Bridgwater Tidal Barrier Environment Agency
Flood Risk Assessment
P02 November 2019
DRAFT Flood Risk Assess men t EDF Ener gy Servic es
Flood Risk Assessment
Bridgwater Tidal Barrier
Project no: ENVIMSW002039 Document title: Flood Risk Assessment Document No.: ENVIMSW002039-CH2-00-00-RP-HY-00130 Revision: P02 Date: November 2019 Client name: Environment Agency Project manager: Russell Corney Author: Rebecca Bailey/Rachel Hopgood File name: BTB Scheme Flood Risk Assessment
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Document history and status
Revision Date Description By Review Approved
P01 16 Sept Initial Draft FRA RB 2019
P02 15 Nov Final RH RC RC 2019
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Contents 1. Introduction ...... 1 1.1 Background ...... 1 1.2 Objectives ...... 2 2. Site Description ...... 4 2.1 Site location ...... 4 2.2 Scheme proposals ...... 5 2.3 Programme ...... 9 2.4 Existing land use and baseline conditions ...... 9 2.5 Hydrology / existing drainage ...... 10 2.6 Geology / hydrogeology ...... 10 3. Planning Policy ...... 11 3.1 National Planning Policy ...... 11 3.2 Assessment of flood risk...... 11 3.3 Local Policies ...... 14 4. Assessment of Flood Risk ...... 16 4.1 Hydraulic modelling ...... 16 4.2 Climate change ...... 16 4.3 Flood risk from the sea ...... 18 4.4 Flood risk from rivers ...... 28 4.5 Flood Risk from surface water ...... 38 4.6 Flood Risk from groundwater...... 41 4.7 Flood risk from reservoirs ...... 42 4.8 Flood risk from sewers ...... 43 5. Summary and Conclusions ...... 44
Appendix A Mapping
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1. Introduction
1.1 Background
1.1.1 Bridgwater in Somerset lies on the estuary of the River Parrett some 17 km upstream of the Severn Estuary (Figure 1-1). It is a market town with a population of approximately 45,000; is the administrative centre of Sedgemoor District; and is the leading industrial town in the county. Bridgwater is the focus for accelerated housing growth; new employment opportunities (including those directly supporting the new nuclear build at Hinkley C); expansion of further education opportunities; and new retail and leisure investment.
Figure 1-1: Location of Bridgwater
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1.1.2 Much of Bridgwater and the surrounding area lies below high tide level and there are some 11,300 homes and 1,500 businesses at risk from tidal flooding. For development to continue in accordance with the National Planning Policy Framework, there needs to be a plan to ensure that it is safe from flood risk for its lifetime. In the case of housing, this is for 100 years. It is therefore essential to have an appropriate long-term strategic flood defence solution for the town in order to support the continued growth requirements and wider regeneration ambitions set out in both adopted Local Plans and the Bridgwater Vision.
1.1.3 The Bridgwater Tidal Barrier (BTB) is a key element of the strategic tidal defence scheme that has been developed for Bridgwater. The tidal barrier, and associated works form the Bridgwater Tidal Barrier scheme (BTB Scheme) which will provide flood risk reduction to properties, businesses and infrastructure in Bridgwater and within the wider area of the Sedgemoor District. The Environment Agency is seeking powers to construct the BTB Scheme through application for a Transport and Works Act Order (TWAO), under the Transport and Works Act 1992.
1.1.4 The BTB Scheme will also require planning permission under the Town and Country Planning Act 1990; an application will be made for this planning permission to be ‘deemed’ through the TWAO process.
1.1.5 This Flood Risk Assessment provides evidence to support the planning application, as required under the National Planning Policy Framework (NPPF). The NPPF requires that a Flood Risk Assessment is undertaken for developments:
· In flood zone 2 or 3 including minor development and change of use · More than 1 hectare (ha) in flood zone 1 · Less than 1 ha in flood zone 1, including a change of use in development type to a more vulnerable class (for example from commercial to residential), where they could be affected by sources of flooding other than rivers and the sea (for example surface water drains, reservoirs) · In an area within flood zone 1 which has critical drainage problems as notified by the Environment Agency
1.1.6 As the BTB Scheme is greater than 1ha and located in flood zones 2 and 3 a Flood Risk Assessment is required.
1.2 Objectives
1.2.1 In accordance with the National Planning Policy Framework (NPPF), the aim of this Flood Risk Assessment (FRA) is to determine whether the proposed scheme is appropriate for the level of flood risk at the site and confirm that it will not increase the flood risk to third parties. A summary of how the NPPF requirements are addressed in this flood risk assessment is shown in Table 1-1.
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Table 1-1. Site-specific Flood Risk Assessment (FRA) objectives How the objective is addressed in this Objective from the NPPF FRA Establish whether a proposed development is Assessment of the existing sources of risk to likely to be affected by current or future the proposed scheme. flooding from any source. Establish whether it will increase flood risk Assessment of the impact on flood risk to elsewhere. third parties as a result of the development of the scheme. Establish whether the measures proposed to Identification of any mitigation measures to deal with these effects and risks are manage the above risks that could be appropriate. incorporated in the detailed design. Establish the evidence for the local planning Summary of previous work undertaken which authority to apply (if necessary) the supports the Sequential Test. Sequential Test. Establish whether the development will be Assessment of whether the Exception Test is safe and pass the Exception Test, if required and, if required, whether the applicable. proposed development meets the requirements of the Exception Test.
1.2.2 In addition to the requirements of the NPPF, which requires an assessment of whether flood risk to third parties will increase as a result of development, this Flood Risk Assessment includes an assessment of any beneficial impacts as a result of the new flood risk management infrastructure.
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2. Site Description
2.1 Site location
2.1.1 The main tidal barrier site is located on the River Parrett, adjacent to Express Park business park, approximately 1.7km to the north west of Bridgwater town centre. The planning application boundary includes the main tidal barrier site as well as sites for downstream defence works at Chilton Trinity, Pawlett and Combwich. The boundary also includes sites required for mitigation works for the tidal barrier including navigation signs and fish and eel pass improvements upstream. The application boundary is located across a number of sites lying within three Local Planning Authority areas: Table 2-1. Bridgwater Tidal Barrier site locations Works (see Section 2.2) Local Planning Authority Tidal Barrier Sedgemoor District Council Downstream Defences – Chilton Trinity Sedgemoor District Council Downstream Defences - Pawlett Sedgemoor District Council Downstream Defences - Combwich Sedgemoor District Council Navigation Sign - Dunball Sedgemoor District Council Navigation Sign – Bridgwater Docks Sedgemoor District Council Fish Pass Site – Ablake Weir South Somerset Council Fish Pass Site – Ham Weir South Somerset Council Fish Pass Site – Long Load Sluice South Somerset Council Fish Pass Site – Monk’s Leaze Clyce South Somerset Council Fish Pass Site – Midelney Pumping Station South Somerset Council Fish Pass Site – Thorney Mill Sluice South Somerset Council Fish Pass Site – Witcombe Bottom South Somerset Council Fish Pass Site – Bishop’s Hull Gauging Somerset West and Taunton Council Station Weir Fish Pass Site – Bradford-on-Tone Weir Somerset West and Taunton Council Fish Pass Site – Firepool Weir Somerset West and Taunton Council Fish Pass Site – French Weir Somerset West and Taunton Council Fish Pass Site – Longaller Weir Somerset West and Taunton Council
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2.1.2 The planning application boundary can be found in Appendix A.
2.1.3 The site is situated in predominantly undeveloped agricultural land with the exception of the ancillary building required for the tidal barrier operation which are to be situated in the Express Park.
