS DCD 14/07/15

Scott House, Alençon Link, Basingstoke, Hampshire, RG21 7PP, United Kingdom

Telephone: 01256 310 200 Website: http://www.aecom.com

Job No Date Created

47070074 ............................................................. 14th

July 2015

AECOM River Itchen Flood Alleviation Scheme ii


Limitations

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Southampton City Council Limited (“Client”) in accordance with the Agreement under which our services were performed

[EC09/001/21862nd

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AECOM River Itchen Flood Alleviation Scheme iii


Executive Summary .................................................................................................................................................................... i

Study ............................................................................................................................................................................................. i

This Report .................................................................................................................................................................................... i

Problem ......................................................................................................................................................................................... i

Strategic Management of Flood Risk ........................................................................................................................................... ii

Long term vision ........................................................................................................................................................................... ii

Interim solution ............................................................................................................................................................................. ii

Next Steps ................................................................................................................................................................................... iv

1 Introduction ............................................................................................................................................................................ 1

1.1 Background ....................................................................................................................................................................... 1

1.2 Commission ...................................................................................................................................................................... 2

1.3 This Report ....................................................................................................................................................................... 2

2 Commission and Location .................................................................................................................................................... 2

2.1 Location ............................................................................................................................................................................ 2

3 The Problem – what if we ‘Do Nothing’ ................................................................................................................................ 4

3.1.1 Nature of flooding .............................................................................................................................................................. 4

3.1.2 Modelling flood risk ........................................................................................................................................................... 4

3.1.3 Consequences of tidal flood risk ....................................................................................................................................... 4

3.2 Strategic Management of Flood Risk ................................................................................................................................ 8

3.2.1 Long term vision ................................................................................................................................................................ 8

3.2.2 Interim options to manage risks ........................................................................................................................................ 8

4 Supporting studies and data collection ............................................................................................................................... 9

4.1 Data requirement .............................................................................................................................................................. 9

4.2 Surveys ............................................................................................................................................................................. 9

4.2.1 Surveys of existing defences ............................................................................................................................................ 9

4.2.2 Surveys to gather ground condition data......................................................................................................................... 11

4.2.3 Topographic surveys ....................................................................................................................................................... 11

4.2.4 Environmental surveys .................................................................................................................................................... 11

4.3 Stakeholder Engagement ................................................................................................................................................ 12

5 Option Development ............................................................................................................................................................ 13

5.1 Provisional scheme concepts developed in The Strategy ............................................................................................... 13

5.2 Option development in this study .................................................................................................................................... 13

5.2.1 Scheme route alignment options considered .................................................................................................................. 13

5.3 Defence Options ............................................................................................................................................................. 18

5.3.1 Front line defences ......................................................................................................................................................... 18

5.3.2 Set-back defences .......................................................................................................................................................... 19

5.3.3 Floodgates ...................................................................................................................................................................... 21

5.3.4 Land raising .................................................................................................................................................................... 22

5.4 Scheme design life .......................................................................................................................................................... 22

5.5 Options summary ............................................................................................................................................................ 22

6 Economic Appraisal of Options .......................................................................................................................................... 24

6.1 Approach ......................................................................................................................................................................... 24

6.2 Methodology ................................................................................................................................................................... 24

6.2.1 Quantifying benefits ........................................................................................................................................................ 24

6.2.2 Option Costing ................................................................................................................................................................ 25

Table of Contents

AECOM River Itchen Flood Alleviation Scheme iv


6.3 Option Benefits ............................................................................................................................................................... 27

6.3.1 Do Nothing Damages ...................................................................................................................................................... 27

6.3.2 Option Benefits ............................................................................................................................................................... 27

6.3.3 Residual Damages .......................................................................................................................................................... 27

6.4 Benefit Cost Assessment ................................................................................................................................................ 28

6.5 Funding ........................................................................................................................................................................... 28

6.5.1 Partnership funding ......................................................................................................................................................... 28

6.5.2 Partnership funding assessment and sensitivity testing .................................................................................................. 28

6.5.3 Summary ......................................................................................................................................................................... 29

7 Wider Implications of Options and Environmental Appraisal .......................................................................................... 31

7.1 Overview ......................................................................................................................................................................... 31

7.2 Key services / utilities ...................................................................................................................................................... 31

7.3 Surface Water and drainage ........................................................................................................................................... 32

7.4 Highways ........................................................................................................................................................................ 32

7.5 Landscape ...................................................................................................................................................................... 33

7.6 Screening the requirement for Environmental Impact Assessment ................................................................................. 34

7.6.1 Front route alignment option ........................................................................................................................................... 34

7.6.2 Set-back route alignment option ..................................................................................................................................... 34

8 Health, Safety and Environmental Risks............................................................................................................................ 38

9 Summary............................................................................................................................................................................... 48

9.1 Conclusion ...................................................................................................................................................................... 48

9.2 Next Steps ...................................................................................................................................................................... 49

10 References ............................................................................................................................................................................ 50

Appendix 1 – Flood Modelling ................................................................................................................................................ 51

Appendix 2 – Non Intrusive Survey Report ............................................................................................................................ 52

Appendix 3 – Ground Investigations and Interpretive Report .............................................................................................. 53

Appendix 4 – Environmental Baseline and Surveys ............................................................................................................. 54

Appendix 5 – Option Drawings ............................................................................................................................................... 55

Appendix 6 – Economic Assessment ..................................................................................................................................... 56

Appendix 7 –Option Appraisal ................................................................................................................................................ 57

Appendix 8 – EIA Screening Report ....................................................................................................................................... 58

Appendix 9 – Landscape improvement / enhancement ideas and opportunities for consultation................................... 59

AECOM River Itchen Flood Alleviation Scheme v


List of Tables

Table 3-1. Peak Return Period Water Levels ................................................................................................................................4

Table 3-2. Summary of people and assets of risk under ‘Do Nothing’. ..........................................................................................5

Table 5-1. Options taken forward for detailed appraisal. ............................................................................................................. 22

Table 5-2. The two route alignment options for detailed appraisal. ............................................................................................. 23

Table 6-1. Summary of option capital costs (including 30% optimism bias). ............................................................................... 26

Table 6-2. Summary of whole life PV costs for each option (including optimism bias) ................................................................ 26

Table 6-3. Do Nothing Damages ................................................................................................................................................. 27

Table 6-4. Option benefits (2015-2085) ....................................................................................................................................... 27

Table 6-5. Option residual damages (2015-2085) ....................................................................................................................... 27

Table 6-6. Cost benefit analysis .................................................................................................................................................. 28

Table 6-7. Partnership funding scores for the front alignment ..................................................................................................... 29

Table 6-8. Partnership funding scores for the setback alignment ................................................................................................ 29

Table 6-9. Summary of economic assessment of options. .......................................................................................................... 30

Table 7-1. Summary of wider option impacts .............................................................................................................................. 35

Table 8-1. Health, Safety and Environmental Risks .................................................................................................................... 39

Table 9-1. Summary of economic assessment of options. .......................................................................................................... 48

List of Figures

Figure 1-1. Southampton Costal Flood and Erosion Risk Management Strategy Frontage ..........................................................1

Figure 2-1 Study Area ...................................................................................................................................................................3

Figure 3-1 Flood Map for a 1:200 year (0.5%AEP) event under a “Do Nothing” scenario at 2015. ...............................................6

Figure 3-2. Flood Map for a 1:200 year (0.5%AEP) event under a “Do Nothing” scenario at 2060. ..............................................7

Figure 4-1. MBES survey output from the aggregate wharfs, Itchen (July 2014). .........................................................................9

Figure 5-1. Indicative typical section for the front quay raising solution (Not to scale). ............................................................... 19

Figure 5-2. Indicative typical section for the set-back piled floodwall solution (Not to scale). ...................................................... 20

Figure 5-3. Indicative sketch of a typical flood gate configuration (not to scale). ......................................................................... 21

Figure 7-1. Example visualisation to show potential landscape interest and enhancement ideas which will be displayed for

consultation. ................................................................................................................................................................................ 33

AECOM River Itchen Flood Alleviation Scheme i


Study

Following the Environment Agency approval of the Southampton City Coastal Flood and Erosion Risk Management

(FCERM) Strategy, Southampton City Council commissioned AECOM to undertake a preliminary study for a River Itchen

Flood Alleviation Scheme to protect parts of Northam, St Marys and the City Centre from tidal flooding. The scheme is

primarily required to mitigate the significant risk to residential areas which lie behind the waterfront industry.

The preliminary study is required to gain the necessary data and evidence to build a robust business case for the scheme

which encompasses a 4km frontage from the Mount Pleasant Industrial Estate to just south of the Itchen Bridge. The

outputs of the study include a technical, economic and environmental appraisal of the options available and outline

designs for the option which can be funded public from Grant in Aid (GiA) monies. This will allow Southampton City

Council to develop and submit a Project Appraisal Report to the Environment Agency to apply for grant aid funding for the

scheme.

Although not specifically required in the scope of the commission, the deliverables of the preliminary study will contribute

towards a planning application for the scheme.

This Report

This report presents the feasible options to mitigate tidal flood risk (alignments and defence types) for public consultation.

For each option the technical, economic and environmental requirements, considerations and implications are appraised

and discussed.

The options put forward have been developed following extensive technical assessment, stakeholder engagement,

economic appraisal and environmental consideration.

Problem

The existing quay walls of the study frontage are relatively low meaning the onset of tidal flood risk occurs from a 1:10

year event (10% Annual Exceedance Probability - AEP) in some locations, and the flood envelope increases significantly

with more extreme events, meaning over 400 residential properties, many of which are in more deprived areas, are at risk

from a present day 1:200 year (0.5% AEP) event. As recently as 14th February 2014 flooding of commercial properties,

pathways and vehicles was experienced along parts the Itchen from a 2.86mODN event (Dock Head).

With climate change future sea levels are predicted to rise, and this will bring about a significant increase in the risk of tidal

flooding from the same frequency event, and 430 homes would then be at risk from as little as a 1:10 year return period

(10% AEP) event at this time. If no action was taken to address flooding in this area then flooding could reach depths of

greater than 1.5m for several areas of Northam.

In addition to the residential properties and people at risk, there is also a significant number of commercial properties and

business potentially at threat. Some of these industries are located along the frontage and rely heavily on the water for

their existence. Some of these waterfront properties and infrastructure are by their nature more flood resilient than others,

however the potential for significant damage and disruption exists.

This evidence demonstrates a compelling case to implement a scheme to reduce this risk.

Executive Summary

If no action is taken to address tidal flood risk total Present Value (PV) damage is predicted to be £214M by 2085

with 1157 residential properties at risk by this time. This brings with it the significant risk to life due to flooding.

AECOM River Itchen Flood Alleviation Scheme ii


Strategic Management of Flood Risk

When developing schemes to address flood risk, it is necessary to understand the wider strategic constraints,

opportunities and implications for the works. The approved Southampton Coastal Flood and Erosion Risk Management

Strategy (URS, 2012) set the preferred strategic approaches, and route map, to manage tidal flood risk in the City and

deliver the North Solent Shoreline Management Plan policy of ‘Hold the Line’.

Given Southampton City Council’s aspirations for regeneration, improved waterfront connectivity, public realm

improvements and overall vision to prosper, the long term management of flood risk is intended to be delivered through

land raising achieved through redevelopment along the waterfront.

Long term vision

As opportunities arise through regeneration and the planning process, developers will raise existing ground levels so by

2060 it is intended that the strategic defence to the city will largely be achieved through a continuous strip of raised land.

Such an approach has many advantages including passive, maintenance free defences with very low residual risk. Once

implemented there will also be no lasting footprint as the defences will be built upon. This replaces the need for raised

defences such as walls which require access provisions and hinder views and leave a mark on the landscape.

Interim solution

Given the existing urban environment, it is recognised that this long term strategic intent to raise land to address flood risk

through redevelopment takes time to implement. However, with large parts of the Itchen at significant risk of flooding now,

The Strategy recognised the need and made the case for a priority scheme to provide the interim solution to address flood

risk until the land raising can be achieved to form the primary defence. Through consideration of technical and economic

aspects at the strategic level a setback floodwall was identified as the preferred option to implement interim flood risk

mitigation.

This study has reviewed and refined the options and this report presents the feasible options and evaluates their technical,

economic and environmental merits and implications.

The options

Following numerous surveys, technical studies, economic assessments and environmental appraisals, coupled with

extensive stakeholder engagement, two feasible route alignment options for a scheme have been developed:

Option 1 - A front route alignment (raising and/or replacing existing front line defences) which delivers maximum benefits

but at greatest cost.

Option 2 - A set-back route alignment (typically behind waterfront industry) which offers a publically fundable option which

delivers the same benefits to residential property, but leaves a significant number of commercial and waterfront

organisations at the current risk level.

It is recommended that the most viable and technically sound option to implement a front alignment is to construct new

sheet piles immediately in front of the existing quay walls.

Given potential seepage issues and groundwater interaction, construction footprints and space constraints, a sheet piled

floodwall has been identified as the most feasible solution to implement defences along the set-back route.

To accommodate access requirements under both options a number of floodgates and ramps will be required.

As in line with the recommendations of the approved Southampton CFERMS, the developer is raising the land at the

former Meridian Studios site, and implementing raised front line defences, and land raising at Chapel Riverside to the 100

year design water level (4.15m ODN). These raised areas and development sites will then form part of the strategic

defence, tying into the wider scheme to provide an increased standard of protection to the areas behind. Although detailed

plans and design drawings have not yet been made available for these sites, the final outline design for the scheme will

incorporate the plans and tie in details.

AECOM River Itchen Flood Alleviation Scheme iii


Discussion and Conclusion

1) The appraisal of feasible options demonstrates a sound economic case to implement a tidal flood alleviation

scheme for this part of the Itchen. Both feasible options deliver benefits in excess of costs.

The optimal Standard on Protection (0.5% AEP SoP at 2060) has been confirmed though an Incremental Benefit

Cost Ratio (IBCR) test.

The proposed design life is 2085, accepting beyond 2060 the SoP will begin to fall if sea levels rise as currently

allowed for.

By 2060 it is anticipated that the longer term Southampton CFERMS preferred option of land raising implemented

through redevelopment will largely provide the required longer term protection against tidal flooding.

2) A frontline route alignment (Option 1), replacing and raising existing defences, has a benefit: cost ratio of 5.1 and

delivers maximum benefits through significantly improving the Standard of Protection against tidal flooding to nearly

all of the commercial and residential properties in the flood zone; however this comes at a significant capital cost

(£40.2 M PV).

With the current National Flood and Coastal Erosion Risk Management (FCERM) funding system in place a high

level of non GiA contribution (approx. £23M) would be required to make the frontline alignment option viable.

