Protecting Bridgwater and the Somerset Levels & Moors from Tidal Flooding

Flood Risk Management Review

November 2014

Environment Agency / Sedgemoor District Council Flood Risk Management Review

Protecting Bridgwater and the Somerset Levels & Moors from Tidal Flooding Flood Risk Management Review

CONTENTS

1. EXECUTIVE SUMMARY 3

2. AIM OF THE REVIEW 7

3. THE FLOODING CHALLENGE 7 3.1 Tidal and Fluvial Flood Risk ...... 7 3.2 Flood Risk Today and in the Future ...... 9 3.3 Supporting Development and Growth in Somerset ...... 12

4. FLOOD RISK MANAGEMENT OPTIONS 13 4.1 Options Assessed ...... 13 4.2 Other Options and Descriptions ...... 16

5. CRITERIA TO BE CONSIDERED IN OPTION ASSESSMENT 17

6. OPTION ASSESSMENT 20 6.1 Flood Risk ...... 20 6.2 Economics ...... 21 6.3 Environment ...... 23 6.4 Design and Delivery ...... 25

7. CONCLUSION OF ASSESSMENT 27

8. THE WAY AHEAD 29 8.1 The Delivery Stages and Programme ...... 29 8.2 Cost ...... 30 8.3 Benefits and Funding ...... 30 8.4 Legislative and Planning Issues ...... 31 8.5 Structure Design and Operation ...... 31

APPENDICES 33 Appendix A: Frequently Asked Questions 33 Appendix B: Key Features Plans 35

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Details of document preparation and issue: Version Prepared Checked Reviewed Approved Issue date Issue status no. 1 P Morley J Mason F Spaliviero F Spaliviero 27.11.14 Final

B&V project no. 122320/1830 Client’s reference no. IMSW002039

Notice: This report was prepared by Black & Veatch Limited (BVL) solely for use by the Environment Agency and Sedgemoor District Council. This report is not addressed to and may not be relied upon by any person or entity other than the Environment Agency and Sedgemoor District Council for any purpose without the prior written permission of BVL. BVL, its directors, employees and affiliated companies accept no responsibility or liability for reliance upon or use of this report (whether or not permitted) other than by the Environment Agency and Sedgemoor District Council for the purposes for which it was originally commissioned and prepared. In producing this report, BVL has relied upon information provided by others. The completeness or accuracy of this information is not guaranteed by BVL.

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1. EXECUTIVE SUMMARY

Background

The extensive flooding of the Somerset Levels and Moors in 2012 and between December 2013 and March 2014 has brought into sharp focus the flood risk to Bridgwater and the Somerset Levels and Moors. Communities, businesses, agriculture and the environment rely upon effective flood risk management. The sustainable development of Bridgwater in particular relies upon flood risk infrastructure.

Reports have previously been developed relating to the options and opportunities for continuing to manage flood risk from the River Parrett to Bridgwater and the surrounding areas:

1 The Sedgemoor District Council Bridgwater Flood Defence Infrastructure Planning report 2009, the findings of which were incorporated into Bridgwater Vision 2009 – a 50 year strategic vision and Framework to bring about transformational change and regeneration of the town; 2 The Environment Agency 2006 Parrett Tidal Flood Defence Technical Review and the Parrett Estuary Flood Risk Management Strategy (both considered the range of options for how to manage the flood risk to Bridgwater).

The built up banks of the River Parrett provide defences against tidal flooding coming up the river. These protect approximately 10,000 homes and over 600 businesses in Bridgwater and surrounding villages, plus the low lying land up and downstream of Bridgwater. The 2009 report concluded that the best approach from a flood risk management perspective was to maintain the existing defences, before supplementing them at some point between 2030 and 2050 with a tidal surge barrier constructed somewhere between Chilton Trinity and Dunball.

The flooding in early 2014 led a partnership of key authorities to prepare ‘The Somerset Levels and Moors Flood Action Plan’, which was presented to the Secretary of State in March 2014. This identified the critical role a ‘tidal barrier or sluice’ could play in reducing flood risk and was the focus of an initial workshop in June 2014 held by Sedgemoor District Council and the Environment Agency. The Flood Action Plan has a clear objective of building a barrier or sluice at Bridgwater by 2024.

Following the June 2014 workshop, several related studies were commissioned:

- A review of strategic options for managing flood risk from the tidal River Parrett (this report); - An update to the hydraulic model of the river, incorporating updated cross-sections; - A peer review of the geomorphological impacts of a barrier.

Review of options for managing flood risk

This review was commissioned to re-examine the previous studies, to determine whether the recent flooding, updated data or other evidence might influence the preferred option or timescale for such an intervention. It is an initial review of existing reports and information; it is not a full business case for investment, which will be produced during the next phase of the project.

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It used the latest tide level predictions, flood event data from 2012 and 2013/14, the new Asset Survey carried out during this winter’s floods, the current dredging operation and learning from the 2011 major collapse of West Quay in Bridgwater.

The findings that are presented will be confirmed or revised when the updated river model and geomorphology analysis become available early in 2015, and during the future appraisal process. The emerging information is not expected to change the preferred option, but could affect the timing of an intervention.

We considered options including raising flood walls and embankments, tidal surge barrier, tidal exclusion sluice, and the Bridgwater Bay Lagoon (a developer-led idea for extracting power from the extreme tidal range in the Severn Estuary). The review assessed these options against four main criteria: flood risk, economics, environment and delivery.

Option assessment

A tidal surge barrier is designed to close when required to exclude extreme tides. It is comparable to the Thames Barrier in London, which is normally open but closes when necessary.

In contrast a tidal exclusion sluice permanently stops tides flowing upstream of the structure.

A Bridgwater Bay Lagoon would enclose part of Bridgwater Bay and the Parrett estuary within a sea wall. Potentially Bridgwater Bay Lagoon could be operated to minimise flood risk to the Parrett estuary, although this would be a secondary function, the main function being renewable electricity generation. This would mean there is no need to have a Parrett barrier as well.

The 2009 Sedgemoor District Council report identified a preference for a tidal surge barrier. This current review confirms the need for a major intervention and continues to support the option of a tidal surge barrier. This is based on flood risk management requirements only but does align with the previous planning and regeneration preferences that took account of the development opportunities for Bridgwater. The tidal surge barrier is the most appropriate and deliverable structure to build, with less adverse impacts, resulting in lower costs and greater confidence in a successful outcome. The Environment Agency continues to consider this as the most viable option, which concurs with the Council’s approach set out in the Sedgemoor District Council Core Strategy (adopted 2011).

The review has identified significant environmental impacts if a tidal exclusion sluice was adopted and there are technical difficulties associated with bank raising. The Bridgwater Bay Lagoon option (where flood risk management may be a secondary benefit) should be kept under review, in case options for building it come forward in the future.

Assessment of Current Flood Risk

The review has identified that flood defences in Bridgwater are currently offering protection from a tidal flood with around a 1 in 100 (1%) chance of happening in any one year. This standard of protection will continue to decline over time with the combined effects of sea level rise and deteriorating defence condition. This is a slightly lower standard of protection than our 2009 assessment, due to changes in the estimate of the extreme tides at the mouth of the River Parrett into the Severn Estuary. Increases in sea level caused by climate change are predicted to increase tidal flood risk in the future.

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Exceptionally high rainfall over prolonged periods of time in 2012 and late 2013/ early 2014 caused river flows in the River Parrett and River Tone catchments that exceeded the capacity of the river channels. This resulted in overtopping of the spillways and river banks upstream, which was the main cause of the recent flooding in the Somerset Levels and Moors. Whilst modelling has confirmed that high tides and high river flows do interact, and there were a number of high spring tides in 2014 that coincided with the extensive flooding, the total impact of the tide on recent flood levels in the Somerset Levels and Moors was limited. High tides were not the main cause of the 2012 and 2014 flooding in the Somerset Levels and Moors.

Construction Timescales

The construction of any tidal barrier in the Parrett Estuary will be a major civil engineering project. Careful design and planning is essential to ensure that safe, effective and reliable protection against extreme tidal events is provided for the next 100 years or more at a cost that represents good value. Experience from similar projects elsewhere indicates that any barrier would have a lead time of at least 8 years from inception to completion. For a tidal exclusion sluice this will be extended by at least 2 years to allow for technical and environmental investigations, approvals and mitigation. However the expected scale of the negative impacts associated with a tidal exclusion sluice may mean that effective mitigation is not ultimately possible.

Structure Costs

As of 2014, our cost estimate for construction of a surge barrier is £32.4 million, which includes £5 million for essential work to upgrade downstream embankments. Our cost estimate for construction of an exclusion sluice on the same basis is £35.1 million. However, for the exclusion sluice, it must be noted that this cost excludes any major environmental mitigation or water quality mitigation costs. We understand these additional costs could easily match or exceed the costs for building the actual sluice.

Next steps

The Environment Agency and Sedgemoor District Council are aiming to deliver the tidal barrier as soon as is practical, subject to confirmation of the business case and funding.