2.2 Scheme proposals
2.2.1 The Bridgwater Tidal Barrier Scheme will comprise a tidal barrier across the River Parrett between Express Park and Chilton Trinity with new flood defences (secondary defences) and improved existing defences (primary defences) downstream of the tidal barrier at Chilton Trinity, Pawlett and Combwich. The BTB Scheme will provide a 1 in 200-year Standard of Protection against tidal flooding to Bridgwater, Chilton Trinity, Pawlett and Combwich, with an allowance for projected sea level rise, up to 2055.
2.2.2 After this date, further work will be required to maintain this Standard of Protection. The Environment Agency’s strategy is to maintain this standard through to 2125 through phased raising of existing flood defences downstream of the tidal barrier site. The tidal barrier will include an allowance for sea level rise through to 2125. As the future works to flood defences do not form part of the current TWAO and planning application they have not been included in this assessment of current or future flood risk as a result of the scheme.
2.2.3 In addition to the main tidal barrier and downstream defence works, there are environmental mitigation works proposed as part of the scheme relating to navigation and fish and eel passage which are also described below.
2.2.4 Tidal Barrier
2.2.4.1 The tidal barrier is to be positioned across the River Parrett between Chilton Trinity and the Express Park. The location of the tidal barrier was chosen following an appraisal of construction costs, operational viability, and environmental impacts. The tidal barrier is designed to be operated for tidal flood risk management at the onset of a surge tide, anticipated to be on average between 1 and 5 times a year when the tidal barrier first becomes operational. The structure requires ancillary facilities for control equipment, administration, materials and equipment storage and staff parking; these will be located primarily within Express Park.
2.2.4.2 The tidal barrier will include two openings, each not less than 14m wide, with vertical lift gates which will be lowered into position when a tidal surge event is forecast. The gates will each be approximately 10m high and, when in “closed” position, will have a crest level of around 10mAOD. The tidal barrier abutments will also be raised to above 10mAOD and they will be tied into adjacent raised platforms (see below). During normal operational conditions, when the tidal barrier is open, the gates will be raised so that their base level is at 9mAOD or above to provide a clear opening for navigation and to allow flow through the tidal barrier.
2.2.4.3 The tidal barrier structure will include a high level bridge, supported by three towers, to support the gates and allow operational access across the structure. This will be located significantly above expected flood levels, with a soffit at around 23.5mAOD. A foot and cycle bridge will be constructed across the tidal barrier structure, upstream of the gates, to provide a permissive route for members of the public to cross the river. This bridge will have a single span between barrier abutments and will have a minimum soffit level of 9mAOD. This bridge is therefore above the flood levels upstream of the tidal barrier and is not expected to have an impact on flood risk.
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2.2.4.4 The river bed at the tidal barrier will be protected from erosion by the concrete base/pile cap which will extend 13.5m downstream and upstream of the gates. For a distance of 15m upstream and downstream of the concrete base, the river bed will be protected with rock rip- rap.
2.2.4.5 Raised platforms will be provided on both banks of the tidal barrier to provide access to the towers and to link the tidal barrier structure to the adjacent primary flood defence embankments. This will avoid flood flows bypassing the tidal barrier.
2.2.4.6 Permanent access tracks will be provided to both sides of the tidal barrier structure. On the right bank, this will be through Express Park. On the left bank, the temporary access track used for construction west of the Chilton Trinity Sewage Treatment Works (STW) will be upgraded to form a permanent operational access route.
2.2.5 Express Park Operational Site
2.2.5.1 Ancillary development is required close to the tidal barrier for operational use by the Environment Agency. This will include the provision of a 2-storey control building, two stoplog storage sheds and car parking within a currently disused plot on the south western corner of Express Park.
2.2.5.2 The operational site will be constructed on a raised platform with a level of at least 9mAOD to provide resilience to the operational buildings. The site is currently partially occupied by a concrete base slab from a former building, meaning that the site is partially covered by impermeable surfacing. The exact landscaping/surfacing for the proposed operational site is to be confirmed at detailed design stage but is likely to comprise a combination of soft and hard landscaping.
2.2.6 Downstream flood defences
2.2.6.1 The downstream flood defences to be constructed as part of the BTB Scheme link the tidal barrier to high ground to prevent flood flows bypassing the structure into Bridgwater and provide a consistent standard of tidal flood protection to communities downstream of Bridgwater.
2.2.6.2 In all locations, raising and widening works on the existing primary flood embankments will take place from the landward side of the defence to avoid any temporary or permanent footprint on the foreshore of the River Parrett. Material for construction of the embankments will be obtained from nearby borrow pits.
2.2.6.3 The downstream flood defence works have been designed to avoid the drainage/ditch network as far as possible.
2.2.7 Chilton Trinity
2.2.7.1 The flood defences around Chilton Trinity will provide a continuous defence line from the tidal barrier to high ground to the west and will reduce tidal flood risk to the village.
2.2.7.2 Works undertaken on the existing primary embankments will involve raising a section on the left bank of the River Parrett by 0.5-1.0m for some 900m downstream of the tidal barrier. The work here will also involve crossing an existing outfall structure at Pims Clyce; this will involve driving sheet piles over a distance of approximately 55m on the landward side and 40m on the riverward side to enable the embankment above the outfall to be raised. The operation of the outfall itself will remain unaffected.
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2.2.7.3 A new secondary flood defence embankment to the north of Chilton Trinity will link from the primary embankment described above to high ground to the west. To the east of Straight Drove, the embankment will be 1.45m in height on average, over a length of approximately 1km around existing field boundaries or along existing tracks. At Straight Drove, localised road raising will extend the flood defence across the highway. To the west of Straight Drove the embankment will also border existing field boundaries over a length of 2.35km with an average height of 1.8m. A sheet piled cut off will be incorporated into the new embankment over a length of approximately 550m at the western end near Perry Wood Farm due to the expected presence of porous soils; this will prevent seepage beneath the embankment during a flood event.
2.2.8 Pawlett
2.2.8.1 The flood defences at Pawlett will reduce tidal flood risk to properties to the south of the village and to the A38 corridor north of Dunball.
2.2.8.2 Works at Pawlett will include raising approximately 950m of the existing primary flood embankment by 0.4m to 0.7m and construction of 440m of 1.8m high secondary flood embankment linking the primary embankment to high ground to the north .
2.2.8.3 The works at Pawlett will also involve raising the defence over an existing outfall structure at Brickyard Clyce. The operation of the outfall structure will not be changed.
2.2.8.4 An existing flood defence wall on the riverward side of Brickyard Cottages will also be rebuilt and raised by approximately 0.7m. The 85m of wall will have bored piled foundations and incorporate 0.8m high glass panels to maintain and open up existing views from the adjacent properties.
2.2.9 Combwich
2.2.9.1 The flood defences at Combwich will reduce tidal flood risk to the village.
2.2.9.2 Works to be undertaken at Combwich include raising the existing primary flood embankment south east of the village by 0.5-1.0m for some 1.2km and construction of a new 1.4m high secondary flood embankment over a length of 800m to the south of the village.
2.2.9.3 Raising and subsequent widening work on the existing primary flood embankment will take place from the landward side of the defence only to avoid any temporary or permanent footprint on the foreshore of the River Parrett. It will also involve passing over Tuckett’s Clyce; the culvert here will be extended to allow for the raised and widened embankment above.
2.2.9.4 At Combwich Pill, repair work will be undertaken to the existing wall and measures to prevent unauthorised access will be incorporated, but there will be no increase in wall height or length. Downstream of Combwich Wharf, the scheme includes raising low spots in the existing flood defence embankment to provide a consistent flood defence level, with no increase in overall defence height, length or width.