3) With the public funding limitations considered, an alternative set-back route alignment (Option 2) offers a viable and

publically fundable option which is likely to gain GiA approval. This is because the Partnership Funding system for

allocating GiA recognises that the residential benefits and outcomes delivered by this option are comparable to the

front alignment option (1157 residential properties better protected under both options), but at approximately one

third of the cost. The set-back option has a cost of approx. £12.8M (PV) and produces a benefit: cost ratio of 11.9.

A Local Enterprise Partnership contribution of £3M has been allocated to the scheme along with a developer

commitment to contribute to the scheme in the form of land raising.

Summary of economic assessment of options.

Option Cost PV (£)* Benefits PV (£)

B:C Likely additional external

contributions required to

gain GiA approval (£)**

No. of residential properties

with improved

SoP

No. of commercial properties

with improved

SoP

Residual damage (PV) £

1. Front line 40,235,000 206,123,000 5.1 23,000,000 1157 679 8,296,000

2. Set-back 12,831,000 152,409,000 11.9 0 1157 507 62,010,000

4) * Including estimated £2.7M land raising / development defence costs.

5) ** Additional contribution required after £3.5M contribution included.

6) It is recognised that the setback option leaves a significant number of commercial properties and waterfront

operations in front of the defence; these will therefore remain at the same Standard of Protection as in a ‘Do

Nothing’ baseline scenario. This means that there is the potential for £62M Present Value (PV) damage to these

properties over the next 70 years, as well as the ongoing risk of wider disruption to their operations.

Having liaised with many of the waterfront organisations during the study it is apparent that some of these

waterfront operations, such as marinas, warehouses and wharves may be relatively water compatible or ‘resilient’

to a degree of flooding, at least compared to residential property or more susceptible commercial property.

7) There are a significant number of utilities and services that will be impacted by a potential scheme, especially with

the set-back route alignment; utilities will need to be accommodated through diversions, protection works or other

appropriate mitigation. Overall the environmental impacts of the set-back option are less onerous than for the front

alignment option.

AECOM River Itchen Flood Alleviation Scheme iv


8) The setback route alignment (Option 2) runs alongside or across several main roads and many access roads.

Therefore an initial high level screening exercise of the concept deign was undertaken by highway engineers to

identify potential highways issues and requirements.

As a result of this review, the concept design has incorporated, where possible, the necessary measures to avoid

impacts to the highways. Where potential issues are unresolved at this stage, these have been signposted and

potential mitigation has been identified for further more detailed consideration

9) A review of the potential landscape impacts of each option has been undertaken for both options. Potential

opportunities and ideas for enhancement of the public realm will be presented during the consultation period.

Should there be a common aspiration for particular enhancements or finishes in certain areas there is the potential

for these to be delivered as part of the final scheme. However, it should be noted that beyond a basic, acceptable

finish to the floodwall, specialist or extravagant enhancement features are not likely to qualify for Grant aid Funding

and will need to be funded from other sources or through contributions.

10) There is a residual risk of defence failure presented by the requirement for a number of floodgates for access.

Suitable mitigation strategies such as telemetry and electronic activation of gates will be required to reduce this risk

to a minimum; specific requirements will need to be confirmed during detailed design.

Next Steps

The options put forward in this report will be presented for 3 months of public consultation and feedback between August

and October 2015. During this period, specific engagement and meetings will be held with key stakeholders and

landowners along the frontage to gauge opinion and further explore potential contributions for the scheme.

Following this consultation, the feedback will be reviewed and evaluated. Should the required external funding not be

obtainable to meet the shortfall to deliver the front line scheme (Option 1), it is intended that the alternative publically

fundable setback option (Option 2) will be delivered.

The outline design for the preferred option will then be updated and finalised following consultation. A planning application

will then be submitted to SCC, and a Project Appraisal Report (PAR), detailing the business case for GiA approval, will be

submitted to the Environment Agency Large Project Review Group in early 2016.

Following funding approval, a detailed design stage will be required with commencement of construction aimed for late

2016/17.

AECOM River Itchen Flood Alleviation Scheme 1


1.1 Background

In 2012, URS (now AECOM) developed the Southampton City Coastal Flood and Erosion Risk Management Strategy

(CFERMS) on behalf of Southampton City Council. The Council adopted Strategy recommends preferred flood risk

management options for the next 100 years for a 22km frontage spanning from Woodmill Lane at the tidal extent of the

river Itchen around the main part of the City to Redbridge on the river Test (Figure 1-1). It is recommended that defences

are to be implemented across the City in a phased manner to address and mitigate future flood risk which is set to

increase significantly based on the latest climate change guidance (UKCP09 projections).

Figure 1-1. Southampton Costal Flood and Erosion Risk Management Strategy Frontage

The Strategy promotes the requirement, and makes a sound business case, to implement the Shoreline Management

Plan Policy of ‘Hold the Line’ and to sustain a minimum 1:200 year standard of protection against tidal flooding in the City.

However, some of the most vulnerable parts of the City (Northam and St Marys) are currently at risk of tidal flooding from

as little as a 1:20 year (5% AEP) return period event and consequently there is a sound economic case to implement an

urgent flood risk reduction scheme in these areas, especially as the risk is set to increase significantly due to sea level

rise.

1 Introduction

Redbridge

Woodmill

City of Southampton

Aerial imagery © Southampton City Council

2011

Upper

Itchen

Northam

Lower

Itchen

River Test St

Marys

Itchen

Bridge



1.2 Commission

Following the Environment Agency approval of the Southampton City CFERMS, Southampton City Council has

commissioned AECOM to undertake a Preliminary Study for a tidal flood alleviation scheme along the tidal river Itchen.

The scheme is primarily required to mitigate the significant risk to residential areas which lie behind the waterfront

industry.

The preliminary study is required to gain the necessary data and evidence to build a robust business case for the scheme

which encompasses a 4km frontage from the Mount Pleasant Industrial Estate to just south of the Itchen Bridge. The

outputs of the study include a technical, economic and environmental appraisal of the options available and outline

designs for the option which can be funded from public grant in aid and Local Enterprise Partnership monies. This will

allow Southampton City Council to develop and submit a Project Appraisal Report to the Environment Agency to apply for

Flood and Coastal Erosion Risk Management Grant in Aid (FCERM GiA) funding approval for the scheme.

Although not specifically required in the scope of the commission, the deliverables of the preliminary study will contribute

towards a planning application for the scheme.

1.3 This Report

This report presents the feasible options to mitigate tidal flood risk (alignments and defence types) for public consultation.

For each option the technical, economic and environmental requirements, considerations and implications are appraised

and discussed.

The options put forward have been developed following extensive technical assessment, stakeholder engagement,

economic appraisal and environmental consideration.

2.1 Location

The study area runs from the southern end of the boardwalk near Mount Pleasant Industrial Estate around Northam and

St Mary’s to a southern limit beneath the Itchen Bridge. This is a total frontage length of nearly 4km (Figure 2-1).

2 Commission and Location



Figure 2-1 Study Area



3.1.1 Nature of flooding

At present, there are no formal raised flood defences within the City of Southampton. Quay and dock walls are present

along a large proportion of Southampton frontage and these prevent coastline erosion and currently offer protection

against flooding from every day (astronomical) tides and protection from the small waves which are typical of the sheltered

estuarine environment.

However under extreme tidal events, when storm surges combine with high astronomical tides, water levels can reach

much greater peak levels and this brings the risk of tidal inundation over the quay walls. The present day and future

predicted extreme water levels, accounting for latest UKCP09 climate change allowances are presented in Table 3-1.

Table 3-1. Peak Return Period Water Levels

Return Period Peak Water Level (mODN) (adjusted from Dock Head to account for local site conditions)

2015 2030 2060 2085

1 2.53 2.65 2.84 3.00

2 2.63 2.74 2.93 3.10

5 2.75 2.86 3.06 3.24

10 2.84 2.95 3.16 3.34

20 2.92 3.04 3.24 3.43

50 3.02 3.15 3.36 3.54

100 3.10 3.22 3.44 3.63

200 3.17 3.30 3.51 3.71

500 3.26 3.40 3.62 3.81

1000 3.33 3.46 3.69 3.89

Source: peak levels adapted and updated from the Southampton Coastal Flood and Erosion Risk Management Strategy (2012). These

include a 5cm increase on Dock Head extreme water levels presented in the Strategy to account for the site specific conditions at the

frontage.

3.1.2 Modelling flood risk

To inform a detailed understanding of flood risk and support a robust economic appraisal of the options, a tidal inundation

model was developed by AECOM using DHI’s industry recognised software ‘Mike 21 fm’. The flexible mesh model uses

the latest high resolution LiDAR data, supplemented with topographic survey data to characterise and describe the

topography and existing defence crests within the study area.

A range of present day and future extreme tidal events were then simulated (based on the extreme water levels presented

in Table 3-1). This provided the flood extents and depths under the various conditions with the existing defences in place

and demonstrates the risk under a ‘Do Nothing’ case.

The model was later used to simulate the flood risk and benefits of the ‘Do something’ options being considered to

mitigate tidal flooding. For a detailed description of model setup, validation and full results see Appendix 1.

3.1.3 Consequences of tidal flood risk

The existing quay walls of the study frontage are relatively low meaning the onset of tidal flood risk occurs from a 1:10

year event (10% AEP) in some locations, and the flood envelope increases significantly with more extreme events,

meaning over 400 residential properties, many of which are in more deprived areas, are at risk from a present day 1:200

year (0.5% AEP) event. As recently as 14th February 2014 flooding of commercial properties, pathways and vehicles was

experienced along parts the Itchen from a 2.86mODN event (Dock Head).

3 The Problem – what if we ‘Do Nothing’



With climate change, future sea levels are predicted to rise, and this will bring about a significant increase in the risk of

tidal flooding. By 2060 the number of residential properties at risk will increase to 930 from the same frequency event, and

430 homes would then be at risk from as little as a 1:10 year return period (10% AEP) event at this time. If no action was

taken to address flooding in this area then flooding could reach depths of greater than 1.5m for several areas of Northam.

In addition to the residential properties and people at risk, there are also over 800 commercial properties and business

potentially at threat. Some of these industries are located along the frontage and rely heavily on the water for their

existence. Some of these waterfront properties and infrastructure are by their nature more flood resilient than others,

however the potential for significant damage and disruption exists to these operations.

If no action is taken to address tidal flood risk total PV damage is predicted to be £146M by 2060. There is also a

significant risk of potential fatalities associated with flooding.

This evidence demonstrates a compelling case to implement a scheme to reduce this risk. A summary of key assets at

risk if no scheme is implemented to mitigate tidal flood risk is provided in Table 3-2. For full details of the economic

appraisal see Appendix 7.

Figure 3-1 shows the extent and depth of potential flooding for a 1:200 year (0.5% AEP) event at 2015 with the existing

defences in place.

Figure 3-2 shows the extent of potential flooding for a 1:200 year (0.5% AEP) event at 2060 with the existing defences in

place.

Table 3-2. Summary of people and assets of risk under ‘Do Nothing’.

Summary of people and assets potentially at risk or affected by tidal flooding by 2060*

An estimated 2,500 local residents

Over 1,100 homes; 430 of which are within the highest 20% of nationally deprived areas and 217 are within the 21-40% of most deprived areas.

Over 800 commercial properties

3 churches

45 electricity sub stations

26 education buildings (mainly at Southampton City College)

3 supermarkets

7 engineering works

3 public houses

11 restaurants/cafes

Major roads including the A3024 (Northam Road ), the A3025, the Itchen Toll Bridge and Northern Bridge

The major train route between Southampton Central station and St Denys station/Southampton Airport parkway

Access to St. Marys football stadium

Four marinas including Saxon Wharf, Ocean Quay, Shamrock Quay and Drivers Wharf

Coastal cycleways (including national cycle route 23) and footpaths to the west of Northam Bridge

Several slipways

32 listed buildings

*From a 1:200 year (0.5% AEP) event



Figure 3-1 Flood Map for a 1:200 year (0.5%AEP) event under a “Do Nothing” scenario at 2015.



Figure 3-2. Flood Map for a 1:200 year (0.5%AEP) event under a “Do Nothing” scenario at 2060.



3.2 Strategic Management of Flood Risk

In developing schemes to reduce flood risk, it is necessary to understand the wider strategic constraints, opportunities and

implications for the works. The approved Southampton Coastal Flood and Erosion Risk Management Strategy (URS,

2012) set the preferred strategic approaches, and route map, to manage tidal flood risk in the City and deliver the North

Solent Shoreline Management Plan policy of ‘Hold the Line’.

Given Southampton City Council’s aspirations for regeneration, improved waterfront connectivity, public realm

improvements and overall vision to prosper, the long term management of flood risk is intended to be delivered through

land raising achieved through redevelopment along the waterfront.

3.2.1 Long term vision

As opportunities arise, ground levels along the waterfront are to be raised through regeneration and redevelopment, so by

2060 it is intended that through the planning process the strategic defence to the city will largely be achieved through a

continuous strip of raised land. Such an approach has many advantages including passive, maintenance free defences

with very low residual risk. Once implemented there will also be no lasting footprint as the defences will be built upon. This

replaces the need for raised defences such as walls.

3.2.2 Interim options to manage risks

Given the existing urban environment, it is recognised that this long term strategic intent to raise land to address flood risk

through redevelopment takes time to implement. As described in Section 3, large parts of the Itchen are at significant risk

of flooding now. Therefore the Strategy recognised the need and made the case for a priority scheme to provide the

interim solution to address flood risk until the land raising can be achieved to form the primary defence.

The interim measure provisionally put forward in the Strategy was a reinforced concrete floodwall set-back just behind

existing defences. In two areas where redevelopment is imminent land raising was proposed rather than the floodwall to

make the most of the opportunity to begin the process of implementing the longer term flood risk management solution in

these areas.

The interim solution concept was proposed at the strategic level without detailed information on aspects such as existing

defence structural integrity and residual life or ground conditions. Therefore this preliminary study was commissioned to

review, update and refine the preferred option through collection and analysis of more detailed data, more extensive

engagement and outline design evaluation of options.

This report puts forward options to implement the interim solution and ensures that the options are sustainable, co-

ordinated and fully incorporate and address the following strategic considerations:

• Technical feasibility;

• Climate Change and future uncertainty;

• Regeneration and redevelopment;

• Practical constraints and opportunities;

• Stakeholder / landowner / operator requirements and feedback;

• Access demands;

• Physical barriers;

• Location and types of key services;

• Environmental impacts and legislative requirements; and

• National grant aid funding system and partnership funding.



4.1 Data requirement

In order to develop robust options and carry out a detailed appraisal and subsequent outline design of the preferred

option, the data gaps and outstanding evidence required to support the technical assessments and to guide decision

making needed to be scoped and then captured.