The immediate next step in the process is to disseminate the results of this study and the associated hydraulic modelling and geomorphology work to interested parties and take on board any comments. The results from these studies are not due until early next year. In parallel with this we will define the next stages of the technical work and develop a more detailed delivery programme. We will also seek proposals from suppliers and appoint one to carry out the technical work.

The next stage in the technical process is to identify and agree the preferred option, develop the business case for a barrier and associated works then seek approval for design and construction. The business case stage will include: appraisal of the different viable options; confirmation of the location; optimisation of the timing for construction; environmental assessment; ground investigation and other surveys; consultation; identification of the legislative route needed to gain approval; costing; and outline design. This is an extensive phase of work that could take several years to complete.

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Availability of funding for a barrier

Funding is initially required to get the project to the point where permissions are in place to construct a barrier. This work will include the appraisal, detailed design and approvals processes. Earlier this year the Somerset Flood Action Plan partners made a bid for funding to the Local Enterprise Partnership, which was successful in securing initial funding. This will supplement developer contributions that have been collected by Sedgemoor District Council for long-term flood risk management in Bridgwater. These funds are sufficient to undertake the work required in advance of construction.

A subsequent bid to the Local Enterprise Partnership has been submitted for the construction of a barrier and we are awaiting the results. Funding for the construction phase, however, may alternatively come from a combination of national Flood Defence Grant in Aid funding (through the Environment Agency) and partnership contributions from the Local Enterprise Partnership and Sedgemoor District Council’s developer contributions.

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2. AIM OF THE REVIEW

Considerable work has been undertaken over the last decade by the Environment Agency, Sedgemoor District Council and other partners to identify a sustainable approach to managing flood risk to the Somerset Levels and Moors and Bridgwater from high tide levels in the Severn Estuary. Plans are in place to develop a tidal surge barrier in the future to protect both existing and future development from climate change. This is an integrated solution to both manage the existing flood risk to communities and to facilitate the growth and regeneration of Bridgwater. Preliminary work such as ground investigation has been undertaken. However implementation of the proposed scheme was not originally planned to commence until after 2030. In the interim period the extensive existing flood defences would be maintained as required.

The flood event between December 2013 and March 2014 has highlighted how vulnerable individuals, homes, infrastructure and businesses currently are to weather generated flood events. In response to the flooding a broad partnership of local and national organisations prepared ‘The Somerset Levels and Moors Flood Action Plan’. The Action Plan identified the critical role a ‘tidal barrier’ could play to reduce flood risk.

The Plan identified the following actions:

1. Review design options for Bridgwater tidal barrier/sluice 2. [Undertake] discussions to identify approaches to accelerate the build programme and financing of the barrier/sluice 3. Build Bridgwater tidal barrier/sluice

This review seeks to complete action 1 in relation to the tidal barrier/sluice and to inform actions 2 and 3.

The review draws on a number of reports which were prepared by the Environment Agency and Sedgemoor District Council in the late 2000s and more recent scientific studies including:

 Parrett Tidal Flood Defence: Sluice/Embankments Technical Review December 2006  Bridgwater Strategic Flood Defence Infrastructure Planning June 2009  Parrett Estuary Flood Risk Management Strategy March 2010  Coastal Flood Boundary Conditions for UK Mainland and Islands: Design Sea Levels, Environment Agency 2011  Sedgemoor District Council Strategic Flood Risk Assessment 2008  Recent hydraulic modelling and site investigation data  Flood event data from 2012 and 2013/14  The Asset Survey carried out during early 2014  Analysis of the impacts and costs of the current dredging operation and  Learning from the 2011 major collapse of West Quay in Bridgwater.

Whilst a tidal barrier or sluice is a critical part of future management of flooding in the Somerset Levels and Moors area, it is essential that it is seen as part of an overall flood risk management system. Flood risk varies considerably over the area and an integrated system of flood risk management measures is required to address the various sources of flood risk effectively and provide a sustainable flood risk management approach into the future.

3. THE FLOODING CHALLENGE

3.1 Tidal and Fluvial Flood Risk

The recent flooding has highlighted how vulnerable individuals, homes and businesses are to weather generated flood events. Overall the Somerset Levels and Moors floods in 2012 and 2014 were caused by high fluvial flows, related to exceptionally high rainfall on the River Parrett and River Tone catchment over an extended period of time. Whilst there were some tidal flood events

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earlier in 2014, they were not the main cause of the 2012 and 2014 flooding in the Somerset Levels and Moors, although modelling has confirmed that high tides and high flows do interact. However, the main source of flood risk to Bridgwater and the lower reaches of the River Parrett is tidal flooding. The River Parrett is the only major river in Somerset that does not have a sluice or other structure to exclude high tides. Today approximately 10,000 homes in Bridgwater and other villages in the lower Parrett catchment rely on raised flood defences for tidal protection. In the future the number of properties at risk of flooding from tidal events will increase as Bridgwater continues to develop. The tidal floodplain in the Bridgwater area is shown on Figure 1 below.

Figure 1. The Tidal Floodplain in Bridgwater

This review re-examines a number of options to manage tidal flood risk, building on the considerable work already undertaken by the Environment Agency and Sedgemoor District Council over the last decade. It will highlight pros and cons of various approaches to the management of tidal flood risk without firmly selecting the best way forward as this is part of a wider debate which is beyond the remit of this study. The review relies upon existing studies with limited additional analysis. A full appraisal will be undertaken in a later phase of the project.

Whilst the main focus of this review is on the tidal barrier/sluice, the assessment has taken a step back to review whether a tidal barrier or sluice, as identified by the Environment Agency originally

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in 2006, is the best option, or whether other options such as bank and wall raising may also be viable.

3.2 Flood Risk Today and in the Future

Understanding the flood risk today and how it may change in the future must underpin the approach to flood risk management.

Today Bridgwater and the lower reaches of the River Parrett are protected by extensive tidal walls and embankments. These banks serve to contain high tides and minimise the extent of inland flooding. These defences are illustrated in Figure 2 below.

Figure 2. Existing Flood Walls in Bridgwater and How Raised Defences Could Look in the Future (if adopted)

The flood defences through Bridgwater are significant engineering structures. The present defences were predominately built in the early 1980s. Whilst the defences have generally performed well, they will gradually deteriorate even with the appropriate normal maintenance. Realistically the current defences in Bridgwater have a further life of 20 to 35 years.

Hydraulic modelling confirms that flood banks on the lower reaches of the River Parrett and upstream of Bridgwater protect adjacent areas from both tidal and fluvial events, however further upstream fluvial flood risk is predominant.

The potential influence of the tide on flood risk is threefold:

 High tides present a direct flood risk through overtopping or breaching of defences. Breaching risk increases as the defence condition gradually deteriorates over time

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 Sediment deposition restricts the conveyance capacity of the channel and increases fluvial flood risk. The influence of this factor is part of an ongoing geomorphological review  At high tide, tidal water occupies a volume in the river channel which could be available to receive fluvial flows

The following table illustrates how tidal flood risk in Bridgwater could change over time assuming that the walls and banks in Bridgwater and upstream are maintained at their current level. Climate change will cause sea levels to rise, which in turn will increase the occurrence of tidal bank overtopping in the estuary and in Bridgwater. The calculations do not include any freeboard (an allowance for uncertainty) when comparing existing flood defence levels with current predictions of water levels.

Present day risk of Tidal flood risk 20 year horizon 50 years horizon overtopping (2014) Risk of overtopping Approximately 1 in Approximately 1 in Approximately 1 in in any year 100 (1%) annual 50 (2%) annual 15 (7%) annual probability probability probability

This assessment is based on the recent reappraisal of tidal and climate change data at the Parrett Estuary which supersedes the data used in the reports produced by the Environment Agency and Sedgemoor District Council between 2006 and 2010. The new data was obtained from Coastal flood boundary conditions for UK mainland and islands: Design sea levels, Environment Agency, 2011. The new data indicates that extreme flood levels at the mouth of the River Parrett are approximately 140mm higher than previously estimated. This change should be considered in the context of the 14m tidal range in the Severn estuary and the relatively small difference between flood levels with different annual probabilities. There remain a number of key uncertainties including:

 Climate change  Flood defences with varying levels and construction leading to risk of failure from overtopping  Feedback from raising flood defences in some areas which increases tidal flood levels elsewhere  Underlying uncertainty in tidal flood levels due to science and data limitations.