2.2.10 Navigation Signs
2.2.10.1 The Navigation Risk Assessment undertaken for the tidal barrier has confirmed the need for two navigation signs. These will be located at Dunball (downstream of the tidal barrier) and at Bridgwater Docks (upstream of the tidal barrier). Whilst these are included within the planning application boundary, the proposed works at these locations are not considered to have a significant effect on flood risk and are therefore not considered further in this FRA. 7 Flood Risk Assessment
2.2.11 Fish and Eel Pass Sites
2.2.11.1 Works are required at a number of sites throughout South Somerset and Somerset West and Taunton to mitigate potential impacts on fish and eel passage arising from the tidal barrier. The works are described below (exact proposals are to be confirmed at detailed design stage). Whilst these works would not usually require planning permission, and therefore would not require a FRA under the NPPF, they form part of the overall BTB Scheme and have therefore been included in this FRA. However, due to the scale and nature of the works, and the limited detail available at this stage of the design process, the assessment of these sites is generally qualitative. Table 2-2. Fish and eel pass sites proposed works Site Proposed Works Ablake Weir Replacement of existing side wall with a reinforced concrete structure incorporating a gravity-fed eel pass. Ham Weir Minor modifications to the weir including the addition of eel stud tiles to the weir face. Long Load Sluice Replacement of existing side wall with a reinforced concrete structure incorporating a gravity-fed eel pass. Monk’s Leaze Clyce Installation of a pumped eel bristle trough installed within the existing sluice structure. Midelney Pumping Replacement of existing side wall with a reinforced concrete Station structure incorporating a gravity-fed eel pass. Thorney Mill Sluice Installation of an eel bristle trough fixed to the existing channel side wall. Minor modifications may be required to the weir crest to install the trough. Witcombe Bottom Installation of an eel bristle trough on the existing weir. Bishop’s Hull Gauging Lowering of the right third of the weir crest to improve fish Station Weir passage and installation of gravity fed eel tiles on the existing river wall. Bradford-on-Tone Weir Installation of a rock ramp fish pass in land adjacent to the weir, within the bank of the river. Firepool Weir Replacement of timber stop boards in the existing fish pass and installation of a new gravity eel pass in the side wall of the fish pass. French Weir Replacement of timber stop boards in the existing fish pass, installation of a new gravity eel pass in the side wall of the fish pass and installation of a debris deflector at the fish pass exit. Longaller Weir Installation of an eel board onto the weir face, construction of a rock pre-barrage around the base of the existing fish pass and minor modifications to the weir crest and face.
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2.2.12 Temporary works during construction
2.2.12.1 The BTB Scheme will require numerous temporary works during construction, including compound areas and access routes, some of which will need to be located in the floodplain. As these are not permanent development, they are not considered in this FRA. Any potential flood risk issues arising from these will be dealt with under the Environmental Permitting (England and Wales) Regulations 2016 through application for a Flood Risk Activity Permit, where applicable, prior to construction.
2.2.12.2 During construction of the tidal barrier, a bypass channel will be required at the site to allow navigation on the River Parrett and to maintain flows in the River Parrett. This will be in place between 2022 and 2024 based on the current construction programme. Whilst this is not considered to be permanent development, due to the scale of the works and the potential impacts on flood risk this element has been considered in this FRA. A Flood Risk Activity Permit for these works may also be required under the Environmental Permitting Regulations (England and Wales) 2016 and will need to be based on the final temporary works proposals.
2.2.12.3 The bypass channel has been designed as a 20m wide rectangular cross-section with flow guidance bunds upstream and downstream of the channel. This will temporarily introduce two approximately 45 degree bends into the river at this point.
2.3 Programme
2.3.1 Construction of the tidal barrier, operational site and building, flood defences and fish and eel passes are proposed to take place between 2022 and 2025.
2.4 Existing land use and baseline conditions
2.4.1 Within the planning application boundary, land use is almost entirely agricultural, with a number of access tracks and footpaths crossing the land. There are large areas of permanent grass surrounded by ditches lined with trees and other vegetation which serve to control drainage across the land.
2.4.2 The barrier is to be located across the River Parrett between Express Park and Chilton Trinity. On the left bank the barrier structure will occupy only a small footprint of what is currently arable and open pasture. On the right bank, the structure itself will occupy a small footprint of open land with operational buildings located within Express Park. The Express Park site is currently partially occupied by a concrete floor slab from a former building on the site.
2.4.3 At Combwich and Pawlett the majority of the works affect the existing flood defence embankments. The addition of secondary embankments requires construction over an area of arable land.
2.4.4 At Chilton Trinity the improvements to the primary flood defences fall along the footprint of the existing defence line. The secondary flood embankment construction crosses essentially flat floodplain which is predominantly arable with some open pasture. The main interactions with existing development are the crossing of the highway at Straight Drove.
2.4.5 At the fish and eel pass sites, the works proposed are all to existing water level management structures, with exception of Bradford-on-Tone where works are proposed within adjacent woodland.
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2.5 Hydrology / existing drainage
2.5.1 Bridgwater lies on both banks of the River Parrett which flows north through the Somerset Levels from its source in the hills around Cheddington in Dorset. The river drains an area of around 1,700km2. The Parrett’s main tributaries include the Rivers Tone, Isle and Yeo and the River Cary via the King’s Sedgemoor Drain. The 60km river is tidal for 31km up to Oath. The gradient of the river around Bridgwater is very shallow so it is prone to frequent flooding during high tides. The River Parrett does not have a tidal control structure on it.
2.5.2 Throughout the surrounding area there is a dense network of drains (otherwise known as rhynes). These serve to drain the land allowing it to be used as arable and pasture during the summer. The water management in the area is heavily modified through a number of control structures and spillways.
2.6 Geology / hydrogeology
2.6.1 The Somerset Levels are an area of coastal plain and wetland and consist mainly of marine clays along the coast and inland peat-based moors. Bridgwater is centred on an outcrop of marl in an area dominated by low-laying alluvial deposits.
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3. Planning Policy
3.1 National Planning Policy
3.1.1 National Planning Policy Framework
3.1.1.1 The NPPF was published by the Department for Communities and Local Government (DCLG) in March 2012 and revised on 24 July 2018 and 19 February 20191 sets out the government’s policies for planning in England.
3.1.1.2 Current national planning policy in the National Planning Policy Framework aims to ensure that wherever possible, development is located in those areas at least risk of flooding. Where this is not possible, developers need to demonstrate that there are no alternative sites at lower risk, that the proposal is compatible with the identified flood risk or that an exception can be justified, and that the development is safe from flood risk both now and for the lifetime of the development.
3.1.1.3 The NPPF technical guidance was withdrawn from use in March 2014 and replaced with The Planning Practice Guidance. The Planning Practice Guidance2 is a web-based resource that was launched by the Department for Communities and Local Government (DCLG, March 2014) to support the NPPF.
3.1.2 Flood Risk and Coast Change
3.1.2.1 The NPPF sets strict tests to protect people and property from flooding. The tests are designed to assess and avoid flood risk. In areas at risk of flooding, as identified in a Strategic Flood Risk Assessment (SFRA), developers are required to undertake a site- specific FRA. The assessment should demonstrate avoidance of flood risk by applying the Sequential Test and if needed, the Exception Test.
3.1.2.2 Where development needs to be in locations where there is a risk of flooding, developers need to ensure the development is appropriately flood resilient and resistant, safe for its users for the development’s lifetime and will not increase flood risk overall.