The outstanding data required in order to complete the objectives of the preliminary study included the following areas:

• Structural integrity and residual life of existing defences;

• Information on areas of potential voids;

• Ground conditions and composition;

• Unexploded ordanance;

• Groundwater information and interaction with tidal levels;

• More detailed ground elevation data in key areas;

• More detailed environmental baseline data including trees, ecology, fish, contamination, heritage, landscape, etc.

• Landowner operator requirements and access provisions required;

• Potential for contributions;

• Latest development plans for Meridian Studios and Chapel Riverside; and

• Itchen Riverside Masterplan and SCC initiatives.

4.2 Surveys

A range of surveys were commissioned to obtain the outstanding data required for the study. A summary of the surveys

undertaken and their aims is provided in the following sections. Links to the detailed reports providing more information

are also provided.

4.2.1 Surveys of existing defences

Multibeam Echo Sounder (MBES) Survey

In order to gain an overall understanding of the condition of defences along the frontage (with a focus on the sheet piled sections) EGS International Ltd was commissioned to undertake a Multibeam Echo Sounder Survey. The MBES survey was conducted on the 31st July 2014 using a local vessel.

This survey provided detailed

high resolution data to give an

indication of the underwater

state of the piles and the

interaction between the sea

bed and toe of the frontage,

identifying potential scour or

undermining. This survey also

highlighted features such as

obstructions or obstacles that

could affect any future works

or studies, and also the

requirements to undertake

further dive inspections.

4 Supporting studies and data collection

Figure 4-1. MBES survey output from the aggregate wharfs, Itchen (July 2014).



A summary of the survey details, results and interpretation is provided in

Appendix 2.

Ground Penetrating Radar (GPR) Survey

Select Survey Ltd were commissioned to conduct the survey over a four day period (27th – 31st July 2014). The purpose of the survey was to identify the presence of potential voids behind the defences. The survey provided an approximate depth to the potential void (inferred from ground disturbance) and an approximate size of the void. The survey did not provide the depth to the bottom of the potential void. The GPR survey targeted key locations along the west bank of the River Itchen between Mount Pleasant Industrial Estate and Ocean Village. The survey covered a 5m wide strip set back 1m from the existing defences. The total distance surveyed was 1200m. The survey ran along the survey lines and when a potential void was identified, the area was marked up on a location plan, recording the void extent and depth to the top of the void. If a potential void was identified, a GPS survey was conducted to determine the location of the void at ground level. A summary of the survey details, results and interpretation is provided in Appendix 2.

Concrete Coring

Following the identification of potential voids along parts of the frontage from the GPR survey, a small scale concrete coring survey was commissioned in order to explore further and ratify the presence of voids. Structural Soils Ltd. undertook the survey on 31st October 2014. This targeted four of the largest potential voids identified during the GPR survey. The coring survey confirmed the presence of some shallow voids at three of the four locations where some settlement had occurred below the concrete surfaces.

Sheet Pile thickness testing

A pile thickness survey was conducted on the 29th August 2014 by qualified divers from Quest Surveys Ltd. The purpose of the pile thickness testing was to identify the thickness of the piles and to assist in determining the residual life of the piles. A total of 192 readings were taken at 25 locations between Drivers Wharf and Ocean Quay. At both sites no cathodic protection or sacrificial anodes were observed. The pile thickness survey was supplemented with the results from the dive survey of the Crown to Leamouth Wharf Cemex frontage undertaken by Valkyrie in October 2011. A full set of results can be found in Appendix 2.

Photo 1. GPR survey being conducted along the Itchen

(July 2014).

Photo 2 Concrete Coring survey being conducted along

the Itchen (October 2014).

Photo 3. Diver survey testing the thickness of steel

sheet piles at Drivers Wharf (August 2014).



4.2.2 Surveys to gather ground condition data

Ground investigations

In order to establish typical ground conditions, ground profiles, engineering properties and identify scheme constraints for the consideration of options a desktop analysis and intrusive site investigation were undertaken. Following an AECOM desk study, Structural Soils Ltd were commissioned to undertake:

• Seven cable percussive boreholes to depths between 2.5m and 13.0m;

• Seven machine excavated trial pits to depths between 1.65m and 4.1m;

• Eight Cone Penetration Tests (CPTs) to depths between 1.48m and 20.01m; and

• Standard Penetration Tests (SPT), sampling and groundwater monitoring were carried out in all boreholes

The surveys were undertaken in a two week period in November 2014. The Ground Investigation map, details, results and interpretation are provided in (Appendix 3).

Services and utilities As part of the study a full digital record of services and utilities was also obtained for the study area. The utilities are shown on the Option Drawings in Appendix 5.

Unexploded Ordnance (UXO) 1st Line defence were also subcontracted to carry out the Unexploded Ordnance (UXO) surveys and the UXO assessment findings are provided within Appendix 3.

Other data There was also a preliminary contamination risk assessment included which provided an initial assessment of the contamination risks on site to be carried forward to detailed design. Groundwater monitoring was undertaken in standpipes in five of the boreholes. Data loggers were installed and the data downloaded weekly for six weeks.

4.2.3 Topographic surveys

Ground elevation data is required for the accurate representation of the floodplain in the numerical modelling, and also to determine the design heights of options along their route. A large elevation dataset was obtained during the Southampton CFERMS, however with alternative route alignment corridors identified for exploration in this study, further topographic spot height data was collected by AECOM surveyors. This collected further elevation spot height data in key areas at 2-10m intervals.

4.2.4 Environmental surveys

To ensure that the environmental impacts of options can be adequately determined and compared, surveys were carried out by AECOM specialists to gather detailed environmental baseline data. The findings of the environmental surveys and interpretation of the environmental baseline data is provided in Appendix 4.

Extended Phase 1 Habitat Survey

An experienced AECOM ecologist undertook walkover surveys of the site in April 2014. The objective was to identify and map ecological habitats currently present on the site and identify the presence of, or potential for the site to support protected and/or otherwise notable species, which may provide a constraint to potential flood alleviation works. This information combined with a desk study was used to define the ecological baseline, constraints and opportunities for the study area. The report which can be found in Appendix 4 was also updated and finalised following review by a SCC environmental specialist.

Photo 4. A borehole being drilled at Shamrock Quay

during the ground investigations along the Itchen

(November 2014)

Photo 5. Topographic survey undertaken to obtain

land elevation and defence height data.



Tree survey

A tree survey was undertaken by an AECOM arboriculturalist. The survey was conducted to record the type and nature of trees on site and this is useful data required to help inform the development of options and outline design proposals for the proposed flood alleviation scheme. The tree survey was conducted in accordance with the requirements of BS5837:2012 Trees in relation to design, demolition and construction – Recommendations (BS5837). An initial desktop study of existing aerial photographic information relating to the site was carried out. A field survey was also undertaken on 2nd and 3rd October 2014, and 21

st and 22

nd April 2015, during which dimensional data and

observational information were collected. The survey and the accompanying report (Appendix 4) also provides guidance as to the nature and quality of the existing tree stock, both on, and immediately adjacent to potential route alignment corridors and evaluates the quality and benefits of trees on site.

Landscape survey

A landscape walkover survey and characterisation assessment was carried out by an AECOM landscape architect on 2nd

and 3rd

October 2014, and 21st and 22

nd April 2015. This ensured that the landscape context of the study area was

captured and the key constraints and opportunities for the potential flood alleviation scheme were identified. The details of

the landscape character assessment are provided in Appendix 4.

This information has been used as part of the appraisal of options, and will also be used in the outline design of the

preferred option to ensure that the scheme will not detriment, and where possible will enhance the landscape of the study

area.

4.3 Stakeholder Engagement

Stakeholder engagement formed key part of the Southampton CFERMS Strategy development and this dialogue has been

built upon in the early phase of this study.

Extensive stakeholder engagement meetings have been held with the majority of the land owners and/or operators along

the frontage. These meetings were an opportunity to update occupiers and landowners of the project and scheme

concepts. Permissions were also sought and granted for the surveys.

Further engagement will be undertaken on the options during the three month consultation period which runs from August

to October 2015.



5.1 Provisional scheme concepts developed in The Strategy

The interim defence concept identified in the Southampton CFERMS for the frontage involved a reinforced concrete

floodwall typically set-back a few metres from the existing quay edge. The option was proposed as a cost effective solution

which would not require costly wholesale replacement and raising of quay walls. This alignment was also proposed with

intent to provide maximum scheme benefits and protect as many properties and people from tidal flooding. However this

provisional alignment and solution was put forward at this strategic level in the absence of more detailed survey

information.

Based on strategic level cost estimates and modelled benefit areas, the optimal standard of protection for the scheme was

identified through sensitivity and incremental testing using the FCERM economic appraisal sheets. The scheme was

shown to provide best value for public monies by delivering a design height based on providing a minimum 1:200 year

(0.5% AEP) standard of protection at 2060.

Although some of the pros, cons and risks were identified and considered at this strategic stage, the scheme proposals

were put forward in the context of a commensurate number of outstanding assumptions, uncertainties and recognised

risks. Some additional studies were undertaken under the Strategy commission to further explore the scheme concepts

and help build the business case to progress the scheme design. This additional work provided some useful data and

stakeholder feedback which enabled risks to be more adequately identified and quantified. During this additional work, the

technical and practical difficulties regarding route alignment and the potential solution became apparent. As a result of

these, a potential alternative route alignment set-back further from the quay edge was highlighted for part of the scheme

for further consideration.

5.2 Option development in this study

This preliminary study was commissioned to gain further evidence to underpin more robust option appraisal and selection

of a preferred option for outline design.

Option development is by its nature often an iterative process and the process has been informed though the collection

and technical, economic and environmental evaluation of more detailed data as described in Section 4.

The first phase of the option selection involved identifying feasible route alignments for the scheme. On identification of

feasible route alignments the options were further developed in terms of the potential solutions, design standards and

scheme life.

The route appraisal process comprised a detailed review of the alignment options initially identified in the Southampton

CFERMS (2012), informed by site walkovers from engineers and environmental specialists, and then an analysis of survey

data and evaluation of stakeholder requirements and funding potential.

5.2.1 Scheme route alignment options considered

Three alignment options were considered:

Route 1 – Near the front line (set-back a few metres behind existing defences)

Route 2 – New Front alignment (replacing / raising Front defences)

Route 3 - Set-back alignment (behind waterfront industry)

As a starting point the provisional alignment identified in the Southampton CFERMS (Route 1) was reviewed in the light of

the more detailed data collected in this study. Following this analysis, Route 1 was discarded for further appraisal and two

alternative feasible route alignments were identified (Routes 2 and 3). The key considerations in the decision making

process over route alignment options are discussed below and are shown in Appendix 5.

5 Option Development



1. Near the front line (set-back a few metres behind existing defences)

The key considerations relating to this alignment are discussed below. These key risks are not an exhaustive list, but

summarise the key decision points relating to this alignment, and form the audit trail of route alignment decision making.

Key decision consideration 1 (Technical) - Capacity of existing structures to support the scheme

A key assumption of the provisional scheme alignment proposed in The Strategy related to the residual life and structural

capability of the existing defence structures. For a scheme to be located only a few metres from the quay edge it requires

the existing defences to be able to support the additional loading of a floodwall. It also requires that the existing defence

structures remain in place for the scheme design life as should they fail, the floodwall would also eventually fail leading to

an unacceptable breaching risk.

The Multibeam Echo Sounder survey, steel sheet pile thickness testing and Ground Penetrating Radar Survey all provided

data relating to the structural integrity and residual life of the existing defences. The survey results demonstrated that the

risk of potential failure of front defences before the end of the scheme design life is significant (particularly the sheet piled

quay walls which have been in place for many decades).

Key decision consideration 2 (Technical) – Voids

The surveys indicate the potential for voids behind the existing defences. The GPR survey identified numerous potential

voids and the lack of returns from several metres of borehole at Shamrock Quay also indicates a high risk of a significant

void. The presence of voids does not prohibit the construction of a set-back wall; however they do present a technical

challenge to construction, as well as a health and safety considerations and high settlement potential.

Key decision consideration 3 (Technical) – Seepage potential

A key finding of the ground investigations related to the groundwater interaction with the tide. Monitoring in the surveyed

boreholes showed that groundwater often reacts with little lag to the tidal signature and therefore it is inferred that the

elderly existing piled structures are subject to significant saline intrusion and the made ground behind has high porosity

levels.

Key decision consideration 4 (Stakeholder) – Physical barrier and access

Key landowners and operators were re-engaged and their specific site operations, practical requirements and any

concerns were identified and understood. Initial discussions over potential willingness to contribute to the scheme were

also held to add to the data required for decision making over route alignment.

Many of the sites actively use the quays for pedestrian access to the waterfront, loading, unloading or boat launching.

Each of these activities has the potential to be hindered by a raised defence behind the existing quay walls. On closer

inspection and through gaining a detailed appreciation of site requirements it has been identified that 39 access provisions

(mainly floodgates) would be required to accommodate an alignment near the front. This would lead to a significant

residual risk of failure, especially as many of the gates would be located on private land with little control over their

closure. The set-back wall would also leave a lasting footprint potentially reducing working areas for operations.

Key decision criteria 5 (Heritage) – Impacts on designated heritage assets

Liaison with the heritage officer at SCC has been undertaken during the option development phase regarding American

Wharf. American Wharf and the sea wall fronting it, located a few hundred metres north of the Itchen Bridge, are Grade 2

listed and a wall set-back a few metres from the edge would detrimentally impact this feature. There is also limited space

for this alignment in this area.

Alignment Decision Summary

Given the significant technical risks and implications of the key considerations discussed above, the potential alignment

set-back a few metres behind existing defences is not feasible along many parts of the frontage.



2. Front Alignment (raising front defences)

On establishing that Route 1 is unfeasible, an alternative

front route alignment was identified for further appraisal. This

route runs along (or immediately in front of) the existing

defences and would require new defences to raise the

current crest levels.

The key considerations relating to this alignment are

discussed below. These key risks and are not an exhaustive

list, but summarise the key decision points relating to this

alignment, and form the audit trail of route alignment

decision making.

Key decision consideration 1 (Technical) – Residual life and

capacity of existing structures for raising

Given the aging existing quay walls and defences along the

frontage, and their typically fair to poor condition, raising the

existing structures is not viable as the risk of failure of these

structures is too acute. In order to implement this alignment, new front defences would be required with higher crest

elevations to mitigate the future flood risk and provide the required standard of protection.

Key decision consideration 2 (Technical) – Seepage potential

A key finding of the ground investigations related to the groundwater interaction with the tide. Monitoring in the surveyed

boreholes showed that groundwater often reacts with little lag to the tidal signature and therefore it is inferred that the

elderly existing piled structures are subject to significant saline intrusion and the made ground behind has high porosity

levels. This route alignment will require new front defences and this would mitigate the seepage issues.