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The following table illustrates some of the key flood and defence levels in Bridgwater and the Somerset levels and Moors:

Description Level m AOD Comment Current typical ground level in 7.2 Bridgwater Town Centre Current typical flood defence 8.4 level protecting Bridgwater town centre Current typical ground level in Varies 3.5 - 6.0 Properties tend to be located on the higher North Moor (Moorland area) areas Current typical flood defence Varies but typically Tidal flood levels are slightly lower further levels in North Moor 8.0 to 8.4 inland so tidal defences are frequently lower than in Bridgwater Current 1 in 100 (1%) annual 8.39 probability flood level in Bridgwater Current 1 in 200 (0.5%) annual 8.44 This level compares with the previous probability flood level in analysis which concluded a level of 8.3m Bridgwater AOD. 1 in 200 year (0.5%) annual Higher than 9.0 This level depends on how banks are raised probability flood level in downstream of Bridgwater, with higher Bridgwater in 2114 (without a banks increasing flood levels upstream. barrier)

The previous reports anticipated that the standard of protection provided by existing defences would fall due to asset deterioration and climate change. The standard of protection has fallen somewhat faster than envisaged in the late 2000s. The change is fundamentally driven by a reappraisal of existing extreme tide levels in the Severn Estuary, rather than climate change information. The photograph below illustrates the recent high tides in Bridgwater:

Figure 3: High Tide at West Quay Bridgwater, 3rd January 2014

The flooding between December 2013 and March 2014 in the Somerset Levels and Moors was caused by long duration high fluvial flows which the River Parrett could not carry without overflowing its banks and causing extensive flooding of the adjacent Moors. The costs of the

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flooding and disruption were significant and estimates of the economic impacts are currently being finalised.

3.3 Supporting Development and Growth in Somerset

The sustainable regeneration of Bridgwater within Sedgemoor District Council’s Local Development Framework can only be achieved if flood risk is managed effectively and appropriate flood management infrastructure is provided.

Sedgemoor District Council and the Environment Agency worked together in the late 2000s to commission studies to identify the most appropriate flood risk management solution and to put in place a funding mechanism which would ensure that new development in Bridgwater would contribute financially to the future construction of flood risk management works. The Local Development Framework Core Strategy Shaping the future of Sedgemoor 2006-27 (adopted September 2011) summarises the agreed approach:

“6.4 Large parts of the town are at risk from flooding and there is a need to deliver long term strategic flood defences. The preferred solution is currently a combination of the construction of a tidal surge barrier, ‘The Parrett Barrier’ and downstream bank improvements. These will not be needed until between 2030 and 2050 but it is important that funding is secured and that there is a clear delivery strategy in place in order that development within the areas of risk can continue. At the present time a Supplementary Planning Document for an interim tariff has been adopted that seeks to raise approximately 40% of the current capital costs of the barrier, the remainder will be raised through national or regional funding bids, contributions from major national infrastructure projects, or potentially an Environment Agency capital bid in the future”

The Core Strategy focussed on Bridgwater because of the large number of properties currently in the flood risk area and extensive developments and regeneration proposed for the town. However it should be recognised that the approach was also designed to protect the moors upstream of Bridgwater from tidal flooding.

This study will review whether this approach remains appropriate.

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4. FLOOD RISK MANAGEMENT OPTIONS

4.1 Options Assessed

This report reviews a range of flood risk management options. These are very similar to those considered in the late 2000s with the exception of the additional option of the Bridgwater Bay Lagoon. For the purposes of this report, it has been assumed that a 3.6 GW lagoon as considered in the Department of Energy and Climate Change (DECC) 2010 study could be constructed as described in more detail below. The options focus on the risks associated with tidal flooding to the lower reaches of the Somerset Levels and Moors and to Bridgwater.

The key features in the estuary are shown in Figure 4 and the key features in the Bridgwater area are shown in Figure 5 in Appendix B.

Option Description Do nothing Assessment of the ‘Do Nothing’, (or ‘walk-away’ option), provides a common baseline from which all ‘Do Something’ options can be assessed and is a fundamental requirement in Defra Flood and Coastal Erosion Risk Management Appraisal Guidance (FCERM-AG). The option primarily serves to illustrate the value of the present defences for economic appraisal. If this option was adopted existing flood defences would deteriorate, and any damage would not be repaired. The 2009 assessment suggests that the flood defence system would be in poor condition by about 2030. As sea levels rise and the flood defences deteriorate, flood risk would increase significantly eventually (probably by the middle of the century) leading to the abandonment of much of the centre of town and the loss of existing residential and commercial areas in Bridgwater. Do minimum This option involves carrying out general maintenance of existing defences, but allows the performance of the flood defences to deteriorate in line with sea level rise and ageing of the flood defence system. Work would not include major repairs due to subsidence, channel erosion or material degradation. Regardless of the level of general maintenance to the embankments and walls, the condition of the flood defences deteriorates over time. In response to higher water levels and weaker flood defences, the frequency of overtopping increases, significantly increasing flood risk with time. The risk of sudden tidal defence breach with real risk of loss of life also increases. Raising flood This option involves comprehensive wall and embankment raising throughout the walls and estuary to minimise future risks of overtopping and breach. This option represents embankments the ‘best’ wall and embankment raising option arising from the Parrett Estuary Flood Risk Management Strategy prepared by the Environment Agency in 2010, which followed national project appraisal guidance. A programme of adaptive bank raising and improvement works would be undertaken as required to meet the increasing flood risks from sea level rise and degradation/consolidation of the flood defences with time and maintain an acceptable standard of protection. This re-engineer option is feasible. Due to the nature of the raised defences and historic quay walls, it would become increasingly more costly and difficult to cater for higher flood levels and an increase in height to walls in the town centre is unlikely to be acceptable to the District Council as the planning authority. The collapse of West Quay wall in 2011 also shows that the strength of the historic quay walls cannot necessarily be relied upon to support higher flood defences. Generally defences will require a larger footprint as banks are raised over time.

Tidal Surge This option considers constructing a gated barrier structure on the River Parrett Barrier that would only close during periods of unusually high tides which threaten flooding of Bridgwater and the upstream Somerset Levels and Moors areas. The operating principles of a tidal surge barrier are shown in Figure 6.

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When not in use the barrier gates would be, depending on the design, raised out of the water or flat on the riverbed. The operation of the barrier would be similar to the Thames Barrier in London, although the mechanics of the system would differ reflecting the different conditions in the Parrett Estuary compared with the Thames Estuary. The structure would be situated downstream (north) of Bridgwater to protect most of the town and the Somerset levels and Moors inland. New embankments would also be required downstream of the town and barrier to ensure complete protection of the Bridgwater area. With the barrier in place the flood defences in Bridgwater may become redundant. However when the barrier is closed, flow down the River Parrett would be stored in the river channel. Analysis has shown that a small flood defence would still be needed upstream of the barrier, but it would be significantly lower than it is today. This option offers a lower risk of failure than raising the flood walls and embankments, whilst being cheaper to build and mitigate for than a tidal exclusion Sluice. Tidal This option considers constructing a tidal exclusion sluice on the River Parrett, Exclusion protecting areas upstream from flooding by preventing the incoming tide from Sluice flowing upstream of the structure. The operating principles of a tidal exclusion sluice are illustrated in Figure 6. Similarly to the Tidal Surge Barrier, a Tidal Exclusion Sluice would be located downstream of Bridgwater, with raised tidal defences required downstream of the structure and a small flood defence required upstream of it. The tidal exclusion sluice would provide a significant change to the aquatic environment upstream of the structure, changing the river upstream of the structure to a fresh water watercourse only. Additional works would be required to mitigate the water quality impact of stopping the regular tidal flushing of the River Parrett, as there are a number of industrial discharges and combined sewer overflows upstream of the structure, as well as runoff from agricultural land. The change to the tidal prism (volume) of the Parrett Estuary and prevention of the incoming tide carrying sediment upstream may also lead to significant changes in the distribution of sediment downstream of the structure. There may be a benefit in reduction in siltation upstream, but an increase in the risk of siltation increasing downstream. Bridgwater Studies were undertaken by the Department of Energy and Climate Change Bay Lagoon (DECC) reporting in 2010 which looked at a range of Severn Tidal Power schemes. These schemes would utilise the large tidal range in the Severn estuary to generate electrical power. One of the options investigated was the construction of the Bridgwater Bay Lagoon, the indicative location of which is shown in Figure 5 together with other key features of the estuary. Ultimately DECC confirmed that the Government did not wish to progress any of the Severn Tidal Power Schemes identified. However, it is understood that there is some ongoing interest in some form of Bridgwater Bay Lagoon project by private companies. No firm proposals are yet in place and there are considerable risks associated with this project. Studies undertaken by DECC confirmed that a Lagoon could help reduce tide levels upstream of the Lagoon in the Parrett Estuary dependant on the type and operation of a structure. However there remains considerable uncertainty about how this would operate. There are also a considerable range of technical, environmental and financial issues that need to be resolved if this option was to be progressed. The DECC 2011 estimate of £17.7bn capital cost is orders of magnitude higher than the costs being considered for the other options confirming that any flood management benefit associated with the Lagoon is a secondary issue.