3.2 Assessment of flood risk
3.2.1 Flood Risk
3.2.1.1 The flood risk from fluvial (Main Rivers) and tidal flooding is assessed through the use of the Environment Agency Flood Map for Planning (Rivers and Sea)3. This map defines three zones of different flood risk, the third of which is subdivided into two categories: · Zone 1 “Low probability of flooding” – This zone comprises land assessed as having a less than 1 in 1,000 annual probability of river or sea flooding (<0.1%); · Zone 2 “Medium probability of flooding” – This zone comprises land assessed as having between a 1 in 100 and 1 in 1,000 annual probability of river flooding (1% – 0.1%), or between a 1 in 200 and 1 in 1,000 annual probability of sea flooding (0.5% – 0.1%) in any year;
1 https://www.gov.uk/government/publications/national-planning-policy-framework--2 Ref: ISBN 978-1-5286-1033-9, CP48. [Accessed 16 Sept 2019 2 https://www.gov.uk/government/collections/planning-practice-guidance [Accessed 16 September 2019]
3 https://flood-map-for-planning.service.gov.uk/ 11 Flood Risk Assessment
· Zone 3a “High probability of flooding” – This zone comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%), or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year; · Zone 3b “Functional floodplain” – A sub-part of Zone 3, this zone comprises land where water has to flow or be stored in times of flood. This zone is not normally included within the national Flood Map for Planning but is defined by the Local Planning Authority as part of a SFRA.
3.2.1.2 The NPPF requires that developers consider not just the flood risk to the development but also the impact that the development might have on flood risk elsewhere. As well as main rivers and the sea, it is also necessary to consider flood risk from other sources, including surface water, groundwater, ordinary watercourses, artificial drainage systems, canals and reservoirs.
3.2.1.3 The Planning Practice Guidance describes that Local Planning Authorities should identify in their SFRAs areas of functional floodplain (or Flood Zone 3b), in agreement with the Environment Agency.
3.2.1.4 The SFRA for Sedgemoor District was undertaken in 2008 and updated in 2015. The maps produced for this assessment have been used to determine the separation of Flood Zone 3 into Flood Zone 3a and Flood Zone 3b.
3.2.1.5 The maps published alongside the SFRA4 indicate that all the TWAO order limits are located in Flood Zone 3a with the exception of works required at the tidal barrier itself which fall under Flood Zone 3b. Works to the existing flood defences are considered to fall within Flood Zone 3a as the works are to the landward side of the defences.
3.2.2 Sequential Test
3.2.2.1 The aim of the Sequential Test is to ensure a strategic approach has been followed to steer new development to areas with the lowest probability of flooding. Only where there are no reasonably available sites in Flood Zones 1 or 2 should the suitability of sites in Flood Zone 3 be considered taking into account the flood risk vulnerability of land uses.
3.2.2.2 As flood defence infrastructure, the tidal barrier, operational buildings and downstream defences would not fulfil their purpose if placed in an area at lower risk of flooding. The site selection process was therefore necessarily confined to consideration of sites within Flood Zone 3b. Seven alternative barrier locations, all within Flood Zone 3b, were considered as part of the design process. Consideration was given to a number of technical, environmental, cost and funding criteria to establish the preferred location for the barrier.
3.2.2.3 The analysis showed that the costs of the barrier were sensitive to the width of the river because of the extensive foundations and cut-off walls required. Environmentally there are no strong drivers in relation to the ultimate location, so the minimum cost approach was adopted. The preferred barrier location is 2 km north of Bridgwater town centre between Express Park and the village of Chilton Trinity.
3.2.2.4 The fish and eel pass works are required to improve fish and eel passage at specific structures and therefore cannot be constructed at alternative, lower risk, sites. It is therefore necessary for these to be within Flood Zone 3b.
4 https://www.sedgemoor.gov.uk/sfra 12 Flood Risk Assessment
3.2.2.5 On this basis, there are no reasonably available sites at lower flood risk where the development could alternatively be located. The proposed scheme is therefore considered by the Environment Agency to pass the Sequential Test.
3.2.3 Vulnerability Classification
3.2.3.1 Table 3-1 illustrates the flood risk vulnerability categories and flood zone compatibility matrix for the UK5. The proposed tidal barrier is flood control infrastructure and as such is classified as Water-compatible development (PPG “Flooding & Coastal Change” paragraph 066 and Table 2). The fish and eel pass works are nature conservation and biodiversity measures and are also classified as Water-compatible development. The proposed scheme is therefore considered appropriate for all flood zones and there is no requirement for it to satisfy the Exception Test.
Table 3-1: Flood risk vulnerability and flood zone compatibility Flood Risk Vulnerability Classification Flood Zones Essential Highly More Less Water- infrastructure vulnerable vulnerable vulnerable compatible
Zone 1 ✓ ✓ ✓ ✓ ✓
Exception Zone 2 ✓ ✓ ✓ ✓ Test required Exception Exception Zone 3a ✗ ✓ ✓ Test required Test required Exception Zone 3b ✗ ✗ ✗ ✓* Test required
3.2.3.2 *In Flood Zone 3b (functional floodplain) essential infrastructure that has to be there and has passed the Exception Test, and water-compatible uses, should be designed and constructed to; · remain operational and safe for users in times of flood; · results in no net loss of floodplain storage; · not impede water flows and not increase flood risk elsewhere. As Water-compatible development, the BTB Scheme will need to meet the above requirements.
5 Flood risk and coastal change, Paragraph: 067 Reference ID: 7-067-20140306. [online] Available at: https://www.gov.uk/guidance/flood-risk-and- coastal-change#flood-zone-and-flood-risk-tables [Accessed 16 Sept. 2019] 13 Flood Risk Assessment
3.3 Local Policies
3.3.1 The Flood and Water Management Act 2010 sets out a framework of action for flood risk management authorities (RMAs) to develop, maintain, apply and monitor a strategy for flood and coastal erosion risk management in order to reduce the risk of flooding and coastal erosion and its consequences. The National Strategy prepared by the Environment Agency provides general information on different types of flooding, which organisations are responsible for each and sets out principles for how flood risk should be managed.
3.3.2 The Flood and Water Management Act also introduced the requirement to produce Local Flood Risk Management Strategies. The Somerset Local Flood Risk Management Strategy was developed by Somerset County Council as Lead Local Flood Authority setting out the approach to management of local sources of flooding across the area. In relation to planning, flood risk management is fully considered in the local plan development process which [also] identifies flood defence infrastructure to which development needs to contribute.
3.3.3 Flood risk in Bridgwater has been appraised within the wider strategic context of managing flood risk around the Parrett Estuary.
3.3.4 Approved in 2010, the Parrett Estuary Flood Risk Management Strategy (the Strategy) recommends ‘holding the line’6 and upgrading defences over time, to mitigate for sea level rise as well as constructing a tidal barrier across the River Parrett. It was anticipated that the tidal barrier would be delivered once the condition of defences and sea level rise altered the risk to make a barrier more cost effective. The recommended standard of protection is 2% AEP for overtopping of rural flood defences and 0.5% AEP for urban flood defences. The Strategy recommended construction of a tidal barrier between 2030 and 2050 with a review of the Strategy every 5-10 years to monitor current flood risk and environmental change against the original baseline.
3.3.5 The flooding in early 2014 led to a partnership of key authorities producing a 20 Year Flood Action Plan to promote measures to reduce flood risk, increase resilience and promote business confidence and growth. Those involved included the Environment Agency, Sedgemoor District Council , Somerset Rivers Authority (SRA) and the Local Enterprise Partnership (LEP). The initial parameters and timescale for the Plan were set by the Secretary of State for Environment, Food and Rural Affairs, Owen Paterson. The Flood Action Plan included a requirement to accelerate the construction of a barrier at Bridgwater, with an objective to deliver it by 2024. The Plan has significant ministerial and local support.