Key decision consideration 3 (Environmental) – Environmental impacts

The environmental impacts of a front alignment which would require new defences are much more significant than those

identified for the set-back alignment. The impacts involve direct habitat loss (e.g. through piling in front of existing

structures into the sea bed) and through elevated disturbance. However these impacts do not form a ‘show stopper’ in

terms of the alignment feasibility as they can be avoided, reduced or mitigated sufficiently.


Many of the sites actively use the quays for pedestrian access to the waterfront, loading, unloading or boat launching.

Each of these activities has the potential to be hindered by a raised defence behind the existing quay walls. Through

raising the front defences and tying into the ground behind, these impacts are reduced, although still apparent compared

with a route alignment behind the existing defences. Short term disruption during construction would be significant,

however once constructed, there would be no scheme footprint unlike the set-back route alignment and therefore ongoing

disruption would be minimal. The need for floodgates for access would be greatly reduced, thereby reducing the residual

risk of failure.


Given the technical aspects, assessment of risk and implications of the key considerations discussed above, the potential

front alignment along the existing defence line is feasible; however given the nature of the works required to implement

this route, the costs associated will be vastly increased.

It should be noted that the two redevelopment areas (former Meridian Studios Site, and Chapel Riverside) would form part

of this route alignment, with new defences tying into raised land in these sites to form the strategic defence.

At each end of the study frontage (Mount Pleasant Industrial Estate to Northam Bridge, and from Crosshouse slipway to

Ocean Village) there remains the potential for a route alignment set-back a few metres behind the existing defence as the

site conditions (i.e. existing defences, space, reduced stakeholder requirements) mean that this alignment option can

remain in these areas as part of the wider ‘front’ alignment route.

Photo 6. Front line route alignment would require replacement and

raising of existing defences.



3. Set-back Alignment (typically behind waterfront industry)

In recognition of the significant technical issues and

potential affordability and deliverability issues associated

with a scheme alignment along or near the waterfront,

alternative route alignments options setback further from

the frontage were explored.

The key considerations relating to identification of a

feasible setback route alignment option are discussed

below. These key risks are not an exhaustive list, but

summarise the key decision points relating to this

alignment, and form the audit trail of route alignment

decision making.

Key decision consideration 1 (Technical) – Residual life

and capacity of existing defence structures

Given the aging existing quay walls and defences along

the frontage, and their typically fair to poor condition,

constructing a floodwall within their potential failure envelope is not viable as the risk of failure of these structures is too

acute. Therefore a requirement to explore a set-back route was identified.

Key decision consideration 2 (Economic) – Intent to maximise scheme benefits (but reduce costs)

As stated in Section Figure 5-1, the aspiration is to maximise the benefit area offered by the scheme. To achieve this, a

scheme alignment along the front is optimal and this is delivered by the Front Alignment Route (see above). However to

implement this route, large capital costs will be incurred from raising quay walls and significant private contributions would

be required to facilitate this option as potential grant aid monies are limited. As there is a risk that the required

contributions will not be achieved, an alternative lower cost, publically fundable option was explored.

By setting back the route alignment, alternative lower cost defence options become possible. However in setting back the

scheme, the benefit area reduces and residual risk remains for those assets in front of the defence line. The requirement

to protect residential areas is paramount and a primary driver for attracting public grant aid funding. Therefore it was

recognised that the set-back route must prevent flooding to these areas as much as possible. This is achieved by a route

behind the waterfront industry, but it means some commercial properties will remain at risk (as they are currently).

The St Mary’s Stadium comprises an area of raised land and by tying the defences into this site it has the potential to

reduce the overall scheme length and therefore delivers a cost saving.

Key decision consideration 3 (Technical) – Space restrictions

When assessing potential route alignment options for a set-back scheme, physical limitation such as buildings, working

areas and space were commonly encountered rendering many potential routes unviable without land purchase or

demolition of existing infrastructure.

Therefore the most viable route is to set back the scheme alongside Marine Parade and Milbank Street behind the

majority of the waterfront industry. There are still areas of limited space but generally a wider clear corridor exists here.

There are several localised areas where potential alternative alignments have been identified and the possibilities will be

consulted upon before confirming the final route alignment for the scheme, should this option be taken forward.


Many of the sites in the hinterland of the waterfront actively use their sites for business operations, storage, parking, plant

movement etc. There are also numerous boundary walls, fences and emergency exits. Any form of raised flood defence

dissecting these sites, or running around these sites will cause a physical barrier which is potentially unacceptable to

these operations, and also presents a health and safety risk. Some of these access provisions could potentially be

accommodated in the form of floodgates, but this would lead to a significant residual risk of failure, especially as many of

the gates would be located on private land with little control over their closure. The set-back wall would also leave a lasting

footprint potentially reducing working areas for operations.

Photo 7. A section of the setback route identified at the rear of

Marine Parade.



To avoid these issues, the route has to be set-back further behind many of these sites, running alongside Marine Parade

and Milbank Street. However this still involves numerous access provisions (33), but some of these have the potential to

be ramped instead of gates. Also these access points will be accessible from public areas and therefore greater control

over their operation can be gained.


Given the technical aspects, assessment of risk and implications of the key considerations discussed above, the potential

set-back defence alignment identified is feasible and provides opportunities to implement defences using lower cost

solutions than the front alignment option; however this alignment option carries the potential for significant residual

damage as many commercial and industrial properties remain in front of the defence.

It should be noted that the two redevelopment areas (former Meridian Studios Site, and Chapel Riverside) would form part

of this route alignment, with the floodwall tying into raised land or new defences in these sites to form the strategic

defence.

A map showing the two feasible route alignment options identified for further consideration (Front and set-back) is

provided in Appendix 5.



5.3 Defence Options

With identification of two feasible route alignments, the engineering options and defence types required to implement the

scheme were explored and appraised. This process was informed by the range of data gained from the surveys made in

this study, and particularly through an evaluation of the results of the feasibility stage ground investigations undertaken for

this study (See Section 4.2).

The approved and adopted Southampton CFERMS (URS, 2012) recommended that the scheme should deliver a

minimum 1:200 year (0.5% AEP) Standard of Protection across its design life (2060) to provide the optimal returns on

investment. For the initial purpose of option development and comparison, the SoP recommended by the Strategy was

used as a starting point to guide the required design height for the engineering options being considered. Confirmation of

the optimal SoP to be delivered was later tested and confirmed (see Section 5.4).

Based on various criteria such as typical ground conditions, groundwater interaction, stakeholder requirements, space,

buildability, environmental consideration, failure risks, and health and safety the potential engineering options were

identified and taken forward. For more information on the option development and engineering principles behind the

options see Appendix 3.

5.3.1 Front line defences

In order to implement a scheme along the front route, the crest height of the quay walls which comprise much of the

frontage must be raised. The design water level to achieve a 1:200 year (0.5% AEP) Standard of Protection at 2060 is

+3.50mODN.

A suitable freeboard allowance is required to accommodate factors such as waves and settlement. Given the very short

wave fetches (less than 1km) maximum potential waves are small and wind generated; a freeboard of 300mm was

therefore selected. Therefore the overall design height of +3.80m ODN is required to ensure the required Standard of

Protection is delivered. This requirement means that current defence crests typically need to be raised by approximately

1.0m to 1.3m.

The retained heights (i.e. the vertical distance between the river bed level and the top of the structure) of the existing quay

walls range between 5.1m at Ocean Quay and 7.2m at the Aggregate Wharves. The existing quay walls are typical

anchored sheet piles structures. Reinforced concrete slabs of varying thickness form the surface behind the quay walls.

The proposed increase in cope levels could be achieved through the following options.

• Construction of reinforced concrete gravity walls at the top of the existing quay walls;

• Construction of steep-faced reinforced embankments at the top of the quay walls; and

• Driving a new set of sheet piles in front of the existing quay walls and tying back the new sheet piles via a

suitable method.

Although the first two options are relatively straightforward to build and of relatively low cost, they were deemed not to be

suitable due to the age and relatively poor condition of many of the existing quay walls, meaning their residual life is not

sufficient and the risk of failure is too acute. In addition these options do not address the seepage risk through the existing

quay walls.

As a result, it is recommended that the most viable option to implement a front alignment is to construct new sheet piles

immediately in front of the existing quay walls. Although the details change slightly with each site, an indicative sketch of

the proposed solution is presented in Figure 5-1.



Figure 5-1. Indicative typical section for the front quay raising solution (Not to scale).

5.3.2 Set-back defences

In order to deliver a scheme along the set-back route, a series of floodwalls are proposed. Other raised defence solutions

such as embankments were considered but ruled out due to the significant land take and lasting footprints these options

are undeliverable and/or unacceptable for a space constrained urban environments.

Once again, on the basis of the Strategy recommendations, an initial design water level of +3.50mODN was assumed.

Therefore including a 300mm freeboard allowance to accommodate potential small wind driven waves and settlement, a

design height of +3.80m ODN is required to provide protection up to a 1 in 200 year event at 2060. This requirement

means that typically current defence crests need to be raised by approximately 1.0m to 1.3m.

The following options were considered for the floodwalls.

• Reinforced concrete gravity walls;

• Raising ground levels via earthworks; and

• Sheet pile walls.



Reinforced Concrete (RC) gravity walls are a reliable option in terms of capacity to resist the load imposed by the retained

water but, where the structure is founded on permeable ground, extensive works would be required to provide a

groundwater cut-off. The land take required for the wall base is significantly higher than that required for a sheet pile wall

and this option would be likely to incur even greater services and utilities impacts and therefore costs.

Earthworks could be undertaken to raise ground levels up to the proposed levels along the line of defence. However, with

space often very limited, the increased land take requirements renders this option the least implementable, except in the

redevelopment areas where this option is preferred and can be built upon to form a passive defence.

Utilising sheet piles to form the floodwalls and a groundwater cut-off requires only conventional construction procedures

and provides a solution that can be applied to all areas of the site where access and working space is not a constraint.

The implementation of this option does however incur vibration and noise. Provided that further detailed Ground

Investigations confirm suitable conditions for driving sheet piles to the required depth, the sheet pile solution can provide a

cost effective consistent design and construction procedure for the most, if not all, of the set-back alignment. The sheet

piled floodwall was therefore taken forward to feasibility design stage and an indicative sketch of this option is provided in

Figure 5-2.

Figure 5-2. Indicative typical section for the set-back piled floodwall solution (Not to scale).



5.3.3 Floodgates

Given the numerous access requirements along the scheme’s length, it is inevitable that flood gates will be required.

These gates will be closed during extreme flood events to form a continuous barrier. The height of these structures is up to

1.7m and their width is up to 10m. Where possible the gates should be replaced with more passive measures, especially

for pedestrian access, through ramping or raised sections; this is to reduce the residual risk that floodgates generate.

Either side of, and directly adjacent to the floodgates, the floodwalls will be subjected to higher loading during a flood

event as they will have to resist the water pressure applied to the back of the gates. As a result, the floodwalls at these

locations will be designed for this additional load. It is anticipated that this support can be provided either by a heavier

section and/or longer pile, or that H-piles could be used to support the gates. H-piles that provide a water-tight connection

to sheet piles are available for this purpose.

In order to provide a groundwater cut-off at the location of the floodgates, sheet piles will be driven from ground level to

the same toe depth as the adjacent floodwall sheet piles. An indicative sketch of the typical proposed solution is presented

in Figure 5-3. This floodgate configuration was taken forward to feasibility design stage.

In order to reduce the residual risk of failure, telemetry and electronic activation will be used where possible and will need

further consideration during detailed design of a scheme.

Figure 5-3. Indicative sketch of a typical flood gate configuration (not to scale).



5.3.4 Land raising

As in line with the recommendations of the approved Southampton CFERMS, the developer is raising the land at the

former Meridian Studios site, and implementing raised front line defences and land raising at Chapel Riverside to the 100

year design water level (4.15m ODN).

These development sites will then form part of the strategic defence, tying into the wider scheme to provide an increased

standard of protection to the areas behind. Although detailed plans and design drawings have not yet been made available

for these sites, the final outline design for the scheme will incorporate the plans and tie in details.

The optimal Standard of Protection has been revisited and confirmed through incremental benefit cost testing in this

preliminary study on the basis of the more detailed cost estimates and benefits of the feasible options.

5.4 Scheme design life

Design life

The Southampton CFERMS proposed the scheme is to deliver a minimum 1:200 year, 0.5% AEP, Standard of Protection

until 2060 to provide the interim protection until the longer term strategic option of land raising can be delivered through

redevelopment along the waterfront. This service life is also a reasonable expectation for a reinforced concrete retaining

wall solution which was initially proposed under the Strategy.

The sheet pile floodwall solutions proposed for each option open up the potential to obtain a longer scheme life, if suitable

pile thicknesses and materials are adopted, and adequate future maintenance is carried out. In line with supplier guidance,

allowance for floodgate refurbishment or replacement should also be included at an interval of 25 years.

With these provisions a 70 year design life is achievable. With the recognition that the longer term land raising option may

take considerable time and co-ordination to implement, the potential for an interim solution with the potential to deliver a

longer service life means that beneficial contingency time may be achievable at a marginal additional cost.

Standard of Protection

The approved and adopted Southampton City Coastal Flood and Erosion Risk Management Strategy (2012)

recommended that the scheme should deliver a minimum 1:200 year (0.5% AEP) Standard of Protection across its design

life to provide the optimal returns on investment.

Following the FCERM Appraisal Guidance, the optimal SoP for the scheme has been reviewed in this study by following

the Incremental Benefit Cost Ratio (IBCR) decision rules and thresholds. The details of the appraisal are provided in

Appendix 6, and the optimal SoP based on a 1:200 year (0.5% AEP) event at 2060 has been confirmed.

By adopting the design standard on the 1:200 year (0.5% AEP) design event at 2060, but with the potential for a 70 year

design life (to 2085) there is the potential to provide significant additional benefit, even accepting that the standard of

protection will fall beyond 2060 if sea level rise occurs as currently predicted.

5.5 Options summary

With the feasible route alignments and engineering options identified, the two feasible options for detailed appraisal were

developed and confirmed (Table 5-1).

Table 5-2 summarises the preferred defence types proposed for each area for each route alignment option under

consideration.

Table 5-1. Options taken forward for detailed appraisal.

Option Name Route Alignment

Design life Design Level (including 300mm freeboard)

2060 SoP 2085 SoP

Option 1 Front 70 years 3.8mODN 1:200 year (0.5% AEP) ~1:50 year (2% AEP)

Option 2 Set-back 70 years 3.8mODN 1:200 year (0.5% AEP) ~1:50 year (2% AEP)



Table 5-2. The two route alignment options for detailed appraisal.

