Revision Schedule

Sedgemoor District Council Level 2 Strategic Flood Risk Assessment Final Report

Rev Date Details Prepared by Reviewed by Approved by

00 October 2008 Draft for Nick Bosanko Dr Rob Sweet Jon Robinson comment Assistant Flood Risk Senior Flood Risk Associate Director consultant Specialist

01 January 2009 Final Report Dr Rob Sweet Paul Curwen Jon Robinson with Stakeholder Senior Flood Risk Principal Engineer Associate Director Comments Specialist

Scott Wilson Mayflower House Armada Way This document has been prepared in accordance with the scope of Scott Wilson's appointment with its client and is subject to the terms of that appointment. It is addressed Plymouth to and for the sole and confidential use and reliance of Scott Wilson's client. Scott Wilson accepts no liability for any use of this document other than by its client and only for the PL1 1LD purposes for which it was prepared and provided. No person other than the client may copy (in whole or in part) use or rely on the contents of this document, without the prior written permission of the Company Secretary of Scott Wilson Ltd. Any advice, opinions, or Tel +44(0)1752 676 700 recommendations within this document s hould be read and relied upon only in the context of the document as a whole. The contents of this document do not provide legal or tax Fax +44(0)1752 676 701 advice or opinion.

© Scott Wilson Ltd 2008 www.scottwilson.com 1 District Council

Level 2 Strategic Flood Risk Assessment

Table of Contents

1 Introduction...... 1 1.1 Overview...... 1 1.2 Study Area...... 1 1.3 Aim of Level 2 SFRA ...... 1 1.4 Level 2 SFRA Objectives ...... 1 2 Flood Risk ...... 2 2.1 Overview...... 2 2.2 Tidal Flood Risk...... 2 3 Study Context...... 3 4 Results ...... 4 4.1 Overview...... 4 4.2 Burnham-on-Sea/Highbridge Flood Cell ...... 4 4.3 ...... 6 5 Mitigation Options...... 10 5.1 Purpose of the Costing ...... 10 5.2 Types of Flood Defence...... 10 5.3 Costing Methodology ...... 10 5.4 Potential Mitigation Options ...... 11 6 Discussion and Conclusions...... 13 6.1 Discussion ...... 13 6.2 Conclusions ...... 14 7 References ...... 15 8 Appendices...... A Appendix A – Flood Cells ...... A Appendix B – Modelling Methodology...... B Appendix C – Burnham-on-Sea/Highbridge Flood Cell Figures ...... C Appendix D – Bridgwater Flood Cell Figures ...... D

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1.3.3 This report should be regarded as a “living document” and periodic reviews should be undertaken to 1 Introduction ensure that, as new data becomes available or new/updated guidance is released, the SFRA reflects this information. 1.1 Overview 1.4 Level 2 SFRA Objectives 1.1.1 Scott Wilson Ltd was commissioned to undertake the Sedgemoor District Council Strategic Flood Risk Assessment (SFRA). This Level 2 SFRA forms part of the evidence base for the Local Development 1.4.1 The aims of this Level 2 SFRA are achieved through the following objectives: Framework and meets the requirements of Planning Policy Statement 25: Development and Flood Risk • (PPS25). Undertake 2-D hydrodynamic modelling for breach and overtopping scenarios of the existing tidal flood defences; 1.1.2 Scott Wilson completed the Sedgemoor District Council Level 1 in August 2008. This has allowed Sedgemoor District Council to determine the variations in flood risk for spatial planning purposes. • Provide depth and hazard mapping for tidal modelling outputs to illustrate the distribution of Where decision-makers are unable to allocate all proposed development and infrastructure in flood risk and to enable a sequential approach to site allocation within flood zones; accordance with the Sequential Test (i.e. to steer development to areas at lowest risk of flooding), it • may be necessary to increase the scope of the Level 1 SFRA to provide information necessary for Provide detailed analysis of modelling results for the 12 PDOs, including information on rate of application of the Exception Test (i.e. Level 2 SFRA). onset of flooding, flood depth and flood hazard; 1.1.3 The Level 1 SFRA identified that Sedgemoor District is significantly constrained by tidal flood risk with • Identify locations where improvements in existing flood defences may be beneficial. approximately 50% located within Flood Zone 3. The draft Regional Spatial Strategy (RSS) has 1.4.2 The depth and hazard mapping form the main content of this report and should be used to inform future identified Bridgwater as a Strategically Significant City/Town (SSCT) and is considered suitable to development opportunities. provide sustainable housing growth. Two main areas were identified through liaison with Sedgemoor District Council and the Environment Agency where further information was required on flood risk to inform strategic planning decisions.

1.1.4 This report presents the findings of the Level 2 SFRA and details the results of an extensive tidal modelling and mapping exercise. This has investigated the residual flood risk from both tidal overtopping and breaching of existing flood defences, to provide further information for application of the Exception Test. Furthermore, the report identifies a number of strategic flood defence options available in locations identified through the modelling and provides indicative costs to implement these. This overview provides the basis for an initial flood defence costs exercise within the study area. 1.1.5 The findings of the Level 1 and Level 2 SFRA reports should be used in tandem for both strategic planning and to inform ongoing development control decisions.

1.2 Study Area

1.2.1 The study area is defined by the administrative boundary of Sedgemoor District Council and is described in detail in the Level 1 SFRA report (Scott Wilson, 2008). The Level 2 SFRA focuses on two flood cells, Burnham-on-Sea/Highbridge and Bridgwater. The extent of these flood cells are provided in Appendix A. 1.2.2 These flood cells have extensive areas of relatively level, low-lying land that is managed through networks of man-made ditches and rhynes. The to railway line and the dissect both flood cells. These features are elevated above the surrounding ground level, particularly the M5, with culverts allowing the flow of water beneath.

1.3 Aim of Level 2 SFRA

1.3.1 The aim of this study is to provide supplementary information to the Level 1 SFRA through the ‘increased scope’ referred to in paragraph E6 of PPS25. This will allow Sedgemoor District Council to facilitate the application of the Sequential and Exception Tests through the investigation of tidal flood risk issues and the suitability for development of potential sites with known flooding problems, as identified in the Level 1 SFRA. 1.3.2 The aim of the flood modelling is to simulate flood events to determine and illustrate the potential flood extents, depth and the areas at high, medium and low flood hazard. Detailed analysis of the modelling results for 12 potential development options (PDOs) identified in liaison with Sedgemoor District Council and review of the Issues and Options Report for the Core Strategy (June 2007). The location of each PDO is shown within Appendix A and described further in Section 3.

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• Wave overtopping of the defences at Sands was also identified, resulting in floodwater 2 Flood Risk flowing in a southerly direction, almost reaching Burnham-on-Sea; and, • Minor residual risk areas were also identified by wave-overtopping at Burnham-on-Sea. 2.1 Overview Bridgwater 2.1.1 Due to the large extent of defended tidal floodplain in the Sedgemoor District, each of the PDOs are • Limited areas of inundation due to overtopping at lower sections in existing defences. positioned within areas of residual risk. Residual risk is defined as the risk remaining after flood management or mitigation measures have been put in place. 2.2.8 The 2-D hydrodynamic modelling undertaken for this Level 2 SFRA provides additional detail building on the findings of the Environment Agency North Wessex Area Tidal Areas Benefiting from Defences 2.1.2 Flood defences and urban development can significantly alter the natural flow paths within the floodplain report. The two main areas where this Level 2 SFRA builds upon these findings are: and affect the extent of floodwater inundation. The use of 2-D hydrodynamic modelling allows an • assessment of floodwater inundation for the flood cells identified in Appendix A for a range of scenarios. An appropriate allowance has been made for climate change; and, This provides an appraisal of the probability and consequence of overtopping or failure of flood risk • Breach scenarios allowing the potential consequence of failure of flood risk management management infrastructure, including appropriate allowances for climate change. infrastructure to be considered. 2.1.3 Tidal flooding is the most significant flood risk within Sedgemoor. The Level 2 SFRA is focused upon the

residual risk associated with tidal flooding, which is summarised below.