3.3.6 Table 3-2 lists the overarching strategic documents that support the proposal for a tidal barrier.
6 Hold the Line is a coastal and estuarine policy option which maintains the line of coastal/tidal defence in its existing location. 14 Flood Risk Assessment
Table 3-2: Overarching strategic documents Strategies National Plans Local Plans Defra National Flood and Environment Agency Somerset Levels & Moors Coastal Erosion Risk Corporate Plan 2014 to Flood Action Plan (2014). Management Strategy. 2016. Sedgemoor District Council, Parrett Estuary FRM Environment Agency Flood Local Plan 2011-2032. Strategy (2010). and Coastal Erosion Risk Sedgemoor District Council, Sedgemoor District Council, Management Investment Core Strategy (2013). Programme 2015 to 2021. Green Infrastructure Plan North Devon and Somerset (2011) South West River Basin SMP (2010). Bridgwater Vision (2015). District Flood Risk Management Plan 2015- Sedgemoor District Council 2021 (2016). Strategic Flood Risk Assessment Level 1 (2015) Sedgemoor District Council Strategic Flood Risk Assessment Level 2 (2009) and Level 2 Addendum Report (2019)
3.3.7 The information above demonstrates that the construction of the BTB Scheme is supported by local and national flood risk policy.
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4. Assessment of Flood Risk
4.1 Hydraulic modelling
4.1.1 The 1D-2D model developed for the appraisal has been used to provide evidence to support the FRA. The existing 10m 2D grid resolution is considered acceptable for FRA purposes due to the size and topography of the floodplain. The model has been verified against data from historical flood events and found to have good agreement, increasing the confidence in the model outputs. Further information on model development and verification can be found in the Bridgwater Tidal Barrier Appraisal Hydraulic Modelling Report, Jacobs, 2019.
4.1.2 Hydraulic modelling has already been developed to support the scheme business case assessment, as follows:
4.1.3 1D-2D modelling of the River Parrett which has been updated from the existing 1D Somerset Levels and Moors model as part of this project. The model is a Flood Modeller-TUFLOW model with a 10m 2D grid resolution. The model has been verified against historic flood events and sensitivity testing has been undertaken on key model parameters. Simulations have been completed of the baseline (pre-scheme) conditions and with the barrier and associated flood defence improvements in place. The model includes a tidal downstream boundary and steady-state fluvial inflows. The model uses the same “upper bound fluvial flow” for all simulations as the fluvial flow reaching the barrier is known to be limited by channel capacity and upstream flood risk management infrastructure.
4.1.4 1D-2D modelling of a range of defence failure scenarios (breaches of existing raised defences and barrier failing closed) to determine the residual flood risk to nearby receptors.
4.1.5 Simulations of the present-day configuration of the River Parrett, without the barrier, have been performed to understand the baseline flood risk at the time of the barrier commissioning (2024) and in the future (2125).
4.1.6 The dimensions of the barrier included in the model are based on the current outline design for the scheme. This includes two barrier openings each 15m wide. Further information on the representation of the barrier and downstream defences in the model can be found in the Bridgwater Tidal Barrier Appraisal Hydraulic Modelling Report, Jacobs, 2019. The numerous outfalls which allow water from River Parrett floodplain back into the channel have not been included in the model, giving a conservative representation of flood risk.
4.2 Climate change
4.2.1 The NPPF sets out a requirement for new development to demonstrate that it is safe from flood risk for the lifetime of the development. The tidal barrier has a design life of 100 years. It is expected to be completed and operational in 2025 therefore the proposed epoch for consideration of climate change is up to 2125.
4.2.2 The tidal barrier has been designed to provide 0.5% Annual Exceedance Probability (AEP) (1 in 200-year) Standard of Protection (SoP) in 2125 with allowance for climate change (Upper End estimate7). A high standard of protection was adopted due to the importance of the infrastructure and to allow provision for extending the life of the structure at the end of the 100-year design life. The upper allowance for climate change was adopted on a precautionary basis given the significant uncertainty in future sea level rise.
7 Refer to Table 5 of Adapting to Climate Change: Advice for Flood and Coastal Erosion Risk Management Authorities (Environment Agency, 2016). Note that the “Adapting to climate change…” allowances have been used for design of the scheme as required for projects seeking government flood and coastal erosion risk management grant in aid (FCRM GiA) funding. 16 Flood Risk Assessment
4.2.3 Flood defences adjacent to the tidal barrier and defences around Chilton Trinity, Combwich, and Pawlett have been designed to 0.5% AEP (1 in 200-year) SoP in 2055 with allowance for ‘Change factor’ estimate (medium emission 95% projection)8 sea level rise. The SoP for the downstream defences should ensure properties are outside Flood Zone 3. The 2055 design life reflects the likely need to adapt the defences over time to address expected settlement and changes in sea level due to climate change. It is expected the defences will be modified to extend their design life beyond 2055. The assessment of flood risk for 2125 included in this FRA is based on the proposed bank levels being delivered under the BTB Scheme only, and does not include future defence raising, as these are the works for which planning permission is being sought.
4.2.4 The Flood Risk Assessments: Climate Change Allowances guidance9 have been used to determine the allowances for the FRA document.
4.2.5 As described in Section 4.1, the fluvial flow reaching the barrier is limited by channel capacity and upstream flood risk management infrastructure. The climate change simulations have therefore been run with a variation to the tidal levels only and no variation to the fluvial flows. The upstream fish and eel pass works will not impact on fluvial flows in the River Parrett.
4.2.6 The sea level rise allowances in Table 4-1, below, have been applied for future scenarios.
Table 4-1: Sea level rises for each epoch in millimetres (mm) per year.
Area 1990 to 2025 2026 to 2055 2056 to 2085 2086 to 2115 South West 3.5 mm/year 8 mm/year 11.5 mm/year 14.5 mm/year
4.2.7 As no allowance is provided in the guidance for beyond 2115 it is assumed that the 14.5mm/year allowance can be used up to 2125 (the expected design life of the barrier). Table 4-2 gives the total sea level rise under these allowances compared to 2008, which is the base year for the tidal boundary conditions.
Table 4-2: Allowances applied in the hydraulic model for increase in relative sea levels between 2008 and future epochs
Increase in relative sea level from 2008 (m) Area 2024 2125 South West 0.056 1.225
4.2.8 Consideration has been given to the use of UKCP09 H++ allowances for sea level rise. H++ is a low probability high end climate change scenario which considers effects of climate change which are beyond the likely range but still within physical plausibility. The Flood Risk Assessments: Climate Change Allowances guidance states:
“The high++ allowances will only apply in assessments for developments that are very sensitive to flood risk and with lifetimes beyond the end of the century. For example, infrastructure projects or developments that significantly change existing settlement patterns. This includes urban extensions and new settlements.”
8 Refer to Table 5 of Adapting to Climate Change: Advice for Flood and Coastal Erosion Risk Management Authorities (Environment Agency, 2016). Note that the “Adapting to climate change…” allowances have been used for design of the scheme as required for projects seeking government flood and coastal erosion risk management grant in aid (FCRM GiA) funding. This recommends use of the medium emission 95% projection scenario from UKCP09 to provide the ‘change factor’ allowance. Medium emission refers to the “medium” scenario (A1B) for greenhouse gas emissions used to generate this particular sea level rise scenario. 95% projection refers the 95% confidence interval for this projection. 9 https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances 17 Flood Risk Assessment
4.2.9 One of the purposes of the tidal barrier is to facilitate future development in Bridgwater and it is therefore possible that the proposed scheme could influence development patterns in the area. Additionally, the proposed scheme has a lifetime beyond the end of the century. The H++ scenario should therefore be considered in the FRA. However, this degree of sea level rise is above the proposed SoP for the scheme and it is therefore expected that the tidal barrier and defences would be exceeded in such an event. In this situation flood levels would be predominantly influenced by the still water extreme tide level. The flood risk to receptors in an event of this magnitude will therefore be informed by comparison of the extreme surge tides with local topography – hydraulic modelling is not considered necessary to undertake this analysis. The H++ allowances are shown in Table 4-3.