Area Option 1 - Front route solution Option 2 - Set-back route solution

1 – Mount Pleasant Industrial Estate Sheet pile wall (set-back a few metres from shoreline)

Sheet pile wall (set-back a few metres from shoreline)

2 – Former Meridian Studios Land raising Land raising

3 – Northam Bridge Front sheet pile wall Set-back piled floodwall

4 – Drivers Wharf Front sheet pile wall Set-back piled floodwall

5 – European Metal Recycling Front sheet pile wall Set-back piled floodwall

6 – Saxon Wharf Front sheet pile wall Set-back piled floodwall

7 – Shamrock Quay Sheet pile wall (set-back a few metres from shoreline)

Set-back piled floodwall

8 – Bakers Wharf Sheet pile wall (set-back a few metres from shoreline)

Set-back piled floodwall

9 – Ocean Quay Front sheet pile wall Set-back piled floodwall

10 – Dibles Wharf Front sheet pile wall Set-back piled floodwall

11 – Aggregate Wharves Front sheet pile wall Set-back piled floodwall

12 – American Wharf Front sheet pile wall Set-back piled floodwall

13 – Chapel Riverside Front sheet pile wall / Land raising Front sheet pile wall / Land Raising

14 – Scouts to SWAC Sheet pile wall (set-back a few metres from shoreline)




6 Economic Appraisal of Options

6.1 Approach

The aim of the economic appraisal is to determine the relative economic merits and returns on investment of each of the

options. This assessment forms a key part of the wider option appraisal process and allows informed investment decisions

to be made. It will also underpin the detailed business case for the preferred option and determine the likely levels of

Grant-in-Aid (GiA) funding for the scheme, alongside any potential contributions which may be required under the

Environment Agency’s Partnership Funding system.

To ascertain the comparative economic merits of each option the economic appraisal compares the costs of the options to

the benefits arising from these options. The primary costs associated with the scheme include design, construction and

maintenance. Benefits refer to the potential damages avoided by undertaking the options, through increased flood

protection to people, property infrastructure and the environment.

This comparison is known as a benefit-cost analysis (BCA) and provides a rational and systematic framework for

assessing the advantages and disadvantages of alternative options. This is achieved by expressing all of the potential

costs and benefits of an activity in a directly comparable unit of measurement; in monetary terms. By doing so, the costs

and benefits of different options can be directly compared and an economically preferred option identified. An option is

considered to be ‘justified’ if the benefits outweigh the costs.

Further details of the economic appraisal can be found in Appendix 6.

6.2 Methodology

The BCA of the options was carried out using the framework of the HM Treasury and Environment Agency Flood and

Coastal Erosion Risk Management appraisal guidance (FCERM-AG, 2010). FCERM-AG represents the latest standard of

cost-benefit analysis for all flood and coastal risk projects in England.

An integral part of BCA is establishing the baseline option of the ‘do nothing’ scenario. The ‘do nothing’ baseline

represents an assumed situation where no action is taken to manage flood risk. This hypothetical scenario is essential to

BCA as it provides the baseline from which all other options and scenarios can be compared against in order to

demonstrate the economic benefit of doing something.

6.2.1 Quantifying benefits

To quantify damages under the ‘do nothing’ scenario and establish option benefits (damages avoided) the numerical tidal

inundation model was used to simulate a range of different return period flood events over time. In GIS, National Receptor

Database property data was then overlain with the flood simulations to assign flood depths to each property for the range

of flood return events. The flood depths were then matched to typical flood damages for each property type that are

provided in the Multi-Coloured Manual.

For residential properties the Multi-Coloured Manual provides different flood damages for different types of flood event and

type of residential property (i.e. detached, semi-detached, flat, terraced etc.). For the purpose of this study, the type of

flood event adopted was a ‘Short duration, major flood of salt water’. Additionally a price uplift factor has been applied to

these damages to account for both emergency costs and inflation (through the latest Winter 2014 Consumer Price

Indices).

Commercial property damages were also obtained from the Multi-Coloured Manual. The type of flood event adopted was a

‘Short duration event of salt water, with a warning and no cellars’. A price uplift factor to account for the latest Winter 2014

Consumer Price Index was applied. Emergency costs are not applicable to commercial properties and therefore an

emergency uplift was not applied.

In accordance with the FCERM appraisal guidance residential and commercial properties were defined as written off once

flooded by an event of 1:3 year return period or less (under the assumption that the property would no longer be habitable

or functional). Once written off, these properties no longer accrued flooding damages. The guidance also requires that the

property flood damages over the appraisal period must not exceed the property market value. The cumulative damages



were monitored for each property and once they exceeded the property value further flood damages were capped and the

property was written off.

In addition to direct asset damages, a range of relevant intangible damages were also quantified following the Multi-

Coloured Manual guidelines. This included aspects such as traffic and rail disruption, loss of life risk, critical infrastructure

and school closures.

The direct asset and intangible damages under the ‘do nothing’ scenario were combined to provide the total ‘do nothing’

cash damages. To facilitate comparisons with options whose benefits span different points in time it was necessary to

discount the cash damages to ‘present values’ (PV). Following FCERM appraisal guidance, the following discount rates

were applied; 3.5% for the years 0 to 30, 3% for the years 31 to 75 and 2.5% for the years 76 onwards. Once discounted,

the ‘do nothing’ damages were used as baseline to compare the relative merits of the ‘Do Something’ scheme options.

The benefits (damages avoided) by the options were then estimated and compared to the whole life costs of the options.

6.2.2 Option Costing

The preliminary design concepts for the feasible options were initially worked up based on the available information.

The approved and adopted Southampton City Coastal Flood and Erosion Risk Management Strategy (2012)

recommended that the scheme should deliver a minimum 1:200 year (0.5% AEP) Standard of Protection across its design

life to provide the optimal returns on investment. Therefore initially options were costed based on delivering this standard

of Protection.

In the absence of ‘real’ costs from comparative local works, construction costs were initially estimated for each option

using rates provided in civil engineering price books (i.e. SPONS 2014) coupled with experience of costs from similar

projects.

Budget cost advice was also obtained from the contractor Balfour Beatty to benchmark and ratify the first principle costing.

The contractor estimates were within 10% of the cost estimates worked up from first principles (excluding optimism bias),

increasing confidence in the option cost estimates put forward.

Construction costs

For defence structures considered, the cost of the structure was estimated based on the typical height required. This

information was obtained through GIS analysis for the potential defence alignments. A number of flood gates and access

provisions were identified for each option together with typical width and height details. A suitable cost allowance based

on fitted estimates from three different suppliers was applied to cover these items.

Design Costs, Preliminaries and Scale of Works

In addition to capital construction costs, cost estimates for detailed design, preliminaries (preparation work) and additional

potential costs relating to the scale of the works were included as a percentage of the overall construction costs. A value

of 35% was applied to cover these items in line with industry guidance allowances for a feasibility level project.

Maintenance costs

In addition to capital construction costs, maintenance costs also contributed to the whole life scheme costs that were

estimated. Maintenance costs refer to the costs for periodic or annual maintenance works that are required to maintain the

structural integrity of the defences.

To contribute towards the whole life costs of each option, an estimated maintenance cost of £10,000 was applied annually

from year 10. Allowance for a complete replacement or major refurbishment of all the flood gates was included every 25

years following construction.

Key services easement / diversion

The scheme options have the potential to cut across or run parallel to numerous utilities. These have been identified (see

Appendix 5) and contact has been made with the utilities providers to gain indicative quotations for the easement of

diversions required to accommodate the services affected. Although these estimates have not yet been received a

£2million cost has been added to the setback route option cost, and £1million cost has been added to the front line route

at this stage as an allowance to cover these aspects. These estimated allowances reflect the number, types and

complexities of potential services impacted by the different routes.



Optimism Bias

In line with FCERM appraisal guidance, an optimism bias of 30% was applied to the whole life costs of each option.

Optimism bias;

“is included to account for the tendency for appraisers to be overly optimistic in early assessment of project costs,

timescales and benefits in comparison to the final values. This ‘optimism’ is a result of uncertainty in the final design detail

and implementation as a result of high level strategic approach required at this stage.”

In light of this, the HM Treasury issued guidance in the form of a percentage to increase the present value costs

depending on the uncertainty surrounding the estimates. Given the additional survey data, technical studies, engineering,

contractor cost advice and input obtained at this stage, an optimism bias of 30% is considered a suitable level to cover the

outstanding risks and uncertainties.

The breakdown of estimated capital costs for the options by areas (excluding VAT) is provided in Appendix 6. A summary

of costs for each option are provided in Table 6-1. A summary of the whole life costs for each option are presented in

Table 6-2.

Table 6-1. Summary of option capital costs (including 30% optimism bias).

Area Option 1 - Front route Capital Cost Option 2 - Set-back route Capital Cost

1 – Mount Pleasant Industrial Estate


£552,000 Sheet pile wall (set-back a few metres from shoreline)

£552,000

2 – Former Meridian Studios Land raising £500,000* Land raising £500,000*

3 – Northam Bridge Front sheet pile wall £1,094,300 Set-back piled floodwall £123,000

4 – Drivers Wharf Front sheet pile wall £3,383,400 Set-back piled floodwall £907,000

5 – European Metal Recycling

Front sheet pile wall £6,562,100 Set-back piled floodwall £440,000

6 – Saxon Wharf Front sheet pile wall £1,202,000 Set-back piled floodwall £0

7 – Shamrock Quay Sheet pile wall (set-back a few metres from shoreline)

£2,307,000 Set-back piled floodwall £557,000

8 – Bakers Wharf Sheet pile wall (set-back a few metres from shoreline)

£299,000 Set-back piled floodwall £458,000

9 – Ocean Quay Front sheet pile wall £4,223,000 Set-back piled floodwall £488,000

10 – Dibles Wharf Front sheet pile wall £6,057,000 Set-back piled floodwall £553,000

11 – Aggregate Wharves Front sheet pile wall £7,922,600 Set-back piled floodwall £1,538,000

12 – American Wharf Front sheet pile wall £1,275,600 Set-back piled floodwall £266,000

13 – Chapel Riverside Land raising £2,200,000* Land Raising £2,200,000*

14 – Scouts to SWAC


£852,000 Sheet pile wall (set-back a few metres from shoreline)

£852,000

Additional allowance for gates £487,000 Additional allowance for gates £837,000

Additional allowance for Highway Tables

£0 Additional allowance for Highway Tables

£120,000

Total £38,917,000 £10,391,000

*Indicative estimates and may vary subject to developer designs. These values may be reduced through developer contributions.

Table 6-2. Summary of whole life PV costs for each option (including optimism bias)

Option Name

Route Alignment

Design life PV design and construction cost (inc 30% optimism bias)

PV Indicative services diversion / mitigation

allowance

PV Post construction costs (e.g.

maintenance, gate replacement etc.)

Whole life PV cost

Option 1 Front 70 years £38,917,000 £1,000,000 £318,000 £40,235,000

Option 2 Set-back 70 years £10,391,000 £2,000,000 £440,000 £12,831,000



6.3 Option Benefits

6.3.1 Do Nothing Damages

The ‘Do Nothing’ damages were estimated for a period of 70 years over the duration of the scheme design life (2015-

2085). The ‘Do nothing’ damages are presented in Table 6-3. The Cash Damage relates to the damages in present day

terms. The Present Value Damages are those which include discounting and are used in the economic appraisals.

Table 6-3. Do Nothing Damages

Period Do Nothing Cash Damage (£) Do Nothing Damages (PV) (£)

2015 - 2085 840, 000, 000 214,419,000

6.3.2 Option Benefits

The economic benefits (damages avoided) for both the front and set-back options were generated for the period 2015 to

2085. The option benefits are presented in Table 6-4.

Table 6-4. Option benefits (2015-2085)

Option Option Benefits (PV) (£)

Option 1 - Front alignment 206,123,000

Option 2 - Set-back alignment 152,409,000

6.3.3 Residual Damages

Damages which still occur with the options in place are referred to as ‘residual damages’. Residual damage primarily

occurs from one or more of the following sources:

• damage associated with properties remaining in front of defences;

• damage from above design standard events exceeding defence crests; and

• defence breaching / failure

• outflanking of defences

The residual damage has been estimated for each option and mainly occurs from those properties remaining in front of the

defences (especially with the set-back alignment). Table 6-5 presents the residual damages of each option.

Table 6-5. Option residual damages (2015-2085)

Option Residual Damages (PV) (£)

Option 1 - Front alignment 8,296,000

Option 2 - Set-back alignment 62,010,000



6.4 Benefit Cost Assessment

The economic feasibility and efficiency of a scheme can determined by comparing the costs of implementing an option

with the monetary benefits that it provides. Table 6-6 below presents the costs, benefits and benefit cost ratio of each

option.

Table 6-6. Cost benefit analysis

Option Cost (£) Benefits (£) Benefit : Cost ratio

Front alignment £40,235,000 £206,123,000 5.1

Set-back alignment £12,831,000 £152,409,000 11.9

Table 6-6 indicates that both of the options are economically viable (i.e. Benefit:Cost ratio >1). Both options also provide

avoidance of significant damage, but the greatest benefits result from the frontline alignment; however the Front option is

more than three times the cost of the set-back option.

Consequently, the economically preferred option is the set-back alignment with the most favourable benefit: cost ratio

(11.9). However there is still significant residual damage with this option (PV £62M), but 98% (PV £60.8M) of this residual

damage is to commercial properties in front of the defence.

6.5 Funding

6.5.1 Partnership funding

Central Government funding, known as Grant-in-Aid, is available through Defra to help pay for Flood and Coastal Erosion

Risk Management, including flood alleviation schemes such as the River Itchen Flood Alleviation Scheme.

The amount of potential Grant-in-Aid funding available to each capital scheme largely depends on the number of

properties protected, the damages prevented and any other social or environmental benefits a project would deliver.

Where there is a potential shortfall in funding, additional contributions are required to be sought. This encourages a

partnership approach to funding and by gaining external contributions for a scheme the potential amount and prioritisation

of Grant-in-Aid funding made available to a scheme may be boosted.

A successful application for a £3M Local Enterprise Partnership (LEP) grant has also been made for the scheme. This

funding has been designated to assist the implementation of strategic flood defences to support the wider regeneration of

the area. This money is to be made available for the financial year 2016/17 subject to Environment Agency approval of a

Project Appraisal Report for the Itchen flood alleviation scheme, and this substantial grant can be counted as a

contribution towards the scheme costs which will thereby help ensure Grant in Aid monies can be drawn for the scheme.