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Figure 6: Tidal Surge Barrier and Tidal Exclusion Sluice Cross-Section Schematics

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4.2 Other Options and Descriptions

Recently in the media and elsewhere the possibility of building ‘a barrage’, ‘penning structure’ or ‘sluice’ were suggested. There is some confusion over nomenclature which does not assist understanding of what is proposed. The use of the word barrage has been avoided to prevent confusion with the Severn Tidal Barrage. The term sluice has been avoided in isolation because it is not sufficiently clear about how it would be operated. Instead, the terms ‘tidal surge barrier’, ‘tidal exclusion sluice’ and ‘Bridgwater Bay lagoon’ have been used throughout this report, as described above.

The additional option to pen water back behind a structure may have some perceived landscape benefits although there may also be environmental and drainage disbenefits. A penning structure would ensure that a minimum water level and depth would be retained through Bridgwater thereby covering the silty bed of the river. However river quality and colour would remain the same or deteriorate. In theory a penning structure (or occasional penning structure) could be constructed with a tidal surge barrier or with a tidal exclusion sluice. This option was considered in the Parrett Tidal Flood Defence: Sluice/Embankments Technical Review in December 2006. However it is not a solution to flooding on its own. Therefore penning should only be considered as an option which could be pursued with the main structure options but not alone.

The exact location of any structure has not been confirmed as part of this review. There may be opportunities to combine a flood management structure with a road bridge or other development which may influence the choice of location.

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5. CRITERIA TO BE CONSIDERED IN OPTION ASSESSMENT

The late 2000s studies considered a comprehensive range of issues in choosing the most sustainable approach to flood risk management. A reassessment of the performance of each of the options identified in Section 4 has been undertaken against criteria grouped under the following four headings:  flood risk;  economics;  environment; and,  design & delivery.

These criteria are described in more detail in the table below.

Criteria Description Flood Risk Flood risk to Considers how well the option will protect existing properties in Bridgwater. existing property in Bridgwater Flood risk to the Considers how well the option will protect the Somerset Levels and Moors Rivers Parrett and within the River Parrett and River Tone catchments, referred to as the Rivers Tone moors Parrett and Tone moors. Flood risk to Considers how well the option will protect future development sites identified future development by Sedgemoor District Council. sites Social Impacts of The social impacts of flooding can be severe, as highlighted in the recent Flooding flooding. This criterion considers the likely benefit to the society generally. Economics Flood risk benefits Provides a quantitative and qualitative review of the likely benefits associated with the proposed solution, based on the previous studies. The assessment is not limited to direct economic losses but considers the wider social and community impact associated with flood risk. Somerset County Council are leading on a study of the economic impacts of the 2014 flooding which considers wider perspectives such as community impacts, tourism, health and emergency response which is due to report in the near future. Growth, Considers whether the options help to support growth which is a local priority development and in the Sedgemoor Corporate Strategy and Local Plan. Effective management regeneration of flood risk is essential to support business confidence. Transport Considers the impact on key land transport links within and through the study area including:  A38  M5  Main railway lines Navigation Considers the impact on commercial shipping using the Parrett Estuary, Dunball Wharf (predominantly landing sand/aggregates) and Combwich Wharf (shipments for Hinkley Point), i.e. the Statutory Port of Bridgwater. The Competent Harbour Authority for the port is Sedgemoor District Council. Agriculture Considers the potential impacts on agriculture using knowledge of the 2012 and 2014 floods Costs Summarises the expected capital and maintenance costs (where data are available) to allow comparison with other options. Environment Geomorphology – Considers the impact on geomorphology (sediment movement)

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Criteria Description sediment dynamics Fisheries Considers the impact on the following fisheries interests:  Eel / Elver fishing on the River Parrett  Fish within the Parrett Estuary Recreation Considers the impact on the following recreational interests in particular:  Recreational boating from Combwich, Burnham-on-Sea, Highbridge  Bridgwater and Taunton Canal  Recreational use of footpaths etc. alongside the Rivers Parrett and Tone and on the Rivers Parrett and Tone moors Water Quality Considers the potential impact on water quality.

There are approximately 20 consented discharges to the River Parrett including Chilton Trinity Sewage Treatment Works (see Figures 4 and 5). There are likely to be more within the wider Parrett Estuary. Biodiversity/ Considers the likely impact on key statutory features. Statutory Features The area downstream of Bridgwater is of international ecological importance receiving the following designations (see Figure 4)  Severn Estuary Special Protection Area (SPA; Birds Directive 79/409/EEC, implemented in the UK by several different statutes including the Conservation (Natural Habitats &c) Regulations 1994) designated because the Severn Estuary generally supports an ‘internationally important assemblage of waterfowl’.  Severn Estuary Special Area of Conservation (SAC; Habitats Directive 92/43/EEC, implemented in the UK by the Conservation (Natural Habitats &c) Regulations 1994) designated because the Severn Estuary contains habitats and/or species which are rare or threatened within a European context.  Severn Estuary Ramsar site (Wetland of International Importance, Ramsar Convention 1971) was designated due to the estuary’s immense tidal range (the second-largest in world), which is associated with unusual estuarine communities with reduced diversity but high productivity.  Bridgwater Bay Site of Special Scientific Interest (SSSI; Wildlife and Countryside Act 1981 amended 1991).  Bridgwater Bay National Nature Reserve (NNR; National Parks and Access to the Countryside Act 1949 amended)

The area upstream of Bridgwater is equally ecologically important and includes the following nature conservation designations (only those SSSIs adjacent to the River Parrett are listed):  Somerset Levels & Moors Special Protection Area (SPA)  Somerset Levels & Moors Ramsar Site  North Moor SSSI  Langmead & Weston Level SSSI  Southlake Moor SSSI (within the boundary of the Levels and Moors SPA)  Curry and Hay Moors SSSI which lies upstream of the confluence with the River Tone

UK Biodiversity Action Plan (BAP) priority habitats in the vicinity include intertidal mudflats, coastal saltmarsh and coastal and floodplain grazing marsh (www.magic.gov.uk).

Heritage Considers impact on heritage

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Criteria Description Landscape Considers impact on urban and rural landscape Design & Delivery Location Considers whether there is likely to be a location suitable for construction Maintenance Considers the likely maintenance burden associated with the option. This includes maintenance of related flood defence assets such as dredging of channels and embankment repairs, as well as the maintenance of the structure itself. Renewable Energy Considers the potential for renewable energy generation. Funding Considers high level funding issues and opportunities. Legal Framework Considers how the option would be delivered from a legal / planning perspective Programme Considers the timescale within which the option could be delivered

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6. OPTION ASSESSMENT

A generally qualitative assessment of the options has been undertaken broadly based on information from the 2006/10 studies. Little new assessment has been undertaken for the review except where legislation or data has changed and where this could lead to a different conclusion to the work undertaken previously.

6.1 Flood Risk

Option Factors Flood risk to existing property in Flood risk to the Rivers Flood risk to future Social impacts of flooding Overall View Options Bridgwater Parrett and Tone moors development sites Do Nothing      Very significant flood risk as existing Very significant flood risk as Unacceptable impact Unacceptable impact on existing communities Unacceptable impact. Significant negative impact on property defences deteriorate and climate existing defences deteriorate as sites cannot be and particularly deprived communities in value, and future regeneration potential in deprived change effects increase predicted and climate change effects developed. Bridgwater. communities. flood levels. increase predicted flood levels. Do Minimum      Significant flood risk increasing in the Significant flood risk Unacceptable impact Unacceptable impact in the longer term. Unacceptable impact in the longer term. Significant negative longer term as existing defences age increasing in the longer term in the longer term as impact on property value and future regeneration potential in and climate change effects increase as climate change effects sites cannot be deprived communities. predicted flood levels. increase predicted flood developed levels. Raise flood walls ? ? ? ? ? and Short term reduction in flood risk is Short term reduction in flood Impact uncertain, Impact of high walls in Bridgwater may be Longer term, raising flood walls will make providing an embankments feasible but significant raising of risk is feasible but significant unlikely to be a oppressive, and potential surface water effective flood management solution increasingly difficult. flood defences within the town centre raising of flood defences will sustainable solution in drainage problems may reduce confidence in Urban design implications of raised walls in town centre is likely to be impractical in the become increasingly difficult the longer term. the system. High walls increase the risk of would not be conducive to town centre regeneration and longer term, particularly considering in some areas. rapid flooding with significant risk to life and improving quality of the public realm. Sedgemoor District surface water drainage problems health. Council is seeking to invest in public realm improvements behind defences. from the station through the town centre: the Celebration Mile scheme. Tidal Surge      Barrier Significant reduction in tidal flood Significant reduction in tidal Effective at reducing Effective at reducing the risk of social impact Likely to be effective at managing tidal flood risk, marginal risk over the long term from the date flood risk over the long term. flood risk to future of tidal flooding in Bridgwater and the Rivers reduction in fluvial flood risk to the River Parrett and Tone of construction. Marginal reduction in fluvial development sites Parrett and Tone moors. moors. flood risk. identified by Potential social impacts of fluvial flooding of No significant impact Sedgemoor District the Rivers Parrett and Tone moors will remain upstream of normal tidal Council. as only marginal reduction in flood risk, but limits. will be reduced by shorter duration of flooding. Tidal Exclusion      Sluice Significant reduction in tidal flood Significant reduction in tidal Effective at reducing Effective at reducing the risk of social impact Likely to be effective at managing tidal flood risk, marginal risk over the long term from the date flood risk over the long term. flood risk to future of tidal flooding in Bridgwater and the Rivers reduction in fluvial flood risk to the River Parrett and Tone of construction. Marginal reduction in fluvial development sites. Parrett and Tone moors. moors. flood risk. Potential social impacts of fluvial flooding of No significant impact the Rivers Parrett and Tone moors will remain upstream of normal tidal as only marginal reduction in flood risk, but limits. will be reduced by shorter duration of flooding. Bridgwater Bay ? ? ? ? ? Lagoon Significant reduction in tidal flood Significant reduction in tidal Likely to be effective Likely to be effective at reducing the social Some uncertainties remain about how a tidal lagoon could be risk likely to be achievable, but would flood over the long term. at reducing flood risk impact of flooding, but uncertainties remain. operated. Further assessment may resolve this issue. require compromise with the primary Marginal reduction in fluvial to future development objective of energy generation during flood risk to the Somerset sites, but uncertainties design and operation. Levels and Moors. remain.