2.2 Tidal Flood Risk

2.2.1 The main tidal flood sources within Sedgemoor District consist of the open coast fronting the , the , the and the . Within the two flood cells identified in Appendix A, there is a range of flood management infrastructure protecting existing development, these include embankments, sluice structures, rock revetments and hard engineered wave return walls. Overtopping

2.2.2 Overtopping occurs when water levels exceed the level of the flood defence. High volumes of overtopping may be observed when water levels exceed to Standard of Protection (SoP) of the flood defence. Lower volumes of overtopping may occur due to the action of winds, waves and spray even where water levels are below the flood defence crest. 2.2.3 The impact of wind, waves and spray are not considered within this study. Breaching

2.2.4 Where a flood defence is raised above the surrounding land or there is a control structure, there is potential for breaching to through constructional or operational failure. Breaches are more likely during high water level events including extreme tides when loads on the defence will be greater and can cause rapid inundation of areas benefitting from defence. 2.2.5 The time taken for a breach to be sealed can have a major effect on the extent and depth of flooding. In addition to the flood risk associated with a breach event, there is an implied flood hazard. The highest hazard typically exists in the period immediately following a breach, and usually, but not necessarily, in the areas closest to the breach. 2.2.6 Floodwater flowing through a breach will be of high velocity and volume, dissipating rapidly across extensive low lying areas. Flooding as a result of a breach in defences can be life threatening with far reaching consequences. Areas Benefiting from Tidal Defences

2.2.7 The Environment Agency North Wessex Area Tidal Areas Benefiting from Defences report (September 2007), identifies the nature of tidal flooding throughout the district, which is summarised below for the two flood cells. Burnham-on-Sea / Highbridge to Brean • Low-lying areas east of Berrow are flooded by water overtopping from the River Axe;

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3 Study Context Table 3-2: PDO details and approximate National Grid Reference (NGR). Burnham-on-Sea/Highbridge Flood Cell 3.1.1 This section provides an overview of how the 2-D hydrodynamic modelling has been used to assess flood risk. The detailed methodology applied to undertake the 2-D hydrodynamic breach and Potential Development Site Name Approximate NGR overtopping modelling and mapping is provided within Appendix B. This justifies the approach adopted Location (PDO) and identifies constraints during the development of the model for each flood cell. 1 North of the former Radio Station ST 320 483 3.1.2 Eighteen scenarios were identified in agreement between the Environment Agency, Sedgemoor District 2 Land to the north of Isleport and A38 ST 329 483 Council and Scott Wilson Ltd. These scenarios are based on the five breach locations and existing extreme water level data relevant to the two flood cells (see Table 3-1). 3 Land to the north east of Isleport ST 333 481 4 Land to the east of Isleport ST 332 475 3.1.3 These scenarios allow present (2008) and future conditions accounting for climate change (2108) for two tidal return period events (1 in 200 year and 1 in 1000 year flood events) to be investigated. Further 5 Land to the east of Brean ST 301 557 details of the methodology used to derive the conditions for these scenarios are contained within 6 Land to the north east of Burnham-on-Sea ST 307 509 Appendix B. Bridgwater Flood Cell Table 3-1: Attributes of the 18 scenarios undertaken for the 2-D hydrodynamic modelling 7 Little Sydenham Farm (north sector) ST 313 399 8 Little Sydenham Farm (south sector) ST 315 388 Burnham-on-Sea/Highbridge Flood Cell 9 Bower Lane ST 322 376 Scenario Breach1 Overtopping 2 Return Period Allowance for Climate Change 10 Follets Farm, Dunwear ST 318 361 1 1 Yes 1 in 200 No 11 North of Express Park ST 309 402 2 1 Yes 1 in 200 Yes 12 North East of A38 ST 312 402 3 1 Yes 1 in 1000 No

4 1 Yes 1 in 1000 Yes

5 2 Yes 1 in 1000 No 3 6 2 Yes 1 in 1000 Yes 3 7 3 Yes 1 in 1000 No 3 8 3 Yes 1 in 1000 Yes 3 9 - Yes 1 in 200 No 10 - Yes 1 in 200 Yes 11 - Yes 1 in 1000 No 12 - Yes 1 in 1000 Yes Bridgwater Flood Cell 13 4 Yes 1in 200/1000 No 4 14 4 Yes 1in 200/1000 Yes 4 15 5 Yes 1in 200/1000 No 4 16 5 Yes 1in 200/1000 Yes 4 17 - Yes 1in 200/1000 No 4 18 - Yes 1in 200/1000 Yes 4

3.1.4 The results from these scenarios have been used assess the flood risk and hazard associated with the 12 PDOs identified by Sedgemoor District Council (Table 3-2). This allows Sedgemoor District Council to undertake the Sequential and Exception Tests when allocating land within the Local Development Framework and also to inform their Development Control decisions for sites not allocated within this process.

1 The location of each breach is provided in Appendix A 2 The model allows simultaneous overtopping to occur in those scenarios with breaches. Where no breach is indicated, floodwater inundation occurs solely from overtopping of existing defences. 3 The 1 in 200 year tidal event scenarios were not run for breach location 2 or 3 in the Burnham Flood cell because it was not considered to add significant value to the existing information held at those locations. 4 Within Bridgwater the current extreme water levels for the 1 in 200 and 1 in 1000 year flood levels are equal, therefore simulations provide evidence for both scenarios.

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management infrastructure in liaison with the Environment Agency. Even in the event of a breach, 4 Results overtopping may still occur and was therefore considered within the modelling. 4.2.11 Breach location 1 (Scenarios 1 to 4) north of Brean has resulted in a noticeable, but not significant 4.1 Overview increase upon flood extent and depth in comparison with results from overtopping with no breach. 4.2.12 Similarly, for breach 2 (Scenarios 5 and 6) and breach 3 (Scenarios 7 and 8) flood depths increase due 4.1.1 This section provides the results of the 2-D hydrodynamic modelling and refers to figures in Appendix C to the presence of a breach but the resultant flood extents do not increase significantly. As observed for the Burnham-on-Sea/Highbridge flood cell and Appendix D for the Bridgwater flood cell. These within the overtopping scenarios where no breaches are present, the impact of climate change has a illustrate the flood depth, extent and hazard associated with each of the scenarios provided in Table 3-1. significant influence on flood depth and extent. 4.1.2 A general overview of the flood propagation, depth and hazard is provided for each of the flood cells 4.2.13 The similarities identified above, between overtopping and breach scenarios, are considered to be with a more focussed assessment for the PDOs identified in Table 3-2. For the purpose of this section associated with the local topography within the flood cell. The extensive, relatively flat, low lying ‘present day’ refers to 2008 and ‘climate change’ refers to 2108. topography promotes widespread inundation, in particular, under climate change conditions. The impact of the three individual breach locations is not considered significant in comparison with the potential impacts of climate change. 4.2 Burnham-on-Sea/Highbridge Flood Cell 4.2.14 The inundation extent for breach 2 under present day conditions is not significant due to the limited capacity of the breach (4 m in width). However, increased water levels associated with climate change Flood Propagation and Depth of Overtopping Modelling Results cause wider inundation to occur around the breach location. 4.2.1 The flood depth maps for all modelled scenarios for the Burnham-on-Sea/Highbridge flood cell are Flood Hazard presented in Appendix C. 4.2.15 Flood hazard maps are provided in Appendix C and are based on the methodology provided in Present Day Appendix B. These maps allow areas of low, medium and high hazard to be defined based on model outputs. 4.2.2 During the present day conditions for both the 1 in 200 and 1 in 1000 year events (Scenarios 9 and 11), overtopping is confined to relatively isolated sections of the tidal defences. Overtopping is notable at the 4.2.16 The flood hazard is expressed as a combination of flood depth and velocity. The maximum flood hazard boatyard located on the right hand bank of the River Brue (ST 312 473). Overtopping also occurs in the for a given location can be experienced at any stage of a flood. Where overtopping or a breach occurs, north of the flood cell, near the mouth of the River Axe, along the length of the defences (in the high velocities are likely to be experienced and the highest hazard is likely to be experienced at the time proximity of ST 312 575) that are downstream of the tidal sluice. of peak velocity. Further from the breach the maximum hazard will depend on local factors affecting both the depth and velocity of floodwaters during a flood event. Peak depth and velocities occur 4.2.3 Overtopping is observed during the 1 in 1000 year event (Scenario 11), at the rock armour defences predominantly during the second tidal cycle that is the larger of the three cycles used within the from to the south of Weston Road (in the proximity of ST 297 574). modelling. 4.2.4 In general, overtopping is confined to the north and south of the flood cell in the 1 in 200 year present 4.2.17 The flood hazard is derived using the flood depth data illustrated for the flood propagation and depth day event, whilst the 1 in 1000 year present day event results in a wider flood extent in the north that section (i.e. the depth mapping). Under present day conditions the predominant hazard ratings are Low extends south to Berrow. In the south, the 1 in 1000 year present day event results in increased flooding to Medium. However, under climate change conditions the predominant hazard rating is found to be through overtopping at the boatyard (ST 312 473). High. Climate Change 4.2.18 The results are identified in more detail, in terms of flood propagation, depth and hazard for each scenario and PDO, in the following section. 4.2.5 Accounting for climate change, the extent and depth of flooding is increased significantly for both the 1 in 200 and 1 in 1000 year flood events (Scenario 10 and 12). The Bristol to Exeter railway line provides Tidal Flood Risk - Potential Development Options an informal defence and is demonstrated by the decrease in depth to the east of the embankment. The majority of the flood cell experiences flooding. 4.2.19 The model results have been analysed at each PDO in order to provide a more detailed understanding of tidal flood risk. Six PDOs are located within the Burnham-on-Sea / Highbridge to Brean flood cell, 4.2.6 Overtopping at the boatyard on the right hand bank of the River Brue becomes significantly wider in results for which are summarised in Table 4.1 to Table 4.6 below. extent with increased depths observed. 4.2.7 In the north of the flood cell, overtopping occurs upstream of the tidal sluice on the River Axe and PDO 1 – North of Former Radio Station upstream, north of Eastertown (in the proximity of ST 350 562). 4.2.20 PDO 1 is located to the east of Burnham-on-Sea (see Appendix A) and west of the Bristol to Exeter 4.2.8 The majority of Burnham-on-Sea and Highbridge become inundated by overtopping, however, a ridge of railway line. This area is affected by overtopping from the River Brue under climate change conditions elevated land, which extends along the majority of the coastal frontage remains unaffected. The majority and flood risk is exacerbated by the presence of breach location 3. of this land is developed and benefits from either natural sand dunes or man-made tidal defences that provide protection even under climate change conditions. 4.2.21 Maximum flood depth of between 1.04 and 1.47 m are observed under climate change conditions. The time first inundation is typically experienced on the second tidal cycle with the time of maximum depth 4.2.9 is a prominent high point and is not affected by flooding for any of the scenarios discussed typically experienced on the third tidal cycle. within this report. 4.2.22 An exception is Scenario 7 where breach location 3 promotes flood water inundation and low hazard is Flood Propagation and Depth of Breach and Overtopping Modelling Results experienced with maximum depths of 0.47 m experienced. In addition, although high hazard is typical of all climate change scenarios, the time of first inundation for Scenario 8 is only 2.66 and is associated 4.2.10 It is possible that a breach in the natural or man-made defences could occur. The impact of three with the presence of breach location 3. individual breach locations has been considered based on an assessment of the existing flood