Extensive liaison and discussions have been held (and are ongoing) with potential beneficiaries and developers to explore

the possibility of securing contributions for this scheme. In addition, other funding streams including utilising potential

Community Infrastructure Levy funds have been explored. To date, approximately £500,000 of land raising is anticipated

to be delivered directly via developers and work is ongoing to identify further contribution opportunities.

6.5.2 Partnership funding assessment and sensitivity testing

A partnership funding assessment was undertaken for each of the options, to determine the eligibility and quantity of any

potential Grant-in-Aid (GiA) funding. The assessment followed the Environment Agency’s funding guidance for flood and

coastal protection schemes. The Defra Flood and Coastal Resilience Partnership Funding arrangement defines the level

of GiA a project could achieve based on a series of Defra Outcome Measure (OM) targets.

There are four outcome measures under which projects can attract GiA. These are:

1. All benefits arising as a result of the investment, excluding those valued under the other outcome measures

(outcome measure 1).

2. Households moved from one category of flood risk to a lower category (outcome measure 2).

3. Households better protected against coastal erosion (outcome measure 3).

4. Statutory environmental obligations met through flood and erosion risk management (outcome measure 4).



The Environment Agency has prepared a standard spreadsheet (2014/2015 version) to calculate the level of GiA based on

a series of input parameters including the option costs, option benefits, contributions secured to date, and the number and

level of deprivation of the properties that are currently at risk (2015).

The partnership funding tool produces a partnership funding score (a percentage) which is used to indicate the chance of

attracting GiA funding approval. Following the implementation of the new funding arrangements any score over 100% is

likely to attract funding at some point in the future. The higher the score (over 100%), the higher the chance of attracting

priority funding.

As a sensitivity test a number of Partnership Funding calculations were undertaken for each option to include/exclude a

£3.5million contribution and to include/exclude an approximate £2.7million land raising costs. To view the Partnership

Funding Summary sheets per option see Appendix 6.

Front alignment

Table 6-7Table 6-7 presents the partnership funding scores for the Front alignment (Option 1).

Table 6-7. Partnership funding scores for the front alignment

With £3.5M contribution (LEP and

Developers)

Without £3.5M contribution (LEP

and Developers)

Option 1 – Front (including land

raising)

42% 34%

Set-back Alignment

Table 6-8 presents the partnership funding scores for the set-back alignment (Option 2).

Table 6-8. Partnership funding scores for the setback alignment

Option 2 - Set-back With £3.5M contribution (LEP and

Devleopers)

Without £3.5M contribution (LEP

and Devleopers)

Option 2 – Setback (including land

raising)

110% 82%

6.5.3 Summary

The Partnership Funding assessment demonstrates that Option 2, the setback alignment, has a significantly stronger case

for attracting partnership funding and Grant in Aid approval. This is largely due to the significantly lower cost of

implementing this option. When including the allocated £3.5M contribution (£3.0M LEP contribution and £0.5M private

contribution) the Partnership Funding score is in excess of 100%.

For the front line option it is evident, that given the significantly increased costs involved, a very large external contribution

(shortfall of approximately £23-26.5M) would be required to ensure that the scheme is eligible to gain Grant-in-Aid funding.

However if this could be achieved it would be preferable to deliver this option as a significant level of residual damage

(£53.7M PV) would be avoided at a cost of (£23M PV).

It is possible, that with a sufficient contributions and willingness of a waterfront landowner / landowners, a hybrid route

alignment option could become viable. In this instance part of the setback route could be replaced with a frontline

alignment. However, at this stage there has been no proposed commitment for this option, and without this, an entirely

setback solution (Option 2) route provides best value and significantly higher returns on public money, as well as still

meeting the key scheme objective of protecting residential properties.

A comparison of the economic assessment of options is provided in Table 6-9.



Table 6-9. Summary of economic assessment of options.






with improved

SoP


with improved

SoP


1. Front line 40,235,000 206,123,000 5.1 23,000,000 1157 679 8,296,000

2. Set-back 12,831,000 152,409,000 11.9 0 1157 507 62,010,000

* Including estimated £2.7M land raising costs.

** Additional contribution required after £3.5M contribution included.



7 Wider Implications of Options and Environmental Appraisal

7.1 Overview

In addition to the economic assessment, the wider benefits, drawbacks and implications associated with the options were

considered. The key categories considered in the appraisal included:

• Key services / utilities

• Surface water and drainage

• Cultural Heritage

• Ecology

• Land Quality / Contamination

• Outline Water Framework Directive Assessment (Water body impacts)

• Highways

• Landscape

Appropriate AECOM specialists and subject matter experts undertook an appraisal and evaluation of the scheme options

against each of the above categories, informed by the site investigations, survey data and baseline reviews undertaken

(see Section 4.2).

The construction and post construction phase implications of the different options (alignment choices and engineering

solutions) were identified and qualitatively discussed by area (Frontages 1-14 – See Map in Appendix 5). Although many

of the option implications are area specific and details vary, an overarching summary of the options assessment is

provided below and in an ‘at a glance’ format by category in Table 7-1. The full assessment findings are presented in

Appendix 7. In addition, the options have been screened for the potential requirement of an Environmental Impact

Assessment and a summary of the assessment is provided in Section 7.6.

It should be noted that the appraisals have been undertaken at an appropriate level of detail for the stage of the project

(feasibility) and further, more detailed assessments should be undertaken during outline and detailed design of the

preferred option.

7.2 Key services / utilities

A utilities survey was commissioned by Structural Soils Ltd as part of their ground investigation for Southampton City

Council. Balfour Beatty combined the individual utility plans into a single drawing, which has been shared for use in this

study.

Through assessing both data sets, it has been found that a number of services may be impacted by both the front and set

back alignment options. Although the location of services was considered as part of the route alignment selection process,

in some areas interaction between the defence and services cannot be avoided or designed out of the proposals.

The set-back alignment has greatest potential to impact services and mitigation (e.g. protection works) or services

diversion may be required as part of the scheme. The front alignment option has less potential to impact services;

however some mitigation or diversion should be anticipated as part of this option.

Service plans as well as the long section drawings giving an indication of where utilities could impact on the route

alignment options are provided in Appendix 5. For more detailed commentary on the location and types of services

potentially impacted see Appendix 7.



Southern Water, SSE and SGN have all been invited to comment on the proposals and provide budget cost estimates on

diversion or protection works, where appropriate. BT Openreach and Virgin Media assets may also be impacted by the

proposed schemes. Until this input is received, indicative cost allowances have been estimated and included in the

economic appraisal of options at this stage (Table 6-2).

7.3 Surface Water and drainage

The pluvial flow routes were assessed for the current topography and for each flood defence option. In order to analyse

the pluvial flood flow routing, the DrawNet application within the specialist software package MicroDrainage was used. The

flood flow analysis function in DrawNet simulates a defined quantity of water falling onto the catchment, and details the

direction of flow, areas of ponding/flooding and areas which would experience the greatest flood flow velocities.

A hypothetical 50mm rainfall event was applied across the study area and the analysis time (length of time allowed for the

water to flow) was set at 30 minutes. A flood flow analysis comparison was undertaken for the baseline model and options

models.

This initial assessment of the pluvial flood flow routes indicates that local areas are indicated to be at a slightly greater risk

of surface water flooding with the introduction of flood defences, particularly under the set-back route option. These areas

indicated to be at a greater risk of pluvial flooding will need to be assessed in more detail in subsequent design stages

considering a range of simulated storm events to quantify the increased risk and then design out or develop suitable

mitigation to address the issue.

For more detailed information on potential surface water impacts see Appendix 7.

7.4 Highways

Due to the front route alignment (Option 1) running around the waterfront, there is no interaction with any highways and

consequently the highway implications of this option were not assessed.

Option 2 (the setback route) runs alongside or across several main roads and many access roads. Therefore an initial high

level screening exercise of the concept deign was undertaken by highway engineers to identify potential highways issues

and requirements.

This review provided a check of option deliverability in relation to highways standards, and screened for compliance

against industry highways safety standards. As a result of this review, the concept design has incorporated, where

possible, the necessary measures to avoid impacts to the highways. Where potential issues are unresolved at this stage,

these have been signposted and potential mitigation has been identified for further more detailed consideration (see

drawings in Appendix 5). Estimated additional costs relating to these requirements have been included in the economic

assessment through provision of a highways related contingency lump sum.

Following consultation, if the setback route alignment is confirmed as the preferred option, the following steps will need to

be followed in relation to highways:

i) Preliminary design for planning application

A Stage 1 Road Safety Audit will be undertaken at the completion of outline design which will accompany the

planning deliverables (highway authority requirement). This will consider vehicle, pedestrian and cyclist

movements and will review the form of junctions/amendments proposed, whether they are appropriate and if any

modifications are required. This is the last occasion at which land requirements can be increased.

A series of drawings will need to be prepared which would show the general arrangement for the highways layout

at each location. These should include road markings, traffic islands, visibility splays, swept path analysis (if

required) and annotation to describe the works that would be undertaken. These will be developed on OS

mapping. At this stage, the drawings would indicate what Traffic Regulation Orders and Departures from

Standard would/could be required. These will need to be agreed by the highway authority and other stakeholders.

ii) Potential planning conditions/ancillary works

The highway authority is likely to require a signing strategy to be developed to identify at what locations signs

should be provided and what they would show (e.g. flip down signs that would indicate when the road is closed).



iii) Detailed design for technical approval

A Stage 2 Road Safety Audit will be undertaken at the completion of detailed design to accompany the

submission of a series of detailed highways drawings to the highway authority for their approval. The drawings

would typically show the layout of junctions, position of signs, carriageway markings, lighting provision and other

issues. These would be developed on topographical survey.

7.5 Landscape

A review of the potential landscape impacts of each option has been undertaken for both options. This was carried out by

AECOM landscape architects and was informed by a walkover character area assessment. The findings are summarised

in Table 7-1 and are provides in more detail in Appendix 7.

As the majority of Option 1 runs along the waterfront, which in the most part privately owned, there is limited scope for

landscape impacts or enhancement opportunities.

However, significant stretches of the setback alignment (Option 2) run through public space and along and across

highways, and the potential for detrimental impacts (in terms of views, access and aesthetics) exists, but also the is

significant potential for enhancement of the public realm.

During public consultation on the options a number of potential landscape opportunities and ideas will be presented for

feedback and comment. An example of type of sketches being used to demonstrate ideas and opportunities is provided in

Figure 7-1 and in full in Appendix 9.

Should there be a common aspiration for particular enhancements or finishes in certain areas there is the potential for

these to be delivered as part of the final scheme. However, it should be noted that beyond a basic, acceptable finish to the

floodwall, specialist or extravagant enhancement features are not likely to qualify for Grant in Aid Funding and will need to

be funded from other sources or through contributions.

Figure 7-1. Example visualisation to show potential landscape interest and enhancement ideas which will be displayed for consultation.



7.6 Screening the requirement for Environmental Impact Assessment

7.6.1 Front route alignment option

Based on the findings of the preliminary environmental studies reported in the preceding sections and summarised in

Table 7-1, it is not currently possible to rule out the potential for significant environmental effects in respect of the front

route options. This uncertainty is due to the following potential environmental impacts:

• Piling and other construction works in the river having potential for direct effects on fish habitat and intertidal mud

habitat;

• Construction works on the river frontage having indirect (disturbance) effects due to noise and visibility from the

adjacent designated sites; and

• Piling and other construction works resulting in impacts upon features of heritage importance.

Further studies may conclude that these effects could be mitigated during construction, potentially avoiding the

requirement for statutory EIA if it can be demonstrated that there is no potential for significant environmental effects. If this

cannot be demonstrated and it is concluded that the proposals constitute EIA development, the corresponding EIA would

likely entail a limited scope including just noise and vibration, ecology and potentially heritage assessments.

It should be noted that where a planning application is accompanied by an Environmental Statement reporting the findings

of an EIA in accordance with the statutory process, the determination period for that application is extended to 16 weeks.

7.6.2 Set-back route alignment option

This option does not constitute Schedule 1 development under the EIA Regulations, meaning that EIA is not mandatory.

However, this option is Schedule 2 development (under Part 10 (h) of the EIA Regulations in relation to flood defence

works). This requires that all works over 1 hectare in area should be ‘screened’ for EIA, which would be required if

significant environmental effects are likely.

Using the criteria in Schedule 3 of the EIA Regulations, the likelihood of significant effects on the environment has been

assessed.

The screening indicates that due to the complexity and scale of this option, and its potential impacts are such, that it is not

likely that an EIA will be required. It also demonstrates that significant effects on the Solent and Southampton Water

Special Protection Area (SPA) and Ramsar sites and Lee-on-the-Solent to Itchen Estuary Site of Special Scientific Interest

(SSSI) are unlikely.

Although it is concluded that statutory EIA is not likely to be required (subject to formal screening opinion from the local

planning authority), relevant environmental assessments will be undertaken and the reports of these assessments

included as part of the planning application.



Table 7-1. Summary of wider option impacts

Category

Option

1 - Front Route 2 – Set-back Route

Key Services A number of key services / utilities are impacted by the front route alignment including water mains, low and medium pressure gas mains, low and high voltage electricity cables, foul and surface water sewers, BT owned cables, Virgin Media assets and Vodafone cables. All assets which are impacted on will need to be diverted or protected to a level which will satisfy the authorising body.

The number of key services / utilities impacted on by the set-back alignment is greater than the front alignment. As the defences would run parallel and across a number of roads which act as service corridors, the number of service routes which will need diverting or protecting is significantly greater. The services impacted include water mains, low and medium pressure gas mains, low and high voltage electricity cables, foul and surface water sewers, BT owned cables, Virgin Media assets and Vodafone cables.

Cultural Heritage Preparatory groundwork’s for the construction of the proposed scheme could disturb archaeological deposits of Saxon, medieval and post-medieval date. There is also the potential to impact possible cemetery deposits in the vicinity of Dibles Wharf and American Wharf / Chapel Riverside. Remains of timber revetments or dock structures may be impacted by sheet piling. There is also the potential for an impact upon the Grade II listed quay wall at American Wharf, although it is unlikely that the historic setting of the Grade II* Chapel Mills will be significantly impacted. However, English Heritage should be consulted at the earliest opportunity to discuss the potential impact on the designated structures. Following confirmation of the proposed scheme design, it is recommended that further consultation should be undertaken with English Heritage and a detailed assessment of archaeological impact should be undertaken in consultation with the Archaeological Officer at Southampton City Council.

Preparatory groundwork’s for the construction of the proposed scheme could disturb archaeological deposits of Saxon, medieval and post-medieval date. There is also the potential to impact possible cemetery deposits in the vicinity of Dibles Wharf and American Wharf / Chapel Riverside. Following confirmation of the proposed scheme design, it is recommended that further consultation should be undertaken with English Heritage and a detailed assessment of archaeological impact should be undertaken in consultation with the Archaeological Officer at Southampton City Council.