Table 4-3: H++ sea level rise allowances
Sea level rise Sea level rise Sea level rise Sea level rise Change to relative mm/yr up to mm/yr up to mm/yr up to mm/yr up to mean sea level 2025 2050 2080 2125 H++ scenario 6 12.5 24 33
4.2.10 Since publication of the ‘Flood risk assessments: climate change allowances’ in 2016, the UKCP18 climate change projections have been released. Climate change allowances have not yet been updated to reflect these new projections. Interim guidance provided by the Environment Agency states that, until the guidance has been updated, the existing 2016 allowances should be used for planning decision making where “development proposals are well advanced”. This is the case for the BTB Scheme therefore the projections provided in UKCP18 have not been considered further in this FRA.
4.3 Flood risk from the sea
4.3.1 Tidal flooding is typically caused by high tides coinciding with a storm system that generates a storm surge resulting in raised water levels. It is projected that the risk of tidal flooding will rise in future years due to rising sea levels and increased severity and number of extreme weather events as a result of climate change.. The projected climate change scenarios used for this assessment are discussed in Section 4.2.
4.3.2 The risk of flooding from tidal sources has been assessed using the Environment Agency’s Flood Map for Planning and hydraulic modelling, as summarised in Section 4.1. Table 4-4 lists the scenarios that have been modelled for this assessment. The 0.5% AEP event has been used in this assessment as this is considered the relevant extreme flood event for tidal flood risk. Further scenarios have also been considered in order to assess the residual risk from the scheme – these are discussed separately in Section 4.3.6.
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Table 4-4: Hydraulic model runs for assessment of tidal flood risk
Scenario Geometry Fluvial/tidal boundaries Climate no. change reference year
1 As existing “Upper bound fluvial flow” and 2024 0.5% AEP tidal 2 As existing “Upper bound fluvial flow” and 2125 0.5% AEP tidal 3 Post-scheme (normal “Upper bound fluvial flow” and 2024 operation of barrier) 0.5% AEP tidal 4 Post-scheme (normal “Upper bound fluvial flow” and 2125 operation of barrier) 0.5% AEP tidal
4.3.3 Baseline tidal flood risk
4.3.3.1 This section considers the current and future risk of tidal flooding without the BTB Scheme in place in order to establish baseline conditions with which to compare the scheme.
Existing flood risk
4.3.3.2 The Environment Agency’s Flood Map for Planning10 shows that the tidal barrier, operational building and downstream defences and much of Bridgwater lies within Flood Zone 3 (high probability of flooding). In this area this is due to the high risk of tidal flooding, with a greater than 0.5% annual probability of flooding (0.5% AEP), not accounting for defences. Furthermore, hydraulic modelling shows there are 11,300 homes and 1,500 businesses in Bridgwater and key infrastructure close to the town at risk of tidal flooding.
4.3.3.3 Bridgwater and other settlements downstream on the River Parrett are shown as “Areas benefitting from flood defences” in the Flood Map for Planning as they are defended in the present day 0.5% AEP event, although the residual risk of breach remains.
4.3.3.4 An extract of the Environment Agency’s Flood Map for Planning for the study area is included in drawing 2519 in Appendix A.
4.3.3.5 In November 2011, a major section of river wall supporting flood defences in Bridgwater collapsed without warning (Figure 4-1). This left part of Bridgwater vulnerable to tidal flooding until the wall reconstruction was completed in June 2013.
10 https://flood-map-for-planning.service.gov.uk 19 Flood Risk Assessment
Figure 4-1: Wall collapse at West Quay, Bridgwater, 2011.
4.3.3.6 On 3rd January 2014 a tidal surge event in the Severn Estuary resulted in elevated water levels in the River Parrett within 0.15-0.20m of the crest level of defences in Bridgwater (Figure 4-2). The event has an estimated return period of 5-10% AEP.
Figure 4-2: High Tide in Bridgwater, 8am 3 January 2014.
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4.3.3.7 The existing flood defences are generally directly adjacent to the river channel (defined for the purposes of the BTB Scheme as “primary flood defences”) and consist of mainly flood embankments (approximately 17 km) between the mouth of the Parrett Estuary and Bridgwater, hard defences through Bridgwater and flood embankments upstream of the town. Existing primary flood defences are typically 1 to 2 m high. These embankments are monitored and recorded within the Environment Agency’s AIMS database. Flood risk is a function of the relationship between extreme water levels in the Severn Estuary propagating into the River Parrett, existing flood defence levels and ground elevation.
4.3.3.8 The Environment Agency maintains the flood defences on the River Parrett, including within Bridgwater, at the proposed tidal barrier location, and downstream. The condition of the flood defence assets was assessed after the failure of the flood wall in Bridgwater in 2011. This indicated that the remaining defences were in good or fair condition, with little evidence that the condition would impair the performance of the defences. The assessment noted that the previous condition surveys did not highlight the potential failure mechanism of the river wall in 2011.
4.3.3.9 The existing primary flood defences have been successful in protecting Bridgwater from tidal flooding over the last 30 years. However, recent events and studies have indicated that the risk of flooding is increasing and that there are uncertainties around the factors determining the level of risk and the condition of existing primary flood defences. Table 4-5 provides a summary of the available historic data for extreme events, structural failures and significant maintenance events.
Table 4-5: Chronology of recorded tidal flood events on the Parrett (Bridgwater and downstream)
Date Description 09/01/1936 Main river flooded due to channel capacity exceeded (potential tidal flooding) 28/10/1960 Main river flooded due to channel capacity exceeded (potential tidal flooding) 11/01/1974 Main river flooded due to channel capacity exceeded (potential tidal flooding) 27/10/1978 Main river flooded due to channel capacity exceeded (potential tidal flooding) 18/03/1980 High tidal level leading to overtopping and flooding 13/12/1981 High tidal level leading to operational failure and breaching of defences and flooding 26/02/1990 High tidal level leading to overtopping and flooding 20/01/1995 Main river flooded due to channel capacity exceeded leading to flooding including at Westonzoyland
26/03/2009 Overtopping of defences on the main river leading to flooding 2010 Urgent Works (identified in PEFRMS): Cannington Outfalls 05/11/2011 Wall failure within Bridgwater town centre area 20/11/2012 Overtopping of defences on the main river leading to flooding 09/01/2014 Overtopping of defences on the main river leading to flooding 2015 Wall repairs in Bridgwater (cracks repointed and sealed) 2015 Works to 100 m of embankment past Pims Clyce 2015 Works to 100 m of embankment, Parrett left bank at Chilton Trinity Waters 2016 Cannington Bends bank works 2015 Stallington’s Clyce contingency plan trench sheet
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2017 Stallington’s Clyce bank repairs 2017 Tucketts Clyce bank and clyce re-build 2018 Wall repairs in Bridgwater to address observed leaks 2018 Additional remedial works to 105 m section 2018 Additional remedial works to 493 m section 2019 Planned additional remedial works to 270 m section 2019 Planned additional remedial works to 458 m section
1 in 200-year (0.5% AEP) 2024
4.3.3.10 The 0.5% AEP 2024 event has been modelled to determine baseline conditions immediately following construction of the tidal barrier, for comparison with the “with scheme” scenario. Flood depth mapping for this scenario is shown in drawing 2500 in Appendix A.
4.3.3.11 There is significant flood risk in the 0.5%AEP event to much of the land in the floodplain behind the existing tidal defences. The current defences are overtopped in a number of locations and the flood waters spread out over large areas. The populations of Combwich and Chilton Trinity have the largest number of properties affected. With some properties in Bridgwater at Blake Gardens also at risk from overtopping of existing defences on the left bank of the River Parrett.