Key:  - Impact generally understood and likely to be positive or acceptable ? - Impact may or may not be acceptable  - Impact unlikely to be acceptable or disproportionately costly/legal issues etc

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6.2 Economics

Option Flood Risk Benefits Growth, Development and Transport Navigation Agriculture Costs Overall View Factors Regeneration Options Do Nothing      ?  Unacceptable increase in flood Existing flood defences do not Unacceptable impact Unacceptable impacts on land Unacceptable impact on No direct capital cost of construction Uneconomic. risk related to both overtopping provide 1 in 200 year on major transport transport links will make agriculture including or maintenance costs but significant and breaching of existing standard of protection against links. Statutory Port of Bridgwater significant areas of Good negative effects on the local and defences. tidal flooding required for unviable. to Moderate, Very Good regional economy. Bridgwater would cease to be new development. or Excellent agricultural viable as an urban centre by the Deteriorating flood defences land. middle of the century resulting in and decreasing standard of properties being written off. protection will cause uncertainty and loss of confidence leading to withdrawal of investment and businesses. Do Minimum      ?  Unacceptable increase in flood This is not a significant Unacceptable impact Unacceptable impacts on land Unacceptable impact on No direct capital cost of construction Uneconomic in risk related to overtopping of improvement over the Do on major transport transport links will make agriculture including but significant negative effects on the the longer term. existing defences due to increases Nothing option. links in the longer Statutory Port of Bridgwater significant areas of Good local and regional economy (as well in peak tide levels - not a term. unviable in the longer term. to Moderate, Very Good as ongoing maintenance costs). significant improvement over the or Excellent agricultural Do Nothing option and similarly land in the longer term. Bridgwater would cease to be viable as an urban centre in the long term. Raise flood ?  ?  ?   walls and There would be short term The longer term problems Reduction in flood risk No significant direct impacts There would be short Increasingly difficult to maintain Uneconomic in embankments economic benefits associated with associated with raising to major transport anticipated. term benefits to standard of protection with sea level the longer term. raising the walls and defences would undermine links in the short term. agriculture associated rise. embankments. However benefits growth, development and Significant work with raising the walls Disproportionate cost of raising are likely to be reduced in the regeneration targets. would be required to and embankments. defences significantly within longer term due to problems of raise bridges within However benefits are Bridgwater, with high defences surface water drainage. Bridgwater to avoid likely to be reduced in eventually required. Walls would the need for floodgates the longer term due to need to be rebuilt completely due to on main access routes problems of land limitations of existing foundations. within the town drainage. Existing embankments would need centre. significant rebuilding to achieve acceptable crest width and slopes for stability and maintenance access. Tidal Surge        Barrier Significant flood risk benefits Scheme aligns with current Reduction in flood risk Structure would be within the Effective at protecting The current best estimate of the Economic achieved through reducing risk to planning policy framework to major transport Statutory Port of Bridgwater the Rivers Parrett and capital cost of the tidal surge barrier preferred solution Bridgwater (large number of and technical assessments links. Potential to limits. Tone moors from tidal is £27.4m. It is estimated that an (confirming properties). including future regeneration incorporate new No significant practical flooding and will stop additional £5m will be needed for previous studies). Rivers Parrett and Tone moors plans. crossing of River impacts as barrier would only saline water entering the work to downstream embankments to contain less properties and benefits Improvement of flood Parrett. be operated during surge moors. Also some very address increased tidal water levels. of protecting agricultural land are defences will facilitate tides, or possibly to assist marginal benefits to The current best estimate of the capped under current assessment investment and regeneration. during fluvial flood events. fluvial events. operational cost of the tidal surge guidelines but significant flood The structure could be barrier is approximately £200k per risk benefits can be achieved due designed to permit navigation annum. to the large areas involved and key upstream at all other times if transport infrastructure. required.

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Option Flood Risk Benefits Growth, Development and Transport Navigation Agriculture Costs Overall View Factors Regeneration Options Tidal  ?  ?  ? ? Exclusion Significant flood risk benefits Improvement of flood Reduction in flood risk Structure would be within the Effective at protecting The current best estimate of the No significant Sluice achieved through reducing risk to defences will facilitate to major transport Statutory Port of Bridgwater the Rivers Parrett and capital cost of a tidal exclusion sluice additional flood Bridgwater (large number of investment and regeneration. links. Potential to limits. Tone moors from tidal is £30.1m but this does not include: risk benefits properties). However uncertainty will incorporate new Significant sediment accretion flooding and will stop  significant environmental / compared to Tidal Rivers Parrett and Tone moors continue in the short term crossing of River downstream of structure likely saline water entering the water quality mitigation costs Surge Barrier but contain less properties and benefits during studies to consider Parrett. with potential to impact on moors. Also some very as yet unquantified need for of protecting agricultural land are environmental impacts, port operations. Mitigation marginal benefits to  work to downstream additional capped under current assessment confirm viability of scheme uncertain fluvial events. embankments that will be mitigation works guidelines but significant flood and mitigation required. Amendment to current right needed to address increased increases costs risk benefits can be achieved due of navigation up to town of tidal water levels (estimated and uncertainty to the large areas involved and key Bridgwater. at £5m). very significantly. transport infrastructure. The cost of dredging upstream would reduce, but additional costs would be anticipated due to increased dredging downstream. The operational cost of the tidal exclusion sluice is uncertain but expected to be significantly higher than £200k per annum. Bridgwater ? ? ? ? ? ? ? Bay Lagoon The main driver for the Improvement of flood Reduction in flood risk Structure would be within the Likely to be effective at High capital cost. Associated costs of Economic Bridgwater Bay Lagoon is defences would promote to major transport Statutory Port of Bridgwater protecting the Rivers design and construction of viability of the electricity production. There will investment. However links is likely, but limits. A lock would be Parrett and Tone moors Bridgwater Bay Lagoon to enable it Bridgwater Bay be some economic penalty uncertainty will continue as uncertainties remain. provided to permit navigation from tidal flooding and to function as a flood defence Lagoon is associated with operating it as no firm proposals for the through the structure. will stop saline water structure are not well understood. dependent on flood management infrastructure. lagoon exist, and the design Potential to mitigate impact on entering the moors. power generation, However it is likely that similar and delivery is outside of the navigation due to changes in However uncertainties and flood benefits to the Tidal Surge Barrier control of the Environment sediment regime not fully remain. management is a and Tidal Exclusion Sluice could Agency and Sedgemoor understood. secondary be achieved, but uncertainties District Council. issue/benefit. No remain. firm proposals Additional benefits to other coastal exist at the time of and inland communities in the writing. Bridgwater Bay area.