Surface water and WFD impacts

The surface water run-off modelling to date indicates that the impact from either alignment route (when compared with the baseline) will be negligible. In addition the two alignment routes have a similar (minor) potential to impact groundwater and surface water quality with respect to piling, based on the ground investigation to date. Therefore no preference is given to the set-back or front alignment with respect to groundwater and surface water quality.

The surface water run-off modelling to date indicates that the impact from either alignment route (when compared with the baseline) will be negligible. In addition the two alignment routes have a similar (minor) potential to impact groundwater and surface water quality with respect to piling, based on the ground investigation to date. Therefore no preference is given to the set-back or front alignment with respect to groundwater and surface water quality. The set-back route alignment is further away from the channel of Southampton Water and intertidal mudflats and therefore the potential to impact Southampton Water is limited, making it the preferred option with respect to the biology and hydromorphology WFD related tests.



Category

Option


Contamination Given the historic industrialised use of the area along the front route alignment option, there is the potential for contaminants of concern to be present in river bed sediments and Made Ground. The proposed scheme involves the construction of a sheet piled wall with concrete capping beam which may create vertical migration of contaminants and disturbance of river bed sediments during the construction works. As such, there is considered to be a risk to the underlying Secondary A Aquifer and the River Itchen. Further assessment works where piles are proposed to be driven into Made Ground, tidal flat deposits or river sediments (as specified within Appendix 8 of this Options Report) should be undertaken in advance of any construction works to inform the need for mitigation should there be any potential risks.

Given the historical use of the area along the set-back route alignment option and the presence of reclaimed Made Ground, as indicated by the BGS published maps, there is the potential for elevated chemical concentrations to be present in the Made Ground beneath the proposed scheme. Construction of the proposed scheme may create vertical migration of contaminates in to the underlying Secondary A Aquifer and the River Itchen. Therefore, further assessment works where piles are proposed to be driven into Made Ground (as specified within Appendix 7) should be undertaken in advance of any construction works to inform the need for mitigation should there be any potential risks.

Landscape The front route alignment option would need careful positioning alongside the riverside footpath / cycleway at the northern end of the proposed scheme so as to retain a practical and safe path. This would also present an opportunity to rationalise the vegetation and fence boundaries in this area. Between Northam Bridge and American Wharf (inclusive) the front route alignment option passes through intensively commercial and industrial areas. The proposed scheme is adjudged to be relatively appropriate for this context from a landscape perspective, especially given the lack of amenity trees and planting and the area being largely a utilitarian landscape of limited landscape value. However, at Dibles Wharf, the proposed scheme would need to be considered carefully so as to provide safe and practical widths for the footpath along Belvidere Road, alongside suitable treatment of the proposed scheme itself to mitigate the potential for vandalism. To the south of the Chapel Riverside Site this option would allow for good access to the riverside so as not to disrupt the formalised recreational uses and the transitory visitors to the Crosshouse Road public car park.

The set-back route alignment option is the same as the front route alignment option at the northern and southern ends and so the relevant comments for the front route alignment option apply. The set-back route alignment would need to be given careful consideration around Northam Bridge due to the location’s function as a node (a convergence of footpaths, cycle route, roads and river where Northam Road meets the underpass beneath Northam Bridge). The proposed scheme should aim to avoid inhibiting but instead enhance access and orientation, especially for cyclists and pedestrians. From Northam Bridge to Bond Street the set-back route alignment option passes through intensively commercial and industrial areas and the proposed scheme is adjudged to be relatively appropriate for this context, especially given the lack of amenity trees and planting and the area being largely a utilitarian landscape of limited landscape value. Where the set-back route alignment follows Millbank Street, there are a dichotomy of characters and uses with low-rise residences of the Northam estate to the west and intensive industry and commerce to the east. Much of the eastern roadside boundary of the Northam estate contains mature or semi-mature tree cover which provides a tall, dense vegetative buffer between the opposing uses. The loss of these existing trees could result in a substantial adverse change to the character and appearance of the area. However, where substantial existing trees are lost to the proposed scheme, meaningful tree and shrub replacements could be made and the precise wall alignment should consider the provision of plantable space to achieve this. The set-back route alignment along this street would also require careful positioning so as to retain or improve upon the practicality and safety of the footway.



Category

Option


The set-back route alignment option on Marine Parade behind the aggregate wharves is likely to necessitate the removal of a large number of existing trees, resulting in a substantial change to the character and appearance of this area. However, the opportunity to provide meaningful replacement tree planting could mitigate for this loss. The safety and practicality of footpaths alongside the proposed scheme would be a key consideration as well as the suitable treatment of the proposed scheme itself to mitigate the potential for vandalism

Ecology The front route alignment option includes works within aquatic habitats such as the Natural Environment and Rural Communities (NERC) S41 Priority Habitat mudflats. Any loss of these habitats as a result of the cumulative effects of the works along the length of the proposed scheme may require mitigation in the form of replacement habitat within the local vicinity. In addition, where impact piling takes place within or immediately adjacent to these aquatic habitats, this could impact upon fish populations associated within River Itchen Special Area of Conservation (SAC) located upriver of the proposed scheme. Noise modelling will be required to enable appropriate mitigation to prevent impacting upon these internationally designated fish features. Due to its proximity to the River Itchen and its notable species, in general, it is considered that the front route alignment option is ecologically the less favourable of the two options.

Much of the habitats that the set-back route alignment option will impact are hard standing associated with roads and boating. These habitats have negligible potential for protected species. Where construction works associated with the proposed scheme are to impact directly on suitable trees or suitable buildings, bat roost potential assessments would be required in advance of a planning application to determine their potential to support roosting bats. To avoid impacting nesting birds, any vegetation that is to be removed as a result of the proposed scheme should be undertaken outside of the nesting bird season (April to September inclusive). The set-back route alignment option is generally the option with the least ecological constraints and is therefore the more favourable of the two options from an ecological perspective.



As part of the option appraisal process the key health, safety and environmental risks associated with options have been

identified and are outlined by category (design, construction, operation) in Table 8-1 below.

For each risk identified, the applicability to the route alignment has been identified. This assessment of risks also includes

potential interventions that could be considered by the designer to eliminate, reduce or mitigate the impacts of the risks.

Areas where residual risks may remain are also identified.

Although not included here, a wider project risk register is also being maintained. This is a live document which updated

as necessary by SCC / AECOM thereby keeping a log of the type and scale of risks to the project at large.

8 Health, Safety and Environmental Risks



Table 8-1. Health, Safety and Environmental Risks

Ref. Design aspect or

activity

(see note 3)

Description of constraints, hazards

and associated risks

(see note 4)

Designer’s interventions to

eliminate or reduce risk

(see note 5)

Residual risk, and information to

be provided to enable project

partners to manage the risk (see

note 6)

Risk to

setback

alignment

Risk to

frontline

alignment

DESIGN

D1 Unexploded

Ordnance (UXO)

UXO strike causing explosion

- A UXO desktop survey undertaken to confirm risk rating of areas from historic records.

- GPR scans undertaken. - SI excavations and piling

completed with suitable UXO sensors (magnetometer).

- Extensive Ground Investigations to be undertaken as park of detailed design including the use of magnetometer sensing for UXOs.

- GPR scans to be undertaken in the piling and excavation locations throughout construction

� �

D2 Ground Conditions Unknown ground conditions, - Limited Ground investigation undertaken in proposed areas of works.

- Ground investigation largely identified the underlying ground as made ground of variable quality. Further Ground Investigation to be undertaken at detailed design stage under preferred alignment

� �

Asbestos: Potential for asbestos in

made ground. Risk of

inhalation/ingestion resulting potential

asbestosis.

- Design to limit excavations into made ground wherever possible.

- Where ground investigation results have identified a risk, appropriate specialist asbestos consultants will be consulted.

- Residual risk of encountering asbestos will remain-

- Extent and type of asbestos to be confirmed by detailed design ground investigations.

� �

Voids: Various voids have been

identified in the made ground, creating a

potential risk of subsidence or the

collapse of either existing structure or

proposed defence.

- Some voids have been found to be present along the frontline alignment.

- Design to avoid voids wherever possible

- Where it is not possible to avoid voids, an allowance will have to be made to fill the voids.

- Residual risk remains, although is limited to the frontline alignment.

�

Poor quality ground: The GI identified

made ground of variable quality, risk of

subsidence or collapse

- Suitably sized foundations or piles will have to be incorporated into the design to overcome poor ground conditions.

- Increases the risk of interacting with services in the set-back alignment.

- Potentially very large pile required on the frontline alignment.

� �

- Pile supports or Anchors may be required to support the

- Residual risk of interaction of anchors with existing structures. � �




activity

(see note 3)



(see note 4)



(see note 5)




note 6)

Risk to

setback

alignment

Risk to

frontline

alignment

frontline piled solution.

Groundwater flooding risk: The GI

identified the potential for tidal

groundwater flooding.

- A suitable groundwater cut-off is to be incorporated into the design.

- Piling

� �

D3 Condition of existing

quayside

Sections of existing frontage in poor

condition. Risk of movement / collapse

during construction.

- Pile thickness testing undertaken to understand condition of piled frontage.

- Existing frontages in varying condition the pile thickness survey recommended applying weight limits in some locations.

�

D4 Unknown depth of

existing quayside

piling

Risk of collapse during construction if

existing piles overloaded.

- Design to assume that existing piles are not sufficiently long to support the required loads.

- Frontline alignment design to incorporate piles capable of supporting the quay wall.

- Residual risk of quayside collapse if existing quay is able to support construction plant.

�

D5 Services Known Services in close proximity to the

working areas (particularly setback

alignment) present a risk of striking

services causing electrocution or

explosion.

- Wherever possible avoid interactions with known services in design.

- Plans of services obtained from service suppliers

- Service providers to be contacted at detailed design when preferred route alignment confirmed.

- Residual risk remains, but the Contractor will incorporate service avoidance into their Construction Phase Plan (RAMS etc)

� �

Potential for unknown, private or

abandoned services not registered with

Service providers

- Request additional service information from landowners.

- Locate flood wall along alignment avoiding services where possible

- Residual risk remains, but the Contractor will incorporate service avoidance into their Construction Phase Plan (RAMS etc)

� �

D6 Existing Structures Unknown condition of existing structures,

potential for failure or collapse during

construction.

- Where available request details of existing structures from appropriate landowners.

- Where necessary undertake condition or structural surveys of existing structures.

- Residual risk remains

� �

D7 Piling Existing Structures: - Risk of vibration damage to existing

structures. - Location of existing foundations in

close proximity to pile location risk of damage/failure.

- Where available request details of existing structures from appropriate landowners.

- Where necessary undertake condition or structural surveys of existing structures.

- Wherever possible the design is to allow the maximum possible


� �




activity

(see note 3)



(see note 4)



(see note 5)




note 6)

Risk to

setback

alignment

Risk to

frontline

alignment

clearance from existing structures

- Risk of collapse if the existing quayside cannot support the loads associated with piling plant. (Frontline alignment)

- Maximum load capacity of existing quayside to be confirmed and incorporated into the designs

- Residual risk remains - Potential to undertake piling from

an off-shore working platform.

�

- Burying existing structures behind new frontline piles will prevent access for inspection and maintenance, leading to a risk of subsidence of the new frontage

- Pile survey undertaken to ascertain current condition of piles.


�

Previous structures: - Location and condition of old slipways

and structures underneath existing quaysides and wharfs unknown, risk of damage, failure or collapse. (Frontline alignment)

- Ground investigation identified the main areas of concern.

- Additional GPR to be used where necessary

- Old structures/ obstructions to be avoided wherever possible.


�

D8 Access / Egress: Risk that proposed defences will inhibit

emergency services access. (set-back

alignment)

- Existing boundaries and access points to be maintained wherever possible.

- Local Highways team and Emergency Services to be consulted during detailed design.

- Residual risk remains; however, access is the same as in a flood event without defences.

�

Risk that proposed defences will inhibit

emergency escape routes in the event of

a flood. (set-back alignment)


- Key stakeholders, local businesses and Emergency services to be consulted during detailed design.

- Raise awareness of flood risk and flood warning.


�

Risk that the proposed defences will

introduce additional traffic congestion

during a flood event, potentially causing

greater risk of accidents. (set-back

alignment)


- Local Highways team and local residents/businesses to be consulted during detailed design.

- Residual risk remains, however, traffic congestion is likely to be the same as in a flood event without defences.

�




activity

(see note 3)



(see note 4)



(see note 5)




note 6)

Risk to

setback

alignment

Risk to

frontline

alignment

Risk that proposed defences will inhibit

emergency access to off-shore vessels

(frontline alignment).

- Key stakeholders, local businesses and Emergency services to be consulted during detailed design.


� �

D9 Flooding Installation of flood defences may

increase the risk of surface water flooding

behind the defences.

- Drainage system to be designed to have sufficient capacity to prevent pluvial flooding in the event of a tidal surge.

- Residual risk of flooding if drainage becomes blocked, therefore drainage system to be regularly maintained as part of a maintenance regime.

- Surface water system to be surveyed at detailed design stage

- Existing Surface Water drainage system to be reviewed at detailed design stage.

� �

Breach flooding (i.e. floodgates left open

during flood event)

- Modelled breach analysis to consider the impact of leaving a gate open.

- Limit the use of floodgates wherever possible through passive solutions.

- Locate gates in public domain for easy access.

- Residual risk of floodgates being left open remains.

- Local Authority to implement a flood action plan to ensure floodgates are shut in the event of a flood event.

- Flood warning system to be implemented

- Consider the use of telemetry to monitor gate closures

� �

Risk of drainage system being purged by

tidal flooding, leading to surface water

flooding.

- Designs to incorporate flap valves on all outfalls to prevent tidal purging of the drainage system.

- Residual risk of flooding if flap valves fail, therefore flap valves to be regularly maintained as part of a regular maintenance regime.

� �

D10 Highway works Footway narrowing/closure. - Highway review of design undertaken.

- Flood proofing buildings considered to reduce loss of footway width

- Footway closed and widened on opposite side of highway to maintain footways widths

- St Marys football pedestrian movements to be reviewed at detailed design.

- Pedestrian movements to be reviewed �




activity

(see note 3)



(see note 4)



(see note 5)




note 6)

Risk to

setback

alignment

Risk to

frontline

alignment

- Flood wall pushed back to replace boundary fences when possible to maintain footway width.

Raised highway tables to enable a level

surface to accommodate flood

- Appropriate speed limits, road markings and sign posting to be developed during detailed design.

- To undergo highway design and road safety audit at detailed design stage.

�

Potential for dead space behind 1.5m

flood wall creating corridor effect along

footways.