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Table 4-1 Summary of tidal flood risk at PDO 1 for each scenario Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation (m) (hr) (hr) Scenario Max Depth Time of Max Depth 5 Maximum Time of First (m) (hr) Hazard Inundation 6 (hr) 8 0.69 33.00 Medium 14.83 1 - - - - 9 - - - - 2 1.08 32.917 High 14.00 10 - - - - 3 - - - 11 - - - - 4 1.32 31.417 High 13.66 12 0.62 33.00 Medium 28.25

5 - - - Table 4-3 Summary of tidal flood risk at PDO 3 for each scenario 6 1.30 31.500 High 13.00 Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation 7 0.47 18.500 Low 13.92 (m) (hr) (hr) 8 1.47 16.750 High 2.66 1 - - - - 9 - - - - 2 - - - - 10 1.04 33.000 High 16.33 3 - - - - 11 - - - - 4 - - - - 12 1.30 31.500 High 16.00 5 - - - - 6 - - - - PDO 2, 3 and 4 – Land to north, north east and east of Isleport 7 - - - - 4.2.23 PDO 2 is located to the east of the Bristol to Exeter railway line that forms an informal defence structure. 8 0.24 33.00 Low 29.50 The presence of this informal defence provides protection for all scenarios apart from the 1 in 1000 year event accounting for climate change. This is due to the increased flood levels causing floodwater to both 9 - - - - overtop and flow through the culverts beneath, the railway embankment. 10 - - - - 4.2.24 PDO 3 and 4 are relatively unconstrained. PDO 3 undergoes limited depths of flooding (0.25 m) for Scenario 8, this is associated increased inundation promoted by breach location 3 and occurs under 11 - - - - climate change conditions. In addition, these are experienced on the third tidal cycle and therefore 12 - - - - advance warning is likely to be available. For all other scenarios for PDO 3 and 4, inundation is not experienced. Table 4-4 Summary of tidal flood risk at PDO 4 for each scenario

Table 4-2 Summary of tidal flood risk at PDO 2 for each scenario Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation (m) (hr) (hr) Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation (m) (hr) (hr) 1 - - - - 1 - - - - 2 - - - - 2 - - - - 3 - - - - 3 - - - - 4 - - - - 4 0.48 33.00 Medium 29.08 5 - - - - 5 - - - - 6 - - - - 6 0.41 33.00 Low 28.58 7 - - - - 7 - - - - 8 - - - - 9 - - - -

5 This is attributed to the time of maximum flood depth once floodwater starts to flow through the breach. It provides an indication 10 - - - - of the rate of onset of flooding until maximum depth at the PDO. The time of maximum depth associated with the purely overtopping scenarios is taken from the start of the model run and consequently, should not be directly compared to the time of 11 - - - - the maximum depth from the breach scenarios. However, it provides a useful indication of the nature of flooding. 6 This is attributed to the time of first inundation once floodwater starts to flows through the breach. It provides an indication of how 12 - - - - long floodwater takes to flow across the land and impact the PDO. The time of the first inundation associated with the purely overtopping scenarios is taken from the start of the model run and consequently, should not be directly compared to the time of the maximum depth from the breach scenarios. However, it provides a useful indication of the nature of flooding.

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PDO 5 – Land to the east of Brean Table 4-6 Summary of tidal flood risk at PDO 6 for each scenario

4.2.25 PDO 5 is located to the east of Brean, approximately 1.75 km south of breach location 1. This location is Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation predominantly used for with large areas of static caravans and camping. (m) (hr) (hr) 4.2.26 During the present day 1 in 200 year flood event where there is no breach present (Scenario 9), flooding 1 - - - - from overtopping is not observed. In addition, the hazard is low where a breach occurs at breach location 1 for the present day 1 in 200 year event (Scenario 1). 2 0.86 32.33 Medium 15.92 4.2.27 Low hazard is observed for both overtopping and breaching for all other present day events (Scenarios 3 - - - - 5, 7 and 11) with the exception of Scenario 3 that is medium hazard. Flooding occurs in this location 4 1.13 30.67 High 15.17 predominantly from overtopping of the tidal defences near the mouth of the River Axe and is not affected by inundation from breach locations 2 and 3. 5 - - - - 4.2.28 When considering climate change for both the 1 in 200 and 1 in 1000 year events, the hazard increases 6 1.12 18.83 High 14.50 to ‘high’ with significant increase in the maximum water depths experienced and the time of first 7 - - - - inundation. 8 1.17 18.58 High 15.17 Table 4-5 Summary of tidal flood risk at PDO 5 for each scenario 9 - - - - Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation 10 0.87 31.25 Medium 18.25 (m) (hr) (hr) 11 - - - - 1 0.19 31.58 Low 18.83 12 1.15 18.67 High 17.33 2 2.04 16.66 High 3.33 3 0.52 17.58 Medium 14.00 Summary 4 2.39 16.58 High 2.17 4.2.33 The relatively level and low-lying topography of the flood cell does not exert significant influence on the 5 0.40 18.00 Low 1.50 propagation and depth of tidal flooding. Raised features including the Exeter to Bristol railway line act as an informal defence and exert control upon flood propagation during extreme events accounting for 6 2.32 16.58 High 1.50 climate change. 7 0.41 17.92 Low 14.42 4.2.34 A relatively marked difference was observed for the 1 in 200 year present day scenarios and the 1 in 8 2.33 16.58 High 2.50 1000 year present day scenarios in terms of the tidal propagation and depth across the flood cell. 9 - - - - 4.2.35 Climate change has a significant impact upon flood extent, depth and hazard across the flood cell, due to the increase in water levels of approximately 1 m. 10 1.94 16.75 High 7.00 4.2.36 PDO 2, 3 and 4 are the least constrained by tidal flood risk, primarily due to the protection offered by the 11 0.41 17.92 Low 16.83 Bristol to Exeter railway embankment. However, under climate change conditions flooding is observed 12 2.32 16.58 High 4.00 PDO 2. 4.2.37 It is important that variations in the time of maximum flood depth and the time of first inundation are PDO 6 – Land to the north east of Burnham-on-Sea noted and are a function of the distance of the PDO from the location of overtopping and / or breach.

4.2.29 PDO 6 is located to the north east of Burnham-on-Sea and would form an urban extension of the current settlement. 4.3 Bridgwater 4.2.30 For all present day breach and overtopping scenarios (1, 3, 5, 7, 9 and 11), no flooding is observed at PDO 6. Flood Propagation and Depth of Overtopping Modelling Results 4.2.31 When considering the impacts of climate change, the hazard increases medium or high depending on 4.3.1 The flood depth maps (D1, D2, D5, D6, D9 and D10) for all modelled scenarios for the Bridgwater flood the scenario. Maximum flood depths are typically between 0.86 m and 1.17 m depending on the breach cell are provided in Appendix D. and overtopping scenario. Present Day 4.2.32 For Scenarios 2, 4 and 10, the time of first inundation is experienced on the second tidal cycle but the time of maximum depth is experienced on the third tidal cycle. The time to maximum depth for 4.3.2 During the present day conditions for the 1 in 200 and 1 in 1000 year tidal flood events, overtopping is Scenarios 6, 8 and 12 are experienced on the second cycle and are a product the overtopping and confined to relatively isolated reaches of the tidal defences. Overtopping occurs during the first tidal breaching in the south of the model cell promoting floodwater inundation in this area. cycle along the King’s Sedgemoor Drain, approximately 1 km upstream of (ST 352 376). In this location, the extent and depth of flooding is significant and flooding is exacerbated by flow being routed along the Chedzoy New Cut (ST 357 364). It should be noted that flooding from this pathway does not affect Bridgwater.