4.3.3.12 In Combwich, water overtops the flood defences along the left bank of the River Parrett upstream of the Harbour. The parts of the village to the north of Brookside Road are outside the flood risk extent with the exception of houses around the Nursery Close, Church Hill junctions. The area at flood risk extends to the east via a flow route along the South Main Brook.
4.3.3.13 Flood risk at Pawlett during the 0.5%AEP is limited to land to the south of the village; land adjacent to River Road and South of Vicarage Road.
4.3.3.14 Parts of Chilton Trinity lie in a flow route between the lower lying area to the north where flooding is extensive and lower ground to the south where flood water passing through the village accumulates. The modelling has identified an additional route of floodwater from north to south located to the west of the village. These flow paths cause significant flood risk around the northern fringes of Bridgwater affecting the sports and leisure centre, the park and encroaching on properties south of the Western Way.
Future flood risk
4.3.3.15 The 0.5% AEP 2125 event has been modelled to determine baseline conditions at the time of the end of the barrier’s design life, for comparison with the 2125 “with scheme” scenario. This allows for the assessment of any potential future impacts of the scheme. The model simulations for assessing future flood risk from the sea at Bridgwater have been carried out in accordance with the climate change guidance explained further in Section 4.2 Flood depth mapping for this scenario is shown in drawing 2501 in Appendix A.
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1 in 200-year (0.5% AEP) 2125
4.3.3.16 In the 0.5% AEP scenario for the year 2125 there is extensive flooding in Bridgwater and the surrounding area. As the flood defences in Bridgwater are overtopped, the risk of infrastructure flooding increases for the A38/M5 road corridor, mainline rail link between the South West and Bristol, Express Park, Bridgwater Police Centre and Hinkley Point C construction park and ride. Overtopping also increases the risk of potential defence failure, leading to more significant flooding risk to the town as well as adverse impact to the River Parrett navigation channel and geomorphology.
4.3.3.17 The risk also significantly increases, compared to 2024, for the more rural areas including Combwich and Pawlett from overtopping of existing defences along the estuary and River Parrett due to the increase in tidal levels. Flooding is more extensive and of greater depth in this future scenario.
4.3.4 Development impact on flood risk
1 in 200-year (0.5% AEP) 2024 with scheme
4.3.4.1 A comparison of flood level and extent between the “with scheme” and baseline scenarios for the 0.5% AEP 2024 tidal event is shown in drawing 2503 in Appendix A. The “with scheme” model runs assume the tidal barrier closes for tides above a trigger level. Residual risks from structure failure are considered separately in Section 4.3.6.
4.3.4.2 Operation of the barrier, improvement to existing primary defences and construction of the secondary defences as part of the Scheme removes the areas of tidal flood risk around Bridgwater, Chilton Trinity, Combwich and Pawlett shown in the baseline scenario in this event. The tidal barrier limits the influence of the tidal levels in the River Parrett upstream, the improvements to the primary defences prevent overtopping during the 0.5%AEP event and the new secondary defences act to prevent flood waters spreading out further south.
4.3.4.3 The comparison of flood level shows a decrease in flood level to the south east of the proposed secondary defence at Combwich. This is due to reduced overtopping of the primary defences resulting from crest raising. Other impacts on flood level are typically small in area and magnitude. Areas of decreased flood level are shown in small areas of the left bank floodplain opposite Dunball and opposite Pawlett Hams. There is a small strip of increased flood level adjacent to the proposed flood defences at Chilton Trinity but this only affects a small area of farmland which is already inundated in the baseline and the magnitude of the increase is small (<0.2m).
4.3.4.4 There are also a few small areas of increased flood extent north of Perry Green. These are as a result of shallow overland flows (<0.1m) which result in small areas of ponding of up to 0.3m in low spots in the topography. Experience of conditions on the ground suggests that the small outfalls which drain water from the floodplain back to the River Parrett would allow the water to drain away so the extent would be smaller. small
1 in 200-year (0.5% AEP) 2125 with scheme
4.3.4.5 A comparison of flood level and extent between the “with scheme” and baseline scenarios for the 0.5% AEP 2125 tidal event is shown in drawing 2504 in Appendix A. The “with scheme” model runs assume the tidal barrier closes for tides above a trigger level. Residual risks from structure failure are considered separately in Section 4.3.6.
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4.3.4.6 This scenario considers a tide level that exceeds the design standard of the Scheme when constructed (0.5% AEP to up to 2055). The primary and secondary defences are overtopped by the tide levels. However, the majority of areas behind these defences still benefit from a reduction in flood level, caused by reduced overtopping over the raised crest level. A large area of Bridgwater that was shown to be at tidal flood risk in the baseline is shown not to flood in the “with scheme” scenario, offering a significant reduction in flood risk to properties and infrastructure.
4.3.4.7 There are two areas of floodplain where a small increase in flood level is shown in the “with scheme” scenario, compared to the baseline, around Stockland and Pawlett Hams. This mostly affects farmland although some properties are also affected. These are areas which were already shown to be inundated in the baseline but where there would be a minimal (<0.2m) increase in flood level in the with scheme scenario compared to the baseline. However, it is expected that this adverse impact will be mitigated by the Environment Agency’s planned strategy to raise tidal defences in the area which do not form a part of the BTB Scheme so have not been included in the modelling.
4.3.5 Fish and eel pass sites
4.3.5.1 All of the proposed fish and eel pass sites are located upstream of the tidal limits of the Parrett and its tributaries. Therefore the risk of tidal flooding is not considered to affect these sites.
4.3.6 Residual risks
4.3.6.1 This section discusses the residual risks from tidal flooding that will be present with the BTB Scheme in place. All flood defence schemes pose a residual risk of flooding where they operate in conditions beyond those for which they have been designed. However, such conditions are typically low in probability, meaning that the magnitude of the risk is low. Mitigation measures can be put in place to further reduce these risks.
Overtopping of defences
4.3.6.2 With all flood defence schemes there is a residual risk of overtopping where water levels are higher than those for which the defence was designed. For the purposes of this assessment, a 0.1% AEP 2024 event has been considered. This represents an extreme tidal event with a low probability meaning that the annual chance of occurrence is very small. A 0.1% AEP 2125 event has not been considered as the combined uncertainty of the 0.1% AEP flood estimate and the climate change projections to 2125 mean that there would be limited confidence in the level of risk shown. Furthermore, an event of this magnitude is unlikely to be represented realistically within the hydraulic model.
4.3.6.3 It can be seen from the flood depth mapping for the 0.1% AEP 2024 scenario (see drawing 2502 in Appendix A) no overtopping of the proposed defences is shown. This is because the water levels estimated in this event are typically lower than the crest level of the flood defences which include a residual uncertainty allowance above the design water level. . However, it should be noted that there is considerable uncertainty in the estimation of a 0.1% AEP tide level.
4.3.6.4 This analysis indicates that the residual risk of overtopping of defences is low, although this will significantly increase over time with projected sea level rise. Such risks will be mitigated by future improvements to flood defences following an adaptive approach.
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H++ scenario
4.3.6.5 The Flood Risk Assessment Climate Change guidance indicates that for developments with lifetime beyond the end of the century (e.g. large infrastructure projects or developments that significantly change existing settlement patterns) should also consider the High ++ (H++) climate change allowances. Whilst the scheme has not been designed to offer protection in such an extreme event, consideration of this scenario provides information on the residual risk relating to extreme climate change.
4.3.6.6 A simple analysis has been undertaken by comparing the 0.5% AEP tide level at Hinkley Point, projected to 2125 under the H++ scenario (10.64mAOD), with ground levels taken from LIDAR data.