Key:  - Impact generally understood and likely to be positive or acceptable ? - Impact may or may not be acceptable  - Impact unlikely to be acceptable or disproportionately costly/legal issues etc

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6.3 Environment

Option Factors Geomorphology Fisheries Recreation Water Quality Biodiversity / statutory features Heritage Landscape Overall View Options Do Nothing ? ?     ?  Significant and Significant and Significant impact on Unacceptable risk of saline Unmanaged change, including Potential loss of Unmanaged reversion Unacceptable impact. uncertain uncertain the recreational contamination. potential impacts on the terrestrial or damage to to estuarine floodplain geomorphological change in the opportunities in both components of SSSI/European listed buildings or with associated changes change in the longer term will Bridgwater and the sites in the Parrett Estuary and other historic of vegetation and longer term. lead to Somerset Levels and areas upstream of Bridgwater assets, appearance uncertain Moors through loss particularly in impacts on of, or breach of, Bridgwater. fisheries. riverbank footpaths and cycle routes. Do Minimum ? ?     ?  Significant and Significant and Significant impact on Unacceptable risk of saline Unmanaged change in the longer Potential loss of Unmanaged reversion Unacceptable impact in uncertain uncertain the recreational contamination in the longer term, including potential impacts or damage to to estuarine floodplain the longer term. geomorphological change in the opportunities in both term. on the terrestrial components of listed buildings or in the longer term with change in the longer term will Bridgwater and the SSSI/European sites in the Parrett other historic associated changes of longer term. lead to Somerset Levels and Estuary and areas upstream of assets, vegetation and uncertain Moors in the longer Bridgwater particularly in appearance impacts on term Bridgwater. fisheries. Raise flood   ?      walls and No significant No significant Negative impact on No significant impacts No significant impacts anticipated. Negative effect Unacceptable impact Unacceptable impact. embankments impacts impacts riverside walks and anticipated. on Conservation due to the height of anticipated. anticipated. visual amenity in Area and setting defences required, Bridgwater town of Listed especially within centre. Buildings in Bridgwater. Bridgwater town centre and docks area. Tidal Surge       ?  Barrier Some local No significant No significant impacts No significant impacts as Freshwater, saline and terrestrial Historic assets Local impact only in Impact unlikely to be constriction of impacts as as barrier would only barrier would only be habitats maintained by new are protected. immediate surroundings significant. channel but no barrier would be operated during operated during surge tides. defences. Some minor negative of structure, no other significant impacts only be surge tides. impact due to construction works, significant impacts anticipated as operated during but new opportunities to enhance anticipated. barrier would only surge tides. biodiversity upstream of barrier. be operated during surge tides.

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Option Factors Geomorphology Fisheries Recreation Water Quality Biodiversity / statutory features Heritage Landscape Overall View Options Tidal Sluice       ?  Significant impacts Significant No significant impacts Significant impacts on An impact on fish migration in the Historic assets Local impact only in Significant environmental on sediment negative impact on riverside paths. If existing aquatic regime River Parrett is a potential adverse are protected. immediate surroundings impacts, several of which distribution will on fisheries on water is impounded at upstream of structure will effect on the integrity of the of structure, no other are likely to be occur, but are River Parrett. a high level there are occur, but are poorly Severn Estuary SAC (qualifying significant impacts challenging to mitigate. poorly understood. Eel passes could potential opportunities understood. species of the SAC are Sea anticipated. Potential to Environmental legislation be included but for boating on the Lamprey, River Lamprey, Allis enhance landscape in may mandate use of it is understood Parrett and Tone and Shad and Twaite Shad). Bridgwater by penning practical lower impact elvers rely on a visual amenity Significant changes to existing water at a level above alternative i.e. Tidal the tidal flow to benefit in Bridgwater aquatic habitats upstream of the existing silt banks. Surge Barrier. travel upstream town centre. structure. Terrestrial habitats so mitigating maintained by new defences. this impact is Some minor negative impact due unlikely to be to construction works, but new feasible. opportunities to enhance biodiversity upstream of Tidal Sluice. Environmental legislation may mandate use of practical lower impact alternative i.e. Tidal Surge Barrier. Bridgwater Bay   ? ?   ?  Lagoon Significant Significant Changes to flows Significant impacts on An impact on fish migration in the Historic assets The structure would be Significant environmental changes to impacts on within lagoon and existing aquatic regime River Parrett is a potential adverse are protected. visible from high impacts on the wider geomorphology of fisheries are wider Bristol Channel upstream of structure will effect on the integrity of the vantage points such as area. Further study the Severn Estuary likely, but / Severn Estuary are occur, but are poorly Severn Estuary SAC (qualifying Brean Down; visibility needed to confirm the which would poorly poorly understood. understood. species of the SAC are Sea from beaches may be of feasibility of mitigation. negatively impact understood. Potential negative Chilton Trinity STW, West Lamprey, River Lamprey, Allis lesser significance. Environmental legislation designated sites. Predicted effects on water Huntspill STW and the Shad and Twaite Shad). May help to protect may mandate use of effects include quality affecting overflow from the Significant changes to existing beaches and sand dunes practical lower impact local population bathing beaches. Highbridge Storm Tanks aquatic habitats within and at Burnham-on-Sea, alternative i.e. Tidal collapse/extincti Potential protection of would discharge into lagoon. upstream of lagoon. Terrestrial Berrow and Brean. Surge Barrier if flood on of twaite beaches and sand Outfalls from combined habitats maintained by new risk management is shad and dunes (and associated sewer overflows and sewage defences. considered as the only salmon, and holiday areas) at treatment works that Some minor negative impact due objective. local reductions Burnham-on-Sea, discharge into the tidal River to construction works, but new in sea and river Berrow and Brean Parrett would not be opportunities to enhance lamprey, eel which are otherwise significantly affected. There biodiversity upstream of structure. and twaite shad vulnerable to sea level are no similar outfalls that populations, in rise and storm discharge directly into the Environmental legislation may various rivers. damage. Likely to estuary, inside of the mandate use of practical lower restrict sailing access lagoon. impact alternative i.e. Tidal Surge from Burnham / Discharge plumes from Barrier if flood risk management is Combwich to the Hinkley Point power stations considered as the only objective. wider Severn Estuary (thermal plume) and but lagoon itself could Weston-super-Mare STW be attractive for some. would also be affected, potentially requiring mitigation to address water quality issues.

Key:  - Impact generally understood and likely to be positive or acceptable ? - Impact may or may not be acceptable  - Impact unlikely to be acceptable or disproportionately costly/legal issues etc

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6.4 Design and Delivery

Option Location Maintenance Renewable Energy Funding Legal Framework Programme Overall View Factors Options Do Nothing  Not applicable – ruled out by Flood Risk, Economics and Environmental assessments Do Minimum  Not applicable – ruled out by Flood Risk, Economics and Environmental assessments Raise flood        walls and Disproportionately costly to Significant maintenance issues No renewable energy Unlikely to attract funding Disproportionately difficult to Significant risk to Disproportionately embankments significantly increase flood expected due to problems with generation potential. due to landscape impact make arrangements to increase programme due to difficult to significantly defence levels in Bridgwater surface water drainage. and solution not flood defence levels within complexity of raising increase flood defence considered sustainable. Bridgwater. Legal issues with flood defence levels levels in Bridgwater. land control are likely. within Bridgwater. Tidal Surge    ? ?   Barrier Various potential sites Current best estimate of No renewable energy Scheme aligns with current Scheme aligns with current Outline programme Likely to be deliverable downstream of Bridgwater. structure maintenance costs generation potential. planning policy framework planning policy framework and indicates that a barrier in the short term if Areas safeguarded in Sedgemoor approximately including tariff mechanism technical assessments and could be built by 2022 funding is available. District Council Local £200,000/annum. to collect funds. future regeneration plans. if funding were Development Framework Core Otherwise limited impact on Cost prohibitive in the Wide ranging legal issues available. Strategy. This is currently under maintenance – defences still short term, as assessed would need to be addressed review and additional locations needed upstream and risk is not likely to be including, rights over land, that also facilitate growth and downstream though heights sufficient to justify navigation and environmental regeneration may also be required would be changed. investment when regulations. available. Likely little alteration to considered from a purely dredging needs. flood management perspective. Other funders may wish to bring the construction forward. Tidal Sluice  ? ? ? ? ? ? Various potential sites Significant sediment accretion Very limited renewable Cost prohibitive in the Wide ranging legal issues Significant risk to Considerable downstream of Bridgwater. downstream of structure is energy generation potential, short term, as assessed would need to be addressed desired programme uncertainties due to Areas safeguarded in Sedgemoor likely requiring dredging. especially in summer when risk is not likely to be including, rights over land, due to potential technical challenges and DC Local Development Some reduction in channel fresh water flows are often sufficient to justify navigation and environmental mitigation required. potential legal issues Framework Core Strategy. maintenance upstream due to very low (negligible when investment when regulations. Studies will be associated with excluding sediment. The compared with the considered from a purely required to address environmental impact. balance of costs associated Bridgwater Bay Lagoon). flood management technical issues. with the changing location of Not a key driver for a tidal perspective. The further studies accretion is not known. sluice and would be very Other funders may wish to may conclude that Overall maintenance costs much secondary to flood bring the construction delivering this option including mitigation are likely risk benefits; unlikely to be forward. rather than a Tidal to significantly exceed included. Surge Barrier could structure maintenance costs. not be justified. Bridgwater    ?    Bay Lagoon A potential site has been High maintenance cost of Significant renewable Disproportionately Wide ranging legal issues Programme The delivery of the identified by DECC. structure would be financed by energy generation potential. expensive for flood would need to be addressed. uncertainty due to Bridgwater Bay Lagoon energy generation. protection; renewable third party funding is outside of the control Significant maintenance energy generation potential requirement – flood of the Environment burden of removing sediment would need to attract third risk management Agency and Sedgemoor deposited within lagoon likely. party funding. benefits would be District Council. Flood risk management secondary. Significant There are no firm aspects are secondary from risk to desired proposals at the time of an economic perspective. programme due to writing. potential mitigation required.