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4.3.3 During the second tidal cycle that is greater than the first, overtopping is more widespread and occurs at Tidal Flood Risk - Potential Development Options the following locations: 4.3.13 The model results have been inspected at each PDO in order to provide a more detailed understanding • Between the Bristol to Exeter railway line and the M5 (ST 313 409) of tidal flood risk, results for which are summarised in Table 4.7 to Table 4.12 below. • Locations on and east of the A38 Bristol Road (ST 315 409 and ST 329 399); PDO 7 – Little Sydenham Farm (north) • Adjacent to Express Park, Bridgwater (ST 307 386); and, • 4.3.14 PDO 7 is located to the south east of breach location 4 and to the west of the M5. For scenarios under East Quay, including Castle Field Industrial Estate, Bridgwater (ST 299 375). present day conditions (15 and 17), no flooding is observed as a result of overtopping or breaching. 4.3.4 Flood depths within Bridgwater are relatively shallow and the Bristol to Exeter railway line embankment However, flooding is observed of medium hazard if a breach were to occur at location 4 (Scenario 13). prevents floodwater inundating the area to the east. Only a relatively small part of Bridgwater is affected 4.3.15 When considering climate change, for all scenarios (14, 16 and 18) flooding occurs and is classified as by present day conditions. high hazard. Increased depths are experienced for Scenario 14 where the breach is located close to the site and rapid inundation will occur in a short timeframe (1.33 hours). Climate Change 4.3.16 For Scenario 16, the time of first inundation and maximum depth is greater due to the significant 4.3.5 Accounting for climate change, the extent and depth of flooding is increased significantly for both the 1 distance from breach location 5. in 200 and 1 in 1000 year flood events (Scenarios 10 and 12) with the majority of Bridgwater becoming inundated. The influence of the Bristol to Exeter railway line is significantly reduced and overtopping Table 4-7 Summary of tidal flood risk at PDO 7 for each scenario occurs near to where the Bristol to Exeter railway line crosses the River Parrett (ST 310 356) and Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation upstream of the M5. (m) (hr) (hr) 4.3.6 The M5 represents an informal defence to floodwater propagation and overtopping of this structure does 13 0.508 13.50 Medium 13.00 not occur. Culverts beneath the M5 allow floodwater to flow beneath to the area to the east. 14 1.335 14.00 High 1.33 4.3.7 Express Park located to the north west of Bridgwater town centre and to the west of the A38 is elevated above the resultant flood levels and does not become inundated even under the climate change 15 - - - - conditions. It should be noted that the Ordnance Survey base-mapping does not illustrate this recent commercial development. 16 1.038 26.08 High 12.83 17 - - - - Flood Propagation and Depth of Breach and Overtopping Modelling Results 18 0.926 14.91 High 13.91 4.3.8 Two breach locations have been modelled within this flood cell. The locations of these are provided in Appendix A. PDO 8 – Little Sydenham Farm (south) 4.3.9 Breach location 4 (Scenarios 13 to 14) located north of Bridgwater has resulted in a significant increase upon flood extent and depth in comparison with results from overtopping with no breach. Similarly, for 4.3.17 PDO 8 is located to the north of Bridgwater in the vicinity of the Innovia Site and to the west of the M5. breach location 5 (Scenarios 15 and 16) downstream of Bridgwater, a significant increase upon flood For scenarios under present day conditions (13, 15 and 17), no flooding is observed as a result of extent and depth in comparison with results from overtopping with no breach is observed. Liaison with overtopping or breaching. This concurs with areas identified as benefiting from defence contained within the Environment Agency identifies that this accords well with separate modelling results undertaken by the Environment Agency North Wessex Tidal Areas Benefiting from Defence Report. Peter Brett Associates for a proposed development for Bridgwater. 4.3.18 When considering climate change, for all scenarios (14, 16 and 18) flooding occurs. However, the 4.3.10 The differences identified above, between overtopping and breach scenarios, are considered to be greatest hazard (high) is associated with the breach scenario (14) for breach location 4. For Scenarios associated with the location and width of the breaches within the Bridgwater flood cell. The extensive, 16 and 18, the hazard is medium. The time of first inundation is prolonged, however, the time between relatively flat, low lying topography promotes widespread inundation, in particular, under climate change first inundation and maximum depth occurring is less than an hour for Scenarios 14 and 18. conditions. The impact of the two individual breach locations is not considered significant in comparison 4.3.19 Depths of flooding are greater for the breach scenarios (1.05 m and 0.90 m) and lesser for the with the potential impacts of climate change, however, the impact of flood depth is considered more overtopping with no breach scenario (0.75 m). This is due to the increased volume of water entering significant. through breach locations causing an increase in water levels. Flood Hazard Table 4-8 Summary of tidal flood risk at PDO 8 for each scenario

4.3.11 Flood hazard maps are provided in Appendix D and are based on the methodology provided in Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation Appendix B. These maps allow areas of low, medium and high hazard to be defined based on model (m) (hr) (hr) outputs for the Bridgwater flood cell. 13 - - - - 4.3.12 The flood hazard extents are the same as the extents discussed in the under flood propagation and 14 1.052 14.25 High 13.33 depth. Under present day conditions the predominant hazard rating within Bridgwater and the directly surrounding area is Low. However, under climate change conditions the predominant hazard rating is 15 - - - - Medium. A High hazard rating is observed to the north and south east of Bridgwater urban area, 16 0.903 26.00 Medium 13.33 particularly under breach scenarios. 17 - - - - 18 0.753 15.25 Medium 14.41

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Table 4-10 Summary of tidal flood risk at PDO 10 for each scenario

PDO 9 – Bower Lane Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation (m) (hr) (hr) 4.3.20 PDO 9 is located between the eastern extent of Bridgwater and the M5. For scenarios under present day conditions (13 and 17), no flooding is observed as a result of overtopping or breaching. There are 13 - - - - minimal depths of flooding for Scenario 15 associated with breach location 5. Maximum depths of 0.06 14 1.366 29.83 High 13.66 m are experienced and the hazard is considered low. 15 0.805 25.33 Medium 0.91 4.3.21 When considering climate change, for all scenarios (14, 16 and 18) flooding occurs with flood hazard being medium. The time of first inundation is associated with the first tidal cycle and maximum flood 16 1.366 25.91 High 0.73 depths are experienced during the second tide cycle. 17 - - - - 4.3.22 Depths of flooding are greater for the breach scenarios 14 and 16 (0.76 m) and lesser for the 18 1.109 33.00 High 14.41 overtopping with no breach scenario (0.50 m). Similar to PDO 9, this is due to the increased volume of water entering through breach locations causing an increase in water levels. PDO 11 and 12 – North of Express Park and north east of A38 Table 4-9 Summary of tidal flood risk at PDO 9 for each scenario 4.3.26 PDO 11 and 12 are both north of Express Park and are located to the west and east of the A38 Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation respectively. These sites are approximately 300 m south of breach location 4. (m) (hr) (hr) 4.3.27 For scenarios under present day conditions (15 and 17), no flooding is observed as a result of 13 - - - - overtopping or breaching. There is a medium hazard for Scenario 13 at both locations associated with breach location 5. Maximum depths are 0.49 m and 0.45 m respectively and are experienced within an 14 0.758 30.00 Medium 14.33 hour of first inundation at the site. 15 0.061 25.83 Low 26.58 4.3.28 When considering climate change, for all scenarios (14, 16 and 18) flooding occurs. For Scenarios 16 16 0.762 26.00 Medium 13.08 and 18, the hazard is medium with maximum flow depths of between 0.58 m and 0.68 m. 17 - - - - 4.3.29 The hazard is high for Scenario 14, where the influence of the breach has a significant influence on the depth and time of first inundation at the sites. The maximum flow depths experienced are between 0.85 18 0.502 33.00 Medium 15.66 and 1.00 m. Similar to PDOs 8 to 10, this is due to the increased volume of water entering through breach locations causing an increase in water levels. PDO 10 – Follets Farm, Dunwear Table 4-11 Summary of tidal flood risk at PDO 11 for each scenario 4.3.23 PDO 10 is located between the south eastern extent of Bridgwater and the M5. For scenarios under present day conditions (13 and 17), no flooding is observed as a result of overtopping or breaching. Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation There are relatively significant depths of flooding for Scenario 15 associated with breach location 5. (m) (hr) (hr) Maximum depths of 0.80 m are experienced and the hazard is considered medium. It is noted that rapid 13 0.492 13.66 Medium 12.33 inundation (less than 1 hour) of PDO 10 will result if a breach of flood management infrastructure were to occur but maximum depths would not be experienced until the second tidal cycle. 14 1.007 13.66 High 0.58 4.3.24 When considering climate change (Scenarios 14, 16 and 18), flooding occurs with hazard being high. 15 - - - - The time of first inundation is associated with the first tidal cycle and maximum flood depths are 16 0.666 12.83 Medium 13.00 experienced during the second tide cycle. As before, rapid inundation is associated with breach location 5 (Scenario 16). 17 - - - - 4.3.25 Depths of flooding are greater for the breach Scenarios 14 and 16 (1.36 m) and lesser for the 18 0.678 15.25 Medium 14.08 overtopping with no breach scenario (1.10 m). Similar to PDO 8 and 9, this is due to the increased volume of water entering through breach locations causing an increase in water levels. Table 4-12 Summary of tidal flood risk at PDO 12 for each scenario

Scenario Max Depth Time of Max Depth Max Hazard Time of First Inundation (m) (hr) (hr) 13 0.448 13.00 Medium 12.50 14 0.845 13.83 High 0.83 15 - - - - 16 0.581 12.16 Medium 12.83 17 - - - - 18 0.583 14.41 Medium 13.91

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Summary

4.3.30 The impact of the two breach locations have a significant influence on floodwater inundation patterns in and around Bridgwater for both present and climate change conditions. Climate change is the significant controlling factor on inundation with flood depth and flood hazard increasing significantly and a reduction in time of first inundation. 4.3.31 Under present day conditions all of the PDOs do not experience flooding. PDO 8 is the least constrained by tidal flood risk, as floodwater from neither of the breaches reaches this far inland. The land at Express Park is raised out of the area affected by flood inundation. 4.3.32 The M5 controls the amount of floodwater propagation from one side to the other, which is limited by floodwater that is conveyed through the culverts beneath. 4.3.33 It is important to note that variations in the time of maximum flood depth and the time of first inundation reflect the position of the particular PDO and its proximity to the location of overtopping or breach.