4.3.6.7 It can be seen from the analysis that this event would be likely to inundate a large proportion of Bridgwater. Whilst the proposed defences may act to limit overtopping to some extent, the water levels would be significantly higher increasing the risk of defence breach. The H++ scenario sea level estimate in 2125 is 1.4m higher than the upper end estimate. To maintain the design SoP to 2125 in this scenario, the barrier gates and approximately 8km of flood defences would need to be increased in height. The design of the BTB scheme has taken this into consideration by comprising sectional gates to make future adaptations easier and designing the barrier foundations to resist the H++ scenario.
4.3.6.8 At present there is no strategy in place to defend Bridgwater and the surrounding area in such an event and it would be significantly beyond the scenario for which the Scheme is designed. However, the significant depth of water means that the proposed scheme would not increase flood risk compared to pre-scheme arrangement. This is because flooding would be predominantly driven by the tidal water level.
Breach of defences
4.3.6.9 All raised defences have a residual risk of breach. There are a number of possible mechanisms for defence breach which are dependent on defence design, defence condition and other site-specific factors. The potential risks associated with breach of the proposed defences have been assessed through hydraulic modelling of a number of different breach scenarios. These scenarios have been selected based on guidance contained in the Environment Agency’s Breach of Defences guidance with a location selected on each defence reach that is expected to have the largest potential adverse impact. The breach scenarios are summarised in Table 4-6. All breach scenarios have been run using a 0.5% AEP 2024 scenario.
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Table 4-6: Breach scenarios
Scenario Breach Breach Breach Water level Defence ref. invert width (m) duration at which breached level (time taken breach (mAOD) to repair) commences (hrs) (mAOD)
Breach 1 6.5 50 30 8.0 Chilton Trinity primary Breach 2 6.2 50 30 7.4 Chilton Trinity secondary Breach 3 5.5 50 30 8.0 Pawlett primary Breach 4 6.0 50 30 7.5 Pawlett secondary Breach 5 6.6 50 30 8.0 Combwich primary Breach 6 6.2 50 30 7.1 Combwich secondary
4.3.6.10 In a breach scenario the flood risk arises both from the depth of water and its velocity – breach events are characterised by fast flowing water, potentially with limited warning. For this reason, the risk from breach has been assessed using the Hazard to People Classification (FD2320). This assigns a hazard rating based on the depth and velocity of flood water. The outputs from this can be found in drawings 2507 to 2512 in Appendix A.
4.3.6.11 The levels of hazard are defined as follows: · Hazard Rating <0.75 - Very low hazard – Caution · Hazard Rating 0.75 to 1.25 – Danger for some – includes children, the elderly and the infirm · Hazard Rating 1.25 to 2.0 – Danger for most – includes the general public · Hazard Rating >2.0 – Danger for all – includes the emergency services
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4.3.6.12 In the “Breach 1” scenario there are several areas shown as “Danger for all” – areas adjacent to the breach, where the water has a high velocity, and within several ponds and ditches where there is a high hazard due to flood depth. There is a large area behind the defence classified as “Danger for most” with additional peripheral areas which are “Danger for some”. Properties in and near Chilton Trinity and at the northern edge of Bridgwater are shown to be at risk.
4.3.6.13 In the “Breach 2” scenario there is an overland flow route shown through the Lower Lakes site but this is shown to be “Very low hazard”. This is because localised higher ground limits flow through the breach. There are some areas shown as “Danger for some” and “Danger for most” but these are areas where there is already water in the existing ponds and drainage ditches at the site. In the “Breach 3” scenario there is a large area of farmland behind the defences which is shown as “Danger for all”. This also affects roads, including the Bristol Road, and some nearby properties. A wider area spreading towards Dunball and to the Huntspill River is shown as “Danger for most”, with land beyond the Huntspill generally “Danger for some” or “Very low hazard”.
4.3.6.14 In the “Breach 4” scenario the majority of areas inundated by the breach are shown as “Very low hazard” and the extent of flooding is fairly small. There are areas to the rear of Brickyard Farm which are shown as “Danger for most” and “Danger for some” but this is typically limited to areas of deep water where there are already ponds.
4.3.6.15 In the “Breach 5” scenario there are areas classified as “Danger for all” immediately adjacent to the breach location and within existing watercourses and ponds where there is deep water. There are large areas shown as “Danger for most” behind the breached defence. This mostly affects farmland but some properties are shown to be at risk in Combwich along Estuary Park. Water from the breach spreads to the west of Withycombe Hill via the South Main Brook channel but the areas shown as “Danger for most” and “Danger for some” are mostly low-lying farmland.
4.3.6.16 In the “Breach 6” scenario flooding resulting from the breach affects mostly farmland and public open space with no properties or major roads shown to be at risk. There are some areas of “Danger for most” but the majority of the flooded area has a classification of “Danger for some” or “Very low hazard”.
4.3.6.17 The risk of breach will be mitigated throughout the design process through selection of appropriate geometry and specification of appropriate materials to construct the flood defences. This design will then be implemented by the contractor during construction. Following completion of the flood defences, the risk will be further mitigated through regular inspection and maintenance. This combination of measures means that whilst the hazard arising from flooding is moderate to high, the probability and thus the risk can be mitigated to acceptable levels. Further mitigation measures to reduce the hazard could be considered at detailed design stage. An emergency response plan will also be put in place to minimise risks in the unlikely event of defence breach.
Barrier failure – failed open
4.3.6.18 There is a residual risk that the barrier could fail in such a way that it is unable to be closed during a tidal flood event. This scenario has been modelled by preventing the barrier from closing during the model run. This has been tested on a 0.5% AEP 2024 event. In this scenario the downstream defences would continue to provide protection but water would be able to flow through the barrier openings.
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4.3.6.19 Model results show that, in this event, no tidal flooding propagates upstream of the barrier. However, it is expected that in future the risk of flooding upstream of the barrier would increase if it failed open.
4.4 Flood risk from rivers
4.4.1 Fluvial flooding typically occurs when a river’s capacity is exceeded, and the excess water overtops the river banks. It can also occur when the watercourse has a high level downstream, perhaps due to structures or tidal blocking, thus limiting conveyance. This creates a back-up of water which again may result in water overtopping banks.
4.4.2 The risk of flooding from rivers (fluvial flooding) has been assessed using hydraulic modelling, as summarised in Section Error! Reference source not found.. Table 4-7 lists the scenarios that have been modelled for this assessment. Usually a 1% AEP event would be used to assess the flood risk from fluvial flows. However, the fluvial flow in the River Parrett into Bridgwater does not increase continuously with reducing AEP. This is because the fluvial system upstream of Bridgwater is heavily modified with a number of structures spilling water into adjacent watercourses and land during high flows, limiting the flow through Bridgwater. For example, flood water is rerouted into the River Sowy and King’s Sedgemoor Drain. Further information on this can be found in the hydraulic modelling report for the scheme11.. As a result, the maximum fluvial flow in the river is between 66m3/s to 70m3/s. The hydraulic modelling has therefore adopted an “upper bound fluvial flow” of 70m3/s for assessment of fluvial flood risk.
4.4.3 Within Bridgwater, water levels in the River Parrett are heavily influenced by tide levels, with tidal flood risk (discussed in Section 4.3) being the predominant influence on flooding. In order to isolate the potential effects of the scheme on fluvial flows from the tidal influence, a lower tidal boundary (50% AEP event) has been used for the fluvial flood risk runs.
4.4.4 With climate change, it is expected that the water level management structures upstream of Bridgwater would continue to limit flows in the River Parrett meaning that the “upper bound fluvial flow” is unlikely to significantly change. This has therefore been retained for the 2125 model runs, although the tide level has been increased in accordance with the climate change guidance.
Table 4-7: Hydraulic model runs for assessment of fluvial flood risk