Key:  - Impact generally understood and likely to be positive or acceptable

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? - Impact may or may not be acceptable  - Impact unlikely to be acceptable or disproportionately costly/legal issues etc

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7. CONCLUSION OF ASSESSMENT

The charts and discussion below summarise the review from the four perspectives of flood risk, economics, environment and design & delivery.

Key:  - Impact generally understood and likely to be positive or acceptable ? - Impact may or may not be acceptable  - Impact unlikely to be acceptable or disproportionately costly/legal issues etc

OPTION 1: RAISE FLOOD WALLS AND EMBANKMENTS

Flood Risk Economics Managing tidal flood risk in Bridgwater and in the Somerset Levels and Moors by simply raising walls and banks is technically challenging. Whilst flood risk could be managed by using this approach the cost of this option does not compare favourably with other alternatives. Technically raising walls and bridges is expensive and challenging given the poor ground conditions and frequently constrained sites. Surface water drainage would become difficult with sea level rise and would require pumping of water trapped behind the defences. The urban landscape impact of high walls is unlikely to be acceptable.

Design & This option is not preferred. Delivery Environment

OPTION 2: TIDAL SURGE BARRIER

Flood Risk Economics Technically a tidal surge barrier is effective at managing flood risk. The solution is expensive reflecting the difficult ground conditions and challenging construction environment in the tidal River Parrett. However it is less expensive than the other potentially viable options. Environmentally the tidal surge barrier will be used infrequently and at other times the tidal surge barrier will have no impact on water levels or sediment transport. Therefore the environmental impact is expected to be manageable. The option is likely to be deliverable. The exact timing of construction is in part a function of available funding.

This is the preferred option. Design &

Delivery Environment

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OPTION 3: TIDAL EXCLUSION SLUICE

Flood Risk Economics From a limited tidal flood risk perspective a tidal exclusion sluice will be effective. Equally the construction method would be very similar to the tidal surge barrier. However by excluding all tides there will be considerable changes to the sediment movement in the River Parrett and in the estuary. Unfortunately there remains considerable uncertainty about the scale of the change but the existing significant sediment movement in the estuary would suggest that changes could be dramatic. From an economic perspective a tidal exclusion sluice would stop estuary sediment being deposited upstream in the Somerset Levels and Moors (although fluvial sediment from the upper Design & catchment would remain). This may provide a Delivery Environment saving by reducing the need to dredge upstream. However further dredging will probably be required downstream of the structure. Environmentally the changes associated with the tidal exclusion sluice including the changes to water quality, impact on sediment movement and impact on fisheries is highly significant given the international designation of the Severn Estuary SAC. It is considered unlikely that the changes associated with this type of structure would be acceptable when considered against Water Framework Directive requirements. It is unlikely that suitable environmental mitigation could be identified and delivered.

This option is not preferred.

OPTION 4: BRIDGWATER BAY LAGOON

Flood Economics It is likely that a Lagoon in Bridgwater Bay could be operated to manage tidal flood risk in Risk Bridgwater and the Somerset levels and Moors. However it is uncertain whether this would be fully compatible with the energy generation requirements. Environmentally there are a considerable range of issues to be addressed which may have a significant impact on the Severn Estuary as well as the River Parrett. It is unclear whether suitable mitigation could be achieved. The delivery of the Bridgwater Bay Lagoon is outside of the control of the Environment Agency or Sedgemoor District Council. Design & Option should be kept under review as and Delivery Environment when proposals come forward. However since it is outside of the control of the authorities it cannot be relied upon for future planning.

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8. THE WAY AHEAD

The review of previous studies has confirmed that a tidal surge barrier will address flood risk to Bridgwater, allowing ongoing regeneration and development to continue, and offer a marginal reduction in depth and duration of fluvial flooding to the Somerset Levels and Moors. The review concludes that a tidal surge barrier is the lowest risk and most deliverable of the options considered. The way ahead assumes that this decision is made and outlines the key stages required to deliver the tidal surge barrier, the associated programme and costs.

The barrier would be delivered through a close partnership led by the Environment Agency and Sedgemoor District Council.

8.1 The Delivery Stages and Programme

The delivery programme assumes that partners will wish to deliver the works at the earliest practical time, but following good practice to minimise costs and reduce risks. Good project planning reduces risks for significant and complex capital works such as a tidal surge barrier. Experience from other similarly large and complex projects elsewhere has demonstrated that effective consultation and detailed assessments helps avoid later legal challenges and supports efficient delivery of the necessary approvals. The following main stages are envisaged:

1. Commencement Phase

The commencement phase will start to consider key issues such as the control of land, navigation issues and other aspects which require early consideration. An initial review of the planning process and legislative issues will also be required.

Key outputs will include:  Further understanding of the type of structure to be constructed;  Set-up of the delivery team and agreement of the key roles within the team;  A more detailed funding plan;  Confirmation of potential contributions from appropriate partners and Sedgemoor District Council contributions plan;  Confirmation of the status of the structure within Sedgemoor District Council planning policy.

At the time of writing further hydraulic modelling of the River Parrett is being undertaken. The results of the work are nearing completion. It is not expected that the modelling will have a major impact on the conclusions of this review and it is anticipated that a tidal surge barrier is likely still to be the best option. It may impact on the preferred timing of this intervention. A high level geomorphological study has been commissioned and this will further inform the discussion around siltation and the impacts on river morphology. A review of the economic impact of the 2014 flooding is being undertaken by Somerset County Council and this will also help inform the case for the tidal surge barrier.

2. Feasibility Study and Business Case Preparation

The feasibility study will consider all the main issues including engineering, environment, location, operation, costs and risks. Previous studies have not looked significantly at how frequently the new tidal surge barrier will need to be used taking into account future sea level rise, or included knowledge gained from the fluvial flood events of 2012 and 2014.

The feasibility study will consider how the tidal surge barrier works within the flood risk management system. In particular the tidal surge barrier will rely on the maintenance and improvement of existing tidal embankments downstream of the structure.

The issue of finance of the tidal surge barrier is discussed below in section 8.3. The feasibility study will assess in detail the cost of the tidal surge barrier taking into account potential future delivery risks. The study will reassess the economic benefits of the tidal surge barrier and prepare a business case for the construction of the structure.

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For whoever leads and delivers the project, it will require formal approvals to comply with that organisation’s governance processes. Approval of the business case will be required before the scheme can progress to detailed design and construction.

Concurrently, and to inform the feasibility study and the detailed design, extensive ground investigations should be carried out. The existing ground conditions are known to be poor based on recent ground investigation surveys, requiring major foundation works, and this is a critical element of the work. Once the preferred location is confirmed this work can be undertaken at any time.

3. Detailed Design and Planning

The detailed design will take forward the design principles stated in the feasibility study to prepare construction drawings and specifications etc. At the same time planning will be progressed (this aspect is discussed in section 8.4.

4. Construction

Experience from similar projects elsewhere indicates that construction is likely to take at least 2 years, once construction details are available and planning permission is obtained. A significant period of commissioning would then be required.

Each stage discussed above is essential for the effective delivery of the tidal surge barrier. There may be some scope to modify the stages (for instance by procuring both the detailed design and construction through a single contract). An indicative programme covering all the above stages for delivery is provided below:

Stage Commencement Completion Commencement Phase January 2015 June 2015 Feasibility & Business case July 2015 July 2017 Detailed Design & Planning August 2017 December 2019 Construction January 2020 October 2022 Commissioning and operation November 2022 On going

The programme is based on the information made available in the documents reviewed in this report, and should be considered as indicative only.

8.2 Cost

The capital cost of the tidal surge barrier including all the delivery stages above was estimated in 2009 as £24.9 million. The updated cost is £27.4 million following a review of ground conditions and wider construction costs earlier in 2014.

The operational costs were assessed in 2009 as being approximately £200,000 per annum.

Further investment is required in parallel with the development of the tidal surge barrier to raise and strengthen tidal embankments on the River Parrett Estuary downstream. An estimated investment of £5m is envisaged from 2015 to 2022.

8.3 Benefits and Funding

The review has confirmed the overwhelming need to ensure that Bridgwater and the Somerset Levels and Moors are protected from tidal flooding. The current flood defences protect at least 10,000 households and 600 businesses from tidal flooding. The total value of assets protected is in excess of £1.7 billion. The number of properties protected continues to rise as development continues in Bridgwater, and future growth in the area is dependent on effective flood risk management solutions.

Discussions regarding the funding of the structure are ongoing at the time of writing. The previous studies assumed that the tidal surge barrier would be funded through a combination of central government grant (Flood Defence Grant in Aid) and local investment derived through the Local Development Framework (LDF) infrastructure plan. Under this plan the Bridgwater

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Strategic Flood Defence Tariff was put in place in 2009 to ensure that new development contributes to the strategic flood defence needs in Bridgwater.

Earlier this year the Somerset Flood Action Plan partners made a bid for funding to the Local Enterprise Partnership, which was successful in attracting initial funding. This will supplement developer contributions that have been collected by Sedgemoor District Council through the tariff and will fund the feasibility and planning stages.

A subsequent bid to the Local Enterprise Partnership has been submitted for the construction of a barrier and we are awaiting the results. Funding for the construction phase, however, may alternatively come from a combination of national Flood Defence Grant in Aid funding (through the Environment Agency) and partnership contributions from the Local Enterprise Partnership and Sedgemoor District Council’s developer contributions.

8.4 Legislative and Planning Issues

A range of specific powers are required to allow the construction, operation and maintenance of a tidal barrier. Planning permission for the works will be required, and the application for permission will almost certainly require the preparation of an Environmental Statement given the environmental sensitivity of the area.

Powers will be necessary to:

 Construct, alter or maintain works within an inland waterway  Potentially interfere with existing navigation rights  Compulsory powers to buy land and the right to use land (for access etc)  Powers for making bylaws

There are a range of different legislative routes open for large or nationally important infrastructure including Hybrid Bills, Planning Act 2008 (for nationally significant infrastructure) and Transport and Works Act Orders. For various reasons a Hybrid Bill and use of the provisions of the Planning Act 2008 are not appropriate in this case. The Transport and Works Act order route may be applicable (and has been used successfully elsewhere for similar projects). The commencement phase will consider the powers required and the most appropriate legislative route.

8.5 Structure Design and Operation

The reports reviewed confirm broadly the type of structure envisaged. However the feasibility study would review all aspects of the design. The key design issues are summarised below:

 Managing sediment  Foundation and structure design in poor ground conditions  Gate design and durability  Operational aspects (including expected frequency of operation)  Power, telemetry and control  Safety  Maintenance needs, access and methods  Relationship with other tidal defences  Impact on existing navigation and the Statutory Port of Bridgwater  Reliability, redundancy and risk  Potential links with other infrastructure (e.g. road crossings on the River Parrett)  Environmental impact, mitigation and potential enhancements  Landscape impact and architectural design  Impacts of climate change

Computer generated indicative views of the barrier are shown below:

View of Barrier with gates closed (high tide)

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View of Barrier with gates open (low tide)

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APPENDICES APPENDIX A: FREQUENTLY ASKED QUESTIONS

What difference would a sluice or a barrier structure have made to the recent floods?

Had the tidal barrier been in place in 2012 the impact on the extent and duration of flooding would have been very small. This is also likely to be the situation for the 2014 event. A reduction in peak water levels of about 100mm in severe floods may be possible on North Moor for example (depending on the event). Further modelling is ongoing to confirm the likely impact.

The recent flood events were caused by high fluvial flows. Fluvial flooding is caused by the limited flow carrying capacity of the River Parrett and River Tone. Modelling has shown that any changes to water levels at the downstream end of the River Parrett have a limited impact upstream in the Levels and Moors.

What are the wider flood risk benefits of a sluice or barrier?

Flood risk benefits include reduced risks to railways, highways, agriculture, residential property, businesses, utilities and reduced flood response costs. A sluice or barrier would help reduce the social and health impacts of flooding which has been very significant in 2014 in particular.

Improving flood protection is important for business confidence to retain and attract investment and growth to the local area. The regional economy will also benefit from reducing disruption to the key transport links. Reducing flood risk will support the area’s tourism industry.

How much would a Tidal Surge Barrier cost to build?

The current best estimate of the capital cost of the tidal surge barrier is £27.4m. This excludes work to downstream embankments which will cost approximately £5m from the present day to 2022.

When can it be built?

Assuming funding is available then the structure could be built by 2022.

What are the steps to allow it to be built? What is the programme?

The main steps and programme (assuming funding is available) are summarised below:

Stage Commencement Completion Commencement Phase January 2015 June 2015 Feasibility & Business case July 2015 July 2017 Detailed Design & Planning August 2017 December 2019 Construction January 2020 October 2022 Commissioning and operation November 2022 On going

The above table assumes that work commences in early 2015 and that the Tidal Surge Barrier is the preferred solution. The programme is consistent with experience elsewhere for a structure of this scale and complexity. If a Tidal Exclusion Sluice is promoted the programme would be extended considerably due to additional studies and it is likely that a Tidal Exclusion Sluice would be rejected on technical or environmental grounds.

Are there things we could be doing now, regardless of which option is chosen?

Yes. In particular:

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 Setting up a delivery team  Further work on the proposed location, considering land requirements and access  Further ground investigation to assist with foundation design  Review further hydraulic modelling results  Obtain legal advice on the navigation and port impacts  Undertake environmental surveys

Why can’t we just build it now?

Complex structures require careful planning to reduce risks and ensure that a safe, robust and flexible solution is developed that will provide effective reliable protection for the long term. Only a solution which is proven to be effective is likely to attract funding.

What are the environmental issues associated with different types of structure?

There are relatively few environmental issues associated with a Tidal Surge Barrier as the barrier gates would only be closed relatively infrequently, when required to protect against a forecast tidal surge.

A Tidal Exclusion Sluice would have a greater impact as it would prevent the incoming tide flowing upstream of the structure. The river upstream would become freshwater, without any saltwater inflow, which would change the habitat significantly. It is thought that elvers rely on the flood tide to carry them upstream so it is likely the river would cease to be a viable eel habitat even if eel passes were included. The lack of an outgoing tidal flow would also impact water quality as ‘tidal flushing’ would no longer reduce the effect of consented industrial discharges to the river. There would also be changes downstream of the structure as sediment that is currently carried upriver on flood tides would be deposited in the Parrett Estuary. It is likely that a Tidal Exclusion Sluice would not satisfy the requirements of the Water Framework Directive.

A Bridgwater Bay Lagoon would have significant environmental impacts on Bridgwater Bay and the Severn Estuary as well as the River Parrett. However, the renewable energy generated by the barrage would be a significant environmental benefit.

What impact would a barrier or sluice have on future dredging needs?

A Tidal Surge Barrier would not significantly alter the present dredging and channel maintenance needs. A Tidal Exclusion Sluice would reduce dredging needs upstream of the structure by preventing the passage of silt upstream from the estuary. However, this may lead to rapid siltation downstream of the structure, which would need to be dredged to maintain navigation, and unpredictable effects on sediment transport in the Parrett Estuary. Fluvial derived sediment would still deposit upstream.

There is considerable uncertainty associated with the geomorphological impact of the Tidal Exclusion Sluice. An initial study is underway to inform this, but a full assessment will be required as part of the approvals process.

What are the benefits of penning water at a higher level in Bridgwater?

There is no flood risk benefit of penning water at a higher level in Bridgwater.

Some amenity benefits could be achieved through improving views of the river by covering up the mudflats, and maintaining a depth of water suitable for use by recreational craft. It may make re-opening the lock connecting the River Parrett and the Bridgwater and Taunton canal for use by watercraft more viable. It is likely that navigation would still be difficult due to the

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The level of the pen would need to be balanced to ensure flood fluvial flood storage requirements are not compromised and that gravity drainage and discharges from upstream are maintained.

Why can’t we just increase the height of the existing banks?

The main difficulties with this approach relate to the defences within Bridgwater, which realistically have a further effective life of about 20 to 35 years with appropriate normal maintenance. In many cases raising the level of the defences significantly would require them to be rebuilt completely. Existing bridges, including historic structures such as Town Bridge, would need to be rebuilt or raised. The height of the new defences in many places would have a significant impact on the urban landscape and feel of the town centre environment.

Surface water drainage behind raised defences will become increasingly difficult and is likely to require expensive collection, storage and pumping arrangements.

Many of the tidal defence embankments upstream and downstream of Bridgwater do not meet current standards e.g. for maintenance access and are often very old structures with variable construction. They would require re-building in order to achieve a higher defence level.

How will new development in Bridgwater be protected?

Sedgemoor District Council and the Environment Agency have worked closely together to ensure that both existing properties and new development in Bridgwater will be protected from high tide levels in the future. Through effective planning policy new development is contributing financially to the future flood management needs. The Bridgwater Strategic Flood Defence Tariff was put in place in 2009 to ensure that new development contributes to the strategic flood defence needs in Bridgwater.

Earlier this year the Somerset Flood Action Plan partners made a bid for funding to the Local Enterprise Partnership, which was successful in attracting initial funding. This will supplement developer contributions that have been collected by Sedgemoor District Council through the tariff and will fund the feasibility and planning stages.

A subsequent bid to the Local Enterprise Partnership has been submitted for the construction of a barrier and we are awaiting the results. Funding for the construction phase, however, may alternatively come from a combination of national Flood Defence Grant in Aid funding (through the Environment Agency) and partnership contributions from the Local Enterprise Partnership and Sedgemoor District Council’s developer contributions.

APPENDIX B: KEY FEATURES PLANS

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