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5 Mitigation Options Table 5-2: Indicative Flood defence cost rates – new embankment (Source: Environment Agency)

3 5.1 Purpose of the Costing Volume (m ) Volume 500-5,000 5,000-15,000 >15,000 5.1.1 In order to assess the potential for strategic development, consideration should be given as to whether 3 Cost Range (£/m ) 31-116 29-53 17-31 the development is achievable and deliverable. This includes the provision of strategic flood risk management infrastructure that may include flood defences. This is assessed in a number of ways Average Cost (£/m 3)* 75 40 25 through the Sustainability Appraisal. *An indicative volume of 12 m 3 per metre run of defence has been used within the calculations. 5.1.2 It should be noted that the costs are indicative and costing of any proposed works should be undertaken through formal appropriate procedures including feasibility, detailed design, specification and a final Bill Table 5-3: Upgrading Flood defence Indicative Costs Rates (Source: Environment Agency) of Quantities for estimating costs being undertaken using Civil Engineering Standard Method of Measurement 3 rd Edition (CESMM3), or similar. Type of Upgrade £/m

Rebuild & Raise Hard Defences 4,800 5.2 Types of Flood Defence Strengthen embankment with sheet pile 3,600 5.2.1 Two types of flood defences have been considered to provide simple calculations to be undertaken for Additional capping to sheet pile 900 the indicative costs. These defences are: • Walls Table 5-4 Indicative Costs of New and Refurbished Outfall (Source Environment Agency)

• Embankments Type of Outfall £ 5.2.2 An allowance has been made within the calculations to distinguish between the requirement for new New Small outfall (<400mm) 90,000 defences or the upgrading of existing defences through raising. New Medium Outfall (400-900mm) 192,000 5.3 Costing Methodology New Large Outfall (>900mm) 540,000 Refurbished Small outfall (<400mm) 45,000 Costing Methodology Refurbished Medium Outfall (400-900mm) 96,000 5.3.1 The costing has been based on four main features and is intended to be indicative only. These include: Refurbished Large Outfall (>900mm) 270,000

CAPEX 5.3.3 The values provided by the Environment Agency within Table 5-3 and Table 5-4 have been increased 5.3.2 CAPEX are the capital costs of the required flood defence and includes the construction, preliminaries by 20% to account for potential increases in costs. and site costs, enabling works and temporary works. The CAPEX used has been based on unit cost rates obtained from the Environment Agency 7. The following tables provide indicative costs for the Land Values provision of new flood defence walls (Table 5-1), new flood defence embankments (Table 5-2), upgrading of existing embankments (Table 5-3) and inclusion of outfall structures (Table 5-4). 5.3.4 Where required, the purchase of land may be required for construction of the defence and potentially for compensatory purposes. Due to variations in land value, costs of land per hectare have been identified Table 5-1: Indicative Flood defence costs rates – new walls (Source: Environment Agency) on a regional basis, a 20% increase in land values has been factored into the 2002 values provide approximate values for 2008. Wall Height (m) Table 5-5: Average land values per hectare (Source www.advice-landforsale.co.uk ) Type of Wall <1.2m 1.2m-2.1m 2.1m-5.3m Masonry Wall 406 1,500 1,057 Location Land Costs £/ha Location Land Costs £/ha Retaining wall (Steel reinforced) Concrete 1,565 1,751 2,286 London 6,600,000 East Midlands 1,512,000 Wall with cutoff 916 2,652 3,031 East 3,192,000 North West 1,512,000 Wall with piling - 3,059 2,671 South East 3,000,000 North East 1,212,000 South West 2,040,000 1,176,000 West Midlands 1,728,000 Yorkshire 1,044,000

7 East SFRA (Halcrow) Appendix L ‘Indicative flood defence cost rates’ and Parrett Tidal Flood Defence Technical Review (Black & Veatch)

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5.4.5 A series of indicative costs for a range of flood defence structures have been calculated based on the values provided in Section 5.3. The following assumptions have been made to provide the indicative OPEX costs, these are: • Lifetime of the defence is 100 years; 5.3.5 OPEX are the operational costs for a flood defence including maintenance and refurbishment depending on the proposed lifetime. A number of assumption have been made regarding the operating • Length of the required defence is 300 m; costs, these are: • One large outfall (>900 mm) is required within the defence; • Annual costs of maintenance of the flood defence are approximately 0.5% of CAPEX • The defence is ‘new’ as opposed to an upgrade of existing defences; • Refurbishment costs have been cost as 40% of CAPEX for every 25 years 5.4.6 The lifetime of the defence has been considered as 100 years for this scenario because it will protect Additional Costs new residential property for the lifetime of the development. This approach is inline with PPS25 to ensure that the development is safe for current and future generations. 5.3.6 In addition to the CAPEX, land values and OPEX, additional costs relating to the design and build element of the flood defence have been factored in. These have been considered as a percentage of Table 5-7: Indicative cost for flood defence options in the boatyard area. the capital cost of the flood defence and are provided in Table 5-6. Wall Height Table 5-6: Additional costs as a percentage of the capital cost. Defence Type 1.2 m to 2.1 m 2.1 m to 5.3 m Additional Costs % of Capital Costs 8 Masonry Wall £3.6m £3.2m

Design Fees 10 Wall with piling £4.98m £4.01m Legal Fees 8 Embankment* n/a £3.05m Supervision Fees 5 *There is no height specified for the embankment option.

Other (Archaeology/ecology) 3 5.4.7 The indicative costs in Table 5-7 illustrate that a flood defence solution for the boatyard area is likely to cost between £3m and £5m to implement and maintain for a 100 year lifetime. These provide an indication if future development within Burnham-on-Sea/Highbridge is both achievable and deliverable. 5.4 Potential Mitigation Options Tourism Area – Brean 5.4.1 This section describes the potential mitigation options based on the results discussed in Section 4 and is focussed on the Burnham-on-Sea/Highbridge flood cell. The reasoning for this is because ongoing 5.4.8 The hydraulic modelling results indicate that the Brean area is susceptible to potential overtopping of work being undertaken by the Environment Agency in conjunction with Sedgemoor District Council is flood defences along the River Axe during an extreme event. Tourism currently contributes to the local being undertaken on the feasibility of a strategic flood defence solution for Bridgwater. The preferred economy within the Brean to Burnham-on-Sea area. The continued development of caravan and option based on information provided in the Level 1 SFRA is for a surge barrier and has therefore not camping within this location needs to be considered in terms of long-term sustainability, of which flood been considered within the costing exercise. risk management is a key element. 5.4.2 Based on the results of the Level 2 hydraulic modelling work and the potential development options 5.4.9 The Bridgwater Bar to Bideford Bay Shoreline Management Plan (SMP) identifies that the preferred within the Burnham-on-Sea/Highbridge flood cell, two locations have been identified where strategic management option for Brean Village to Brean Down is to hold the existing defence line with flood risk management options may be considered to protect existing development and enable future observations and monitoring at strategic locations. It is understood that the observations and monitoring development whilst reducing flood risk. These locations are in the vicinity of the boatyard on the River will feed into the current review of the SMP. As identified in paragraph 5.4.4 it is important to ensure that Brue and the land associated with tourism uses at Brean. development aspirations of Sedgemoor DC consider the SMP policies and do not conflict with respect to future flood risk management. The results of the Level 1 and Level 2 SFRA should feed into the SMP Boatyard Area – River Brue consultation process. 5.4.10 In terms of PPS25, paragraph D20 and D21 identifies the following in relation to temporary occupation 5.4.3 The hydraulic modelling results indicate that the area around the existing boatyard on the River Brue is of caravan and camping sites: a location where potential overtopping of flood defences is likely to occur in preference to other locations during an extreme event. D20: Sites intended for temporary occupation are classified as ‘more vulnerable’ because they are usually occupied at times of the year when flood events area less likely to occur, although they may be 5.4.4 The Bridgwater Bar to Bideford Bay Shoreline Management Plan (SMP) identifies that the preferred located for amenity and recreational reasons on coastal or riverside sites with a high residual risk of management option for the north bank of the River Brue is to hold the existing defence line. The SMP is flooding. However, the attractiveness of waterside sites for holiday accommodation also has to be currently under review and will be replaced with the North Devon and SMP. It is important to recognised, provided that proper warning and evacuation arrangements are put in place through ensure that development aspirations of Sedgemoor DC consider the SMP policies and do not conflict appropriate planning conditions. with respect to future flood risk management. The results of the Level 1 and Level 2 SFRA should feed into the SMP consultation process. D21: The Sequential Test and Exception Test should be used by decision-makes (where applicable, - remembering that ‘highly vulnerable’ development should not be permitted in Zones 3a and 3b and ‘more vulnerable’ development should not be permitted in Zone 3b). FRAs should pay particular 8 These percentages should be revised in light of specific mitigation solutions to ensure that they are plausible attention to the management of residual risk, flood warning arrangement and evacuation plans should and realistic be considered (see Annex G – PPS25).