- Highway review of design undertaken and dead space removed

- Flood proofing buildings considered to remove corridor effect, building foundation information to be obtained at detailed design.

- Building foundation information to be obtained at detailed design stage

�

CONSTRUCTION – In addition to the above residual risks the below generic risks have been included for information

C1 Working at height Fall from height causing injury to site

staff.

Wherever possible the design will

limit the need to at height

Residual risk remains for the

Frontline alignment � �

C2 Working in close

proximity to water

Risk of drowning or hypothermia from

falling in water.

Wherever possible the design will

limit the need to work in close

proximity to the quayside

Residual risk remains for the

Frontline alignment

�

C3 Proximity of works

to general public,

traffic and

properties

Potential injuries caused to site staff or

members of the public - From poorly maintained or organised

construction sites - from Construction Plant

Risk of damage to existing

buildings/property

- Flood wall located away from existing property and off the highway where possible.

Residual risk remains

Detailed Design to include:- - Suitably sized working areas with

adequate storage facilities. - Segregation from public - Traffic Management Plan - Site boundary, vehicle and

pedestrian access routes and compound areas to be included on drawings

� �

C4 Noise & Vibration Noise and vibration from construction

activities (including piling) risks to human

and property receptors.

- Works to be designed to limit noise and vibration wherever possible. Where unavoidable mitigation measures will be

- Residual risk of noise and vibration will remain � �




activity

(see note 3)



(see note 4)



(see note 5)




note 6)

Risk to

setback

alignment

Risk to

frontline

alignment

C5 Dust Dust from construction works (silica dust

from concrete, saw dust, imported

material fines, vehicle movements) have

the potential to act as a skin, eyes and

respiratory irritant to site staff and public.

In extreme circumstances, may lead to

long term or event fatal illness / condition.

- Where possible dust creating

activities will be removed or

limited within the design

- Where residual risk remains

appropriate mitigation measures to

be employed throughout

construction (i.e. Dust screens,

damping down haul roads, covering

stock piles etc)

� �

C6 Contamination,

Spillages and

Pollution

- Risk of either encountering, or creating

environmental contamination, spillages,

and or pollution during construction.

- Environmental Action Plan to be

prepared outlining any

significant constraints or risks.

- Site Compound should be

designed with enough space to

accommodate any pollution

prevention measures


- Suitably experienced

Environmental Clerk of Works

(ECOW) to be appointed.

� �

C7 River / Tidal

working / Proximity

to other Vessels

If piling/working from the river is required

there is an additional risk of

accident/injury from working over water in

a tidal zone and in close proximity to

other vessels

- Design is to avoid working from an off-shore platform wherever possible.

- Residual risk remains if off-shore working is required.

�

C8 Inclement weather Risk of accident or injury increased due

to bad weather.

- As far as possible design works to be undertaken during summer months

- Residual risks of bad weather remain, the Contractor will incorporate appropriate mitigation measures into their Construction Phase Plan (RAMS etc)

� �

C9 Excavations

(excluding services)

Risk of excavation collapse causing injury

to site staff.

Risk of excavation being flooded by

incoming extreme tide or surface water

flooding causing injury to, or drowning

staff.

- Excavation work limited

wherever possible

- All excavations to be designed

with sufficient space to avoid

steep slopes

- Tidal and weather information

will be provided.

- Competent and experienced

contractor to be appointed

- Daily tidal information to be

distributed at the start of each shift.

- Good communication of tide and

weather conditions on site.

- GWL to be monitored throughout

works using existing piezometer.

� �

C10 Use of Concrete Handling wet concrete - leading to

dermatitis.

- Concreting activities to be

minimised or pre-fabricated

wherever possible.

- Competent and experienced

contractor to be appointed � �

C11 Use of handheld

tools

Use of pneumatic plant - leading to

vibration white finger.

- Where possible breaking or compaction work to be undertaken using

- Residual risk remains if handheld tools are used, but risks are reduced by selected experienced,

� �




activity

(see note 3)



(see note 4)



(see note 5)




note 6)

Risk to

setback

alignment

Risk to

frontline

alignment

excavators/suitable mechanical equipment instead of handheld tools

trained and competent Contractors.

C12 Manual Handling Manual lifting of materials presents a risk

of injury to site personnel.

- Manual handling is to be designed out wherever possible and by using mechanical lifting aids where it is not possible to be avoided

- Manual handling remains an inevitable activity, but risks are reduced by selected experienced, trained and competent Contractors.

� �

OPERATION / USE

U1 Sheet-Piled

Floodwalls

Vandalism resulting in either failure or

reduced integrity/design life of flood

defence.

Vandal resilience to be

incorporated into the durability of

the design at the detailed design

stage

- Residual risk of vandalism remains, inspection and maintenance regime to be implemented to minimise impact

� �

Accidental off-shore damage from

vessels, resulting in either failure or


defence.

- Where appropriate the impact of vessels will be considered at the detailed design stage.

- Land owners and key stakeholders to be consulted throughout the design process.

- Residual risk remains - Where appropriate a ‘Notice to

Marinas’ will be issued

�

Accidental on-shore damage from

vehicles , resulting in either failure or


defence

- Where appropriate the impact of vehicles will be considered at the detailed design stage.

- Design will incorporate clear signage for traffic.


� �

SP walls under designed leading to

structural failure of defence.

- Design will be undertaken to latest (up-to-date) and relevant British and European Standards.


� �

Incorrect use of designed structure (i.e.

mooring or over loading) leading to

failure.

- H&S File and Operation and maintenance manuals will be prepared for the completed structure.


� �

Future works on existing services in close

proximity to the SP wall lead to the

undermining and failure of the structure.

- H&S File and Operation and maintenance manuals will be prepared for the completed structure.


� �

Future development of quayside

landscape or business activities will put

- H&S File and Operation and maintenance manuals will be prepared for the completed


�




activity

(see note 3)



(see note 4)



(see note 5)




note 6)

Risk to

setback

alignment

Risk to

frontline

alignment

the SP wall structure at risk of failure. structure.

Location/alignment present additional risk

to local traffic

- Local highways, planning teams and other key stakeholders will be consulted throughout the design process


�

U2 Floodgates (See

D9)

Floodgate left open during flood event.

Leading to significant breaching and

inundation.

- Locate gates in public domain for easy access.

- Consider the use of telemetry to monitor gate closures

- Residual risk of floodgates being left open remains.

- Local Authority to implement a flood action plan to ensure floodgates are shut in the event of a flood event.

- Flood warning system to be implemented

� �

Floodgates inadequately maintained,

leading to gate/seal failure during a flood

event.

- H&S File and Operation and maintenance manuals will be prepared for the completed structure


� �

Vandalism or theft of floodgates resulting

in significant breaching and inundation.

- Operation and Maintenance Manual will outline a monitoring and maintenance schedule to limit risks


� �



reduced integrity/design life of floodgate.

- Key stakeholders including Quay operators will be consulted on any changes of design to existing outfalls on the quayside.

- Where appropriate a notice to mariners will be issued


�

Accidental on-shore damage from

vehicles, resulting in either failure or

reduced integrity/design life of floodgate.

- Where appropriate the impact of vehicles will be considered at the detailed design stage.

- Design will incorporate clear signage for traffic.


�

Gate structure/foundations under

designed leading to structural failure of

gate in flood event.



� �

Manual handling of floodgates leads to

injury of gate operators.

- Gates will be designed to limit the amount of manual handling


� �




activity

(see note 3)



(see note 4)



(see note 5)




note 6)

Risk to

setback

alignment

Risk to

frontline

alignment

required. - H&S File and Operation and

maintenance manuals will be prepared for the completed structure

U3 Surface Water

Drainage and

Outfalls

Flood defence prevents existing surface

water drainage from taking place, leading

to pluvial flooding.



� �

In sufficient capacity designed into

drainage system to prevent pluvial

flooding during a tidal flood event.



� �

Outfalls and flap valves inadequately

maintained, leading to blockage or failure

that may result in a surface water flood.

- H&S File and Operation and maintenance manuals will be prepared for the completed structure


� �



blocking of outfall that may result in a

surface water flood.

- Key stakeholders including Quay operators will be consulted on any changes of design to existing outfalls on the quayside.

- Where appropriate a notice to mariners will be issued.


�



9.1 Conclusion

1) The appraisal of feasible options demonstrates a sound economic case to implement a tidal flood alleviation

scheme for this part of the Itchen. Both feasible options deliver benefits in excess of costs (Table 9-1).

The optimal Standard on Protection (0.5% AEP SoP at 2060) has been confirmed though an Incremental Benefit

Cost Ratio (IBCR) test.

The proposed design life is 2085, accepting beyond 2060 the SoP will begin to fall if sea levels rise as currently

allowed for.

By 2060 it is anticipated that the longer term Southampton CFERMS preferred option of land raising implemented

through redevelopment will largely provide the required longer term protection against tidal flooding.

2) A frontline route alignment (Option 1), replacing and raising existing defences, has a benefit: cost ratio of 5.1 and

delivers maximum benefits through significantly improving the standard of protection against tidal flooding to nearly

all of the commercial and residential properties in the flood zone; however this comes at a significant capital cost

(£40.2 M PV).

With the current National FCERM funding system in place a high level of non GiA contribution (approx. £23M)

would be required to make the frontline alignment option viable.

3) With the public funding limitations considered, an alternative set-back route alignment (Option 2) offers a viable and

publically fundable option which is likely to gain GiA approval. This is because the Partnership Funding system for

allocating GiA recognises that the residential benefits and outcomes delivered by this option are comparable to the

front alignment option (1157 residential properties better protected under both options), but at approximately one

third of the cost. The set-back option has a cost of approx. £12.8M (PV) and produces a benefit: cost ratio of 11.9.

A Local Enterprise Partnership contribution of £3M has been allocated to the scheme along with a developer

commitment to contribute to the scheme in the form of land raising.

Table 9-1. Summary of economic assessment of options.






with improved

SoP


with improved

SoP


1. Front line 40,235,000 206,123,000 5.1 23,000,000 1157 679 8,296,000

2. Set-back 12,831,000 152,409,000 11.9 0 1157 507 62,010,000

4) * Including estimated £2.7M land raising costs.

5) ** Additional contribution required after £3.5M contribution included.

6) It is recognised that the setback option leaves a significant number of commercial properties and waterfront

operations in front of the defence; these will therefore remain at the same Standard of Protection as in a ‘Do

Nothing’ baseline scenario. This means that there is the potential for £62M PV damage to these properties over the

next 70 years, as well as the ongoing risk of wider disruption to their operations.

9 Summary



Having liaised with many of the waterfront organisations during the study it is apparent that some of these

waterfront operations, such as marinas, warehouses and wharves may be relatively water compatible or ‘resilient’

to a degree of flooding, at least compared to residential property or more susceptible commercial property. Should

the required external funding not be obtained to deliver a scheme which benefits these operations (Option 1), the

setback option (Option 2) will be delivered. Should this occur, these organisations will be encouraged to individually

improve their resistance and resilience to flood risk; however these measures will need to be privately funded.

7) There are a significant number of utilities and services that will be impacted by a potential scheme, especially with

the set-back route alignment; utilities will need to be accommodated through diversions, protection works or other

appropriate mitigation. Overall the environmental impacts of the set-back option are less onerous than for the front

alignment option.

8) The setback route alignment (Option 2) runs alongside or across several main roads and many access roads.

Therefore an initial high level screening exercise of the concept deign was undertaken by highway engineers to

identify potential highways issues and requirements.

As a result of this review, the concept design has incorporated, where possible, the necessary measures to avoid

impacts to the highways. Where potential issues are unresolved at this stage, these have been signposted and

potential mitigation has been identified for further more detailed consideration

9) A review of the potential landscape impacts of each option has been undertaken for both options. Potential

opportunities and ideas for enhancement of the public realm will be presented during the consultation period.

Should there be a common aspiration for particular enhancements or finishes in certain areas there is the potential

for these to be delivered as part of the final scheme. However, it should be noted that beyond a basic, acceptable

finish to the floodwall, specialist or extravagant enhancement features are not likely to qualify for Grant aid Funding

and will need to be funded from other sources or through contributions.

10) There is a residual risk of defence failure presented by the requirement for a number of floodgates for access.

Suitable mitigation strategies such as telemetry and electronic activation of gates will be required to reduce this risk

to a minimum; specific requirements will need to be confirmed during detailed design.

9.2 Next Steps

The options put forward in this report will be presented for 3 months of public consultation and feedback between August

and October 2015. During this period, specific engagement and meetings will be held with key stakeholders and

landowners along the frontage to gauge opinion and further explore potential contributions for the scheme.

Following this consultation, the feedback will be reviewed and evaluated. Should the required external funding not be

obtainable to meet the shortfall to deliver the front line scheme (Option 1), it is intended that the alternative publically

fundable setback option (Option 2) will be delivered.

The outline design for the preferred option will then be updated and finalised following consultation. A planning application

will then be submitted to SCC, and a Project Appraisal Report (PAR), detailing the business case for GiA approval, will be

submitted to the Environment Agency Large Project Review Group in early 2016.

Following funding approval, a detailed design stage will be required with commencement of construction aimed for late

2016/17.

Given these timeframes, adequate and timely consideration of the procurement route should be made by Southampton

City Council in order to plan and expedite the intended programme of works efficiently. The two procurement approaches

include:

1) a consultant undertaking fully detailed designs of the scheme, then a contractor carrying out construction; or

2) a design and build route where a contractor carries out both stages to deliver the scheme.



Southampton Archaeological Unit (2014) Desk-based assessment of the archaeological potential of land in the Itchen

Riverside Masterplan area, Southampton

TVAS (2014) Meridian Gardens, Summers Street, Southampton, Hampshire Archaeological Desk Based Assessment

MSS14/175

URS (2012) Southampton Coastal Flood and Erosion Risk Management Strategy (Southampton CFERMS). Available

from: http://www.southampton.gov.uk/environmental-issues/flooding/managing-flood-risk/southampton-coastal-

strategy.aspx

Wessex Archaeology, 2005 Birbeck, Vaughan, with Roland J. C. Smith, Phil Andrews, and Nick Stoodley. The Origins of

Mid-Saxon Southampton: Excavations at the Friends Provident St. Mary’s Stadium, 1998–2000. Salisbury, UK: Wessex

Archaeology

10 References



Appendix 1 – Flood Modelling



Appendix 2 – Non Intrusive Survey Report



Appendix 3 – Ground Investigations and Interpretive Report



Appendix 4 – Environmental Baseline and Surveys



Appendix 5 – Option Drawings



Appendix 6 – Economic Assessment



Appendix 7 –Option Appraisal



Appendix 8 – EIA Screening Report



Appendix 9 – Landscape improvement / enhancement ideas and opportunities for consultation

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