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5.4.11 It is understood that Sedgemoor DC and the Environment Agency are discussing the potential to provide a bund in the Brean area to mitigate the risk of overtopping from tidal sources. A series of indicative costs for a range of flood defence structures have been calculated based on the values provided in Section 5.3. The following assumptions have been made to provide the indicative costs, these are: • Lifetime of the defence is 50 years; • Two lengths of defence have been considered – 500 m and 1000 m; • One large outfall (>900 mm) is required every 250 m within the defence; • The defence is ‘new’ as opposed to an upgrade of existing defences; 5.4.12 The lifetime of the defence has been considered as 50 years for this scenario due to the commercial nature of the development. It is suggested that if a flood defence option is progressed, the design should be future proofed to enable the defence to be upgraded through minimisation of future costs and therefore providing a more sustainable solution.

Table 5-8: Indicative cost for flood defence options in the Brean area.

Length of Defence 9 Defence Type 500 m 1000 m Approx Cost (£/m) Masonry Wall* £4.61m £9.21m £10,000 Wall with piling* £6.09m £12.18m £12,000 Embankment* £3.65m £7.30m £7,500

*Assumed height of wall is between 1.2 and 2.1 m, no height is specified for the embankment option.

5.4.13 The indicative costs in Table 5-8 illustrate that a flood defence solution for the Brean area is likely to cost between £3.65m and £12.18m to implement and maintain for a 50 year lifetime depending on the required length.10

9 Based on indicative lengths required to provide mitigation to average sized caravan park 10 Costs per unit length differ between the two proposed mitigation options as they are a function of lifetime, height of flood defence and number of outfalls (1 per 250m)

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6.1.8 Costs for strategic flood defence solutions have been undertaken using a precautionary approach to 6 Discussion and Conclusions provide indicative costs to allow Sedgemoor District Council to consider if future development is both achievable and deliverable within the Burnham-on-Sea/Highbridge area. 6.1.9 It is recommended that more detailed costing of any proposed works should be undertaken through 6.1 Discussion formal appropriate procedures including feasibility, detailed design, specification and a final Bill of Quantities for estimating costs being undertaken using Civil Engineering Standard Method of 6.1.1 Based on the results provided in Appendix C and D, and the analysis for the general areas for the 12 rd Measurement 3 Edition (CESMM3), or similar. PDOs, informed decisions on the application of the Sequential and Exception Test can be made by Sedgemoor District Council based on these findings. 6.1.10 Within the Level 1 SFRA, flood mitigation options provided within the Policy Recommendations identified that a strategy to provide improvements to existing infrastructure to accommodate growth Burnham-On-Sea/Highbridge flood cell within this area was recommended. If allocation of development is progressed, it is recommended that in liaison with the Environment Agency and the Somerset Drainage Board Consortium, a Surface Water 6.1.2 The potential failure of flood risk management infrastructure through overtopping and breaching is a Management Plan (SWMP) is developed for the Burnham-on-Sea/Highbridge Area. This is inline with significant issue within Sedgemoor District. Within the Burnham flood cell, overtopping of flood paragraph 6 of PPS25 and paragraphs 5.33 to 5.42 in the Practice Guide. defences occurs in a number of locations, most notably in the vicinity of the boatyard on the River Brue and along the River Axe. This is more prominent when considering the effects of climate change and Bridgwater flood cell flood events with a higher return period (e.g. 1 in 1000 years). 6.1.11 Within the Bridgwater flood cell, overtopping of the existing defences is insignificant during the present 6.1.3 Due to the large extent of low lying land within this flood cell, the dissipation of flood water across this day scenarios. This concurs with Environment Agency evidence that the flood management area reduces the effects if a breach of flood management infrastructure were to occur. infrastructure along the River Parrett protects Bridgwater for the 1 in 200 year tidal event. However, 6.1.4 Table 6-1 provides an indication of the current and future hazard associated with each PDO. This these defences will only provide adequate protection for existing development until 2030 (see Level 1 illustrates that although sites are available at present that have acceptable levels of hazard, the use of SFRA). It is therefore considered that a strategic flood defence solution (preferred option based on mitigation measures will be required to account for the effects of climate change in the future. Level 1 SFRA findings is for a surge barrier) is required to enable the sustainable development of growth within the Bridgwater area as identified in the draft RSS. 6.1.5 It should be noted that the future flood hazard illustrated in Table 6-1 accounting for climate change is based a 100 year horizon to 2108 and is for the 1 in 1000 year event (0.1% annual probability) as 6.1.12 Depending on the location of a strategic flood defence option, the amount of protection afforded will advocated in paragraph 3.56 of the PPS25 Practice Guide. In addition, the future flood hazard rating for vary. Overtopping of existing may occur downstream of the location of the strategic defence and each PDO in Table 6-1has been based on the hazard for overtopping with no breach. therefore raising of defences may be required in these locations to mitigate against the potential of overtopping. Table 6-1: Existing and future hazard associated with each PDO 6.1.13 Table 6-2 provides an indication of the current and future hazard associated with each PDO. This Potential Development Existing Flood Hazard Future Flood Hazard illustrates that although sites are available at present that have acceptable levels of hazard, the use of Location (Average) (Average) mitigation measures will be required to account for the effects of climate change in the future. 1 - North of former radio - High 6.1.14 It is noted that high hazard is experienced in the northern sector of Little Sydenham Farm and Follets station Farm and therefore most vulnerable development should be steered towards areas of lower hazard.

2 - Land to the north east of - Medium Table 6-2: Existing and future hazard associated with each PDO Isleport Potential Development Existing Flood Hazard Future Flood Hazard 3 - Land to the north east of - Low Location (Average) (Average) Isleport 7 - Little Sydenham Farm (north - High 4 - Land to the east of Isleport - - sector) 5 - Land to the east of Brean Low High 8 - Little Sydenham Farm - Medium 6 - Land to the north east of - High (south sector) Burnham-on-Sea 9 - Bower Lane - Medium 10 - Follets Farm, Dunwear - High 6.1.6 The positioning of the breach locations is subjective but were determined based on defence information 11 - North of Express Park - Medium and liaison with the Environment Agency (Appendix B). Breaches can potentially occur anywhere within a raised defence and the timing during a flood event is not predictable. Due to this, the timing and 12 - North east of A38 - Medium extent of flooding from breaching may vary from those presented in this report.

6.1.7 The results indicate that the impacts of the potential breach locations in this flood cell are not as 6.1.15 The provision of a surge barrier or improvements to existing defences will further reduce the hazard and significant as the effect of climate change. In addition, the location of a breach has an impact on the residual risk, however, it will not totally remove the risk associated with a breach scenario. timing of inundation reaching the PDO. In general, breaches located close to a PDO have a short time to inundation in comparison to those at distances from the breach location. These aspects should be 6.1.16 Where a PDO is allocated for development, a detailed assessment of the impact of tidal flooding will be considered when allocating a site for development and the potential for adequate flood warning and necessary based on these results that can inform the sequential location of development within a site as safe access/egress. opposed to an individual point.

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6.1.17 The results in this assessment have been made at a strategic level and are only intended to inform the allocation of PDOs and provide a synopsis of potential flood risk and hazard. 6.1.18 These results provide a foundation for site specific Flood Risk Assessments where sites are allocated through the LDF process and using the guidance contained within the Level 1 SFRA report.

6.2 Conclusions

6.2.1 The following conclusions have been made: • Under present day conditions (i.e. no inclusion of climate change) tidal overtopping is most significant to the south east of Burnham-on-Sea and to Brean. It is not a significant constraint within Bridgwater; • The effect of potential breaching of flood defences is most significant in the Bridgwater flood cell under present day conditions. The impact of potential breaching is less significant in the Burnham/Highbridge flood cell and does not exert significant control on wider flooding; • The distance of potential breaching from the PDO location has a notable effect on the flood depth, hazard and time to first inundation; • Climate change is the predominant control upon future flooding; • PDOs experience varying degrees of flood depth, hazard and time to first inundation dependant on the flooding scenario. Apart from PDO 5, the PDOs are not affected during present day conditions. However, all PDOs experience medium to high hazard under climate change conditions with the exception of PDO 3 and 4; • The impacts of climate change illustrate that strategic defence options are likely to be required to provide sustainable development within the Burnham-on-Sea/Highbridge/Brean area. Indicative costs for providing flood defence options for the Boatyard area and the area around Brean have been undertaken. These indicative costs provide a steer to Sedgemoor DC regarding the deliverability and achievability of future development and protection of existing development; • It is recommended that a SWMP is undertaken to provide a strategic solution for management of surface water to provide betterment to existing development and allow for future development within the Burnham-on-Sea/Highbridge area. • The impacts of climate change illustrate the requirement for a strategic flood defence solution for the sustainable development of Bridgwater and surrounding area. As identified within the Level 1 SFRA, both the Environment Agency and Sedgemoor District Council have confirmed that a surge barrier, ‘The Parrett Barrier’, is the preferred option accounting for economic and environmental considerations to provide a deliverable solution to flood risk issues in Bridgwater 11 ; • Where land is allocated for development, a site specific Flood Risk Assessment will be required building on the information provided within the Level 1 SFRA and this Level 2 SFRA report. This should incorporate additional information of mitigation of residual risk and emergency planning procedures to ensure safe access and egress for the lifetime of the development.

11 The location and design of the surge barrier will require further technical work to be commissioned by the Environment Agency and will not be available within the timeframe of this SFRA. Scott Wilson understand that Sedgemoor District Council will develop a funding strategy in partnership with the Environment Agency in order to ensure delivery of this flood defence structure by or before 2030.

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7 References

Department for Communities and Local Government, 2006. ‘Planning Policy Statement 25: Development and Flood Risk’. TSO publications, Norwich.

Department for Communities and Local Government, 2008. ‘Planning Policy Statement 25: Development and Flood Risk, Practice Guide’, London.

DEFRA/Environment Agency 2005. ‘Flood Risk Assessment Guidance for New Development – Phase 2: Framework and Guidance for Assessing and Managing Flood Risk for New Development – Full Documentation and Tools’. R&D Technical Report FD2320/TR2.

Environment Agency, 2007. ‘North Wessex Area Tidal Areas Benefiting from Defences’, Royal Haskoning.

Sedgemoor District Council, 2007. ‘Core Strategy - Issues and Options Report, Planning the Future of Sedgemoor 2006 – 2026’.

Sedgemoor District Council, 2008. ‘Strategic Flood Risk Assessment, Level 1 Report’, Scott Wilson Ltd, Plymouth.

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8 Appendices

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Appendix A – Flood Cells

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Appendix B – Modelling Methodology

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The Burnham-on-Sea/Highbridge to Brean flood cell covers and area of 120 km 2, whilst the Modelling Methodology Bridgwater flood cell covers an area of 28 km 2. Table B-3 presents the flood cell references and a brief description of the breach located within each flood cell. This appendix presents the methodologies used in developing the flood outline, maximum flood depth and hazard zone maps for the SFRA. Table B-3: Flood cells and the associated number of analysed breach with overtopping events

Flood Cell Breach Digital Terrain Map (DTM) Generation Location of Breach (NGR) Number A key component in the modelling process for the SFRA is the representation of topography Burnham-on-Sea/Highbridge to Brean 1 329628E, 158533N throughout flood prone regions of the study area. For this purpose, a Digital Terrain Map (DTM) was derived for each of the modelled areas. A DTM is a three-dimensional surface on which the model Burnham-on-Sea/Highbridge to Brean 2 330340E, 149723N simulations are run. Burnham-on-Sea/Highbridge to Brean 3 330470E, 147835N The platform used for the generation of the DTM was the GIS software package MapInfo Bridgwater 4 331025E, 140585N Professional (Version 8.5) and its extension, Vertical Mapper (Version 3.1). Bridgwater 5 331305E, 135655N The DTM is primarily based on filtered Light Detection And Ranging (LiDAR) data provided by the Environment Agency. LiDAR is a method of optical remote sensing, similar to RADAR (which uses Coastal Defences radio waves instead of light). In this case, the LiDAR surveys return data at a horizontal resolution of 2 m with a vertical accuracy of +/- 0.1 m. Filtered LiDAR data represents the “bare earth” elevation The Environment Agency supplied details of a survey (undertaken by Halcrow in 2005/2006) of all with buildings, structures and vegetation removed. raised coastal defences; levels were taken approximately every five metres. The data was used to update the National Flood and Coastal Defence Database (NFCDD), which was provided as part of The LiDAR data were used to create a DTM grid covering each flood cell. the Level 1 SFRA data request. However, the NFCDD was not updated to reflect each of the survey points. Therefore, the detailed survey data was requested during the Level 2 SFRA and the elevation of the flood defences within both flood cells were defined in five metres intervals, using this Flood Cell Definition information. Consequently, the flood defences have been defined very accurately.

The breach locations were determined and agreed in conjunction with the Environment Agency Culverts/Bridges based on local knowledge of the condition of the defences, the location of future development sites and the vulnerability of local communities. Figures A1 and A2 show the locations of the breaches Strategic bridges and culverts were included within the model domains, these were significant modelled. structures that convey flood water during times of inundation. It was considered impractical and unnecessary to include smaller culverts identified within each flood cell to ensure model stability, Two flood cells were defined based on the areas of location of future development and breach especially considering the strategic nature of the assessment. locations (Bridgwater and Burnham-on-sea/Highbridge). Each flood cell defines the geographical extent of the model. While it is possible to run each of the breach models using all of the derived Sluice structures at Brean Cross Sluice, Highbridge Clyce (Burnham-on-sea/Highbridge cell) and DTM topographical data, it is far more sensible to define a smaller area on which to run each (Bridgwater Cell) for initial model runs were modelled as ‘open’ to provide a precautionary scenario. approach. Flooding observed in the northern area of the Burnham-on-Sea/Highbridge cell and flooding within the Bridgwater cell concurred well with other strategic flood mapping studies (North Flood cells are typically defined by considering the topography of the area inland of the breach and Wessex Areas Benefiting from Defence, Bridgwater Hospital Flood Risk Assessment). However, it the peak levels of the tidal events to be tested. GIS can be used to show areas of potential flooding was considered that additional model runs with the structure at Highbridge Clyce as ‘closed’ were by only displaying areas of the DTM that are below the predicted peak inundation levels inland of the necessary due to the strategic location and requirement to identify potential breach/overtopping coastline, plus a freeboard of several hundred millimetres. Areas of the DTM that are not shown (that locations within the existing defences in this area. is, areas that are well above the tidal levels of interest) do not need to be considered in the model. However, Sedgemoor District is very flat and therefore the local topography does not clearly define a Extreme Water Level Derivation complete, enclosed flood cell. In these cases it is necessary to artificially enclose certain parts of the flood cell. This should only be done for areas that are not near the coastline or any important areas The extreme sea water levels associated with tidal flood events are, in general, specific to each of the model, and will typically be outlying or empty areas of the flood cell. For example, estuaries or breach location. The extreme sea water levels for the breach locations along the coastline are based flat, open fields at the boundaries of the flood cell. Since the model treats the boundaries of flood on information obtained from Environment Agency from their hydraulic modelling studies cells as ‘glass walls’ it is vital that any artificial boundaries do not affect levels in the important areas (‘Environment Agency North Wessex Tidal Areas Benefiting from Defences’, September 2007). of the flood cell. However, this is typically not an issue in models where the inflows are based on tidal levels rather than a specific volume. The width of flood cell within the model was extended Climate Change significantly beyond the M5 to remove any inaccuracies associated with ‘glass walls’, which may affect the area of interest. In the UK the effect of climate change over the next few decades is estimated to result in increased sea levels. The rise in sea levels will increase the duration and magnitude of overtopping of coastal

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defences and is expected to have a major influence on the potential for future flooding. Breach Location of Breach (British Description Consequently PPS25 requires the potential impacts of climate change to be accounted for the Number National Grid) lifetime of proposed developments (100 years for residential development as indicated in the Sea. Depth of breach contained to level of access Practice Guide). Using the guidance provided in PPS25, an increase in peak tide levels of 0.98 m ramp. 4 m breach, controlled by ramp width. was incorporated as an allowance for climate change to 2108. The peak tide levels for each breach location simulated in this assessment are presented in Table B-4. Embankment within the mouth of River Brue Pill, adjacent to Holiday Park, Highbridge. Land 3 330460E, 147850N Table B-4: Maximum Tidal water levels behind defence relatively low resulting in relatively deep breach. Width of breach is 20 m. 1 in 200 year 1 in 1000 year 1 in 200 year + 1 in 1000 year Embankment south of King Sedgemoor Drain Breach Flood Cell CC + CC Sluice (confluence with River Parrett). Relatively 4 331077E, 140534N [mAOD] [mAOD] [mAOD] [mAOD] high land behind defences controlling depth of Burnham-on- breach. Width of breach is 20 m. 1 8.13 8.40 9.11 9.38 sea/Highbridge Hard defence, northern bank of River Parrett, at Burnham-on- Plum Lane. Land behind defence relatively low 2 8.09 9.07 5 331315E, 135662N sea/Highbridge resulting in relatively deep breach. Width of breach is 20 m. Burnham-on- 3 8.09 9.07 sea/Highbridge Bridgwater Model and Software Selection 4 8.28 9.25 To achieve the study objectives, the model used to estimate the maximum flood conditions was Bridgwater 5 8.12 9.10 required to: • Accommodate the effects of a flood flow (propagation of a flood wave and continuous change of Tide Curve water level); • The tide curves used in the models were also derived from the Environment Agency models, as Simulate the hydraulics of the flow that breach the flood defences; and used in the report ‘Environment Agency North Wessex Tidal Areas Benefiting from Defences’ • Generate detailed information on the localised hydraulic conditions over the flooded area in (September 2007). To account for the effects of climate change, 0.98 m were added to all values in order to evaluate flood hazard. the present day curve (Table B-2). MIKE21-HDFM simulates water level variations and flows for depth-averaged unsteady two- dimensional free-surface flows. MIKE21-HDFM is specifically oriented towards establishing flow Breach Modelling patterns in complex water systems, such as coastal waters, estuaries and floodplains.

Breaching of flood defences has the potential to generate the greatest flood risk hazard for an area. MIKE21-HDFM is a new modelling system based on a flexible mesh (FM) approach. The FM model To assess flood propagation in events where the flood defences are breached, a hydraulic modelling has the advantage that the resolution of the model can be varied across the model area. The model analysis has been undertaken using the two-dimensional hydraulic modelling software MIKE21- utilises the numerical solution of the two-dimensional shallow water equations. HDFM (version 2008). Model Extent and Resolution Five breach locations were specified in agreement between the Environment Agency, Sedgemoor District Council and Scott Wilson based on local knowledge of the condition and vulnerability of the For each flood cell, a MIKE21-HDFM model has been developed using the MIKE21 program Mesh defences, the potential location of future development sites and the vulnerability of local Generator. The mesh generator creates a mesh over the flood cell DTM using triangular elements. communities. Breach locations and descriptions are provided in Table B-5 and shown in Figures A1 The element size varies throughout the model domain and depends upon the complexity of and A2. floodplain topographic features and/or areas of interest. Table B-5: Breach locations Using the FM module it is possible to generate a highly resolved mesh in areas of particular interest Breach Location of Breach (British or in areas that are hydrodynamically important within the model. In other areas, a lower resolution is Description Number National Grid) used to reduce computational demands.

Rock Armour and adjacent access ramp at Brean To represent the hydraulics around a breach location with a relatively high level of accuracy, a Down car park. Relatively high land behind 1 329634E, 158554N comparatively small element size has been applied in the vicinity of breaches. The breach has been defences controlling depth of breach. Width of represented by a minimum of four elements. Urban areas and structures within the floodplain have breach 20 m. the potential to affect the free flow of floodwater. Embankments, flood defences, significant 2 330347E, 149688N Access ramp at Maddocks Slade, Burnham-on- watercourses and other linear features have been incorporated into the flexible mesh by creating

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break lines parallel to the feature. Break lines allow the definition of linear features to be resolved Boundary Conditions during mesh generation to ensure these features are represented. The MIKE21 breach models require at least one boundary condition to be defined. This is a time By adding break lines (lines delineating surface features), the mesh orientation is forced to follow the dependent head boundary (HT) at the seaward side of the breach location, which replicates the alignment of the features and the localised elevations of structures are used by the mesh generator. extreme tide levels/cycle during a tidal flood event. The break lines of linear man-made features were schematised by reference to the DTM, 1:25000 OS maps and high-resolution aerial photographs. The crest levels of linear features, such as The Burnham-on-Sea/Highbridge flood cell is extremely large and the extreme water levels varied secondary flood embankments, road embankments and railway embankments, have been between Highbridge to the south and Brean to the north. At the northern end of the flood cell (north established by interrogation of the DTM. It should be noted that the majority of the features of 152200N), a slightly higher level was used in the previous Environment Agency modelling (code described above have been identified through a desktop analysis only, and have not been verified ch30). This can be seen in the higher levels for Breach 1 compared to Breaches 2 & 3 (see Table B- on the ground. Results from the breach modelling which show strong dependence on barriers should 4). For consistency with the Environment Agency models, this higher level was used as the therefore be used with caution. boundary condition north of 152200N, while the lower values were used to the south (code ch42).

For the Bridgwater flood cell, there were also two peak flood levels derived from the Environment Agency models. These two peaks can also be seen in Table B-4. In the northern (downstream) reach of the River Parrett, the levels for code Par20 were used. At the southern (upstream) reach of the River Parrett, the levels for code Par32 were used. These curves were extracted directly from the Environment Agency models.

Boundary conditions for different breach scenarios were modified to reflect the variation in extreme water level where required. Increased resolution Breach mesh in key areas location Hydraulic Roughness

Hydraulic roughness represents the conveyance capacity of the vegetation, bed and bank material, channel, sinuosity and structures in the floodplain. Within the MIKE21 model, hydraulic roughness is defined by the dimensionless Manning Number ‘N’.

The assigned hydraulic roughness coefficient is based on engineering judgement and available Flood Cell literature (e.g. Chow, 1979). The Manning Number, N, used for the land in the study area was 22, extent while all sea/river areas were set at 33.

Model Simulations Undertaken

A total of 18 scenarios were agreed with the Environment Agency and Sedgemoor District Council, Figure B-1: Example of Mike 21 HD Flexible Mesh which are defined within the main body of the report. In scenarios that model a breach, overtopping of defences is also likely to occur and therefore has been incorporated. Breach Specifications The results for the individual model simulations have been processed to create flood depth and The flood conditions (i.e. inundation rate, flood extent, depth of flooding) that may be experienced if hazard maps presented in Figures C1 to C24 (Burnham-on-Sea/Highbridge) and Figures D1 to D12 a flood defence were to breach are a function of the breach dimensions, time required to repair the (Bridgwater). breach (exposure duration) and tidal conditions. Since it is not possible to set repair time in the modelling software, the breach and tidal details are the two major factors that determine the extent of inundation due to breaching and have been described in the preceding text. Limitations of Modelling Approach

Breach widths were determined in agreement between the Environment Agency, Sedgemoor District The hazard zone maps indicate the product of depth and velocity for each scenario. These hazard Council and Scott Wilson. The breach width is determined based upon the location and the type of classifications do not indicate a change in flood probability and represent hazard arising from one or defence and are as tabulated in Table B-5. The base level of the breaches have been set to the more specific breach locations. It should be noted that hazard is likely to vary spatially dependant on minimum elevation of the land directly behind (landward) the flood defence. the breach locations. This is also applicable for the flood depth maps.

In the hydraulic modelling undertaken for this study, the breach in the flood defence was present Further issues in this respect that should also be considered include the following: during the whole flood event (i.e. it is deemed to have occurred prior to the onset of the extreme tidal • Not all possible breach locations have been considered. Necessarily, the modelling study had to event) as it is not possible to vary the DTM during the simulation period. This is a conservative be limited to those locations thought most likely to lead to flood risk for specific development assumption. areas.

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• Breach width and depths are based upon engineering judgement and do not necessarily Table B-5: Danger to people for different combinations of depth and velocity. (Table 13.1 from represent the actual dimensions of a breach in a given location. ‘Flood Risk Assessment Guidance for New Developments: Phase 2, FD2320/TR2). • Changes in inundation extent or hazard zone are non-linear to changes in breach location. Depth (m) • Vertical inaccuracies inherent in the Lidar data. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.5 2.0 2.5 0.0 5 0 0 0 0 0 0 0 0 0 0 0 0.10 Definition of Hazard Categories 0.20 5 Breach analysis presents data to identify the residual risk of flooding from a failure of local defences. 0.5 0 The mapping of flood hazard zone maps within the study area represents an appreciation of the 1.0 residual risk to provide an additional level of information to local planning authorities allowing them to 1.50 make more detailed consideration of the Sequential Test and PPS25 vulnerability classifications 2.00 within Flood Zone 3a. 2.50 3.00 Flood hazard is a function of both the flood depth and flow velocity. Therefore, to create flood hazard 3.50 maps, the modelled floodwater depth and flow velocities resulting from each model scenario have 0 been assessed. 4.0 4.50 In most flood events the maximum hazard of a flood at a certain location is not experienced at the 5.00 peak of the flood but before the maximum floodwater level occurs. This is the point at which the Velocity(m s-1) 0 greatest flood depths and velocities typically occur. Thus, in order to determine the maximum flood hazard, the hazard level was assessed by using an in-house tool (HazardIndex) which assigns one of three hazard categories (low, medium or high) to each element in the mesh at every time step of Key: the model simulation, then determines the maximum for that element. This is consistent with the Danger for some information provided in Table 13.1 from ‘Flood Risk Assessment Guidance for New Developments:

Phase 2, FD2320/TR2’ and is provided in Table B-5. Danger for most Danger for all

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Appendix C – Burnham-on-Sea/Highbridge Flood Cell Figures

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Appendix D – Bridgwater Flood Cell Figures

January 2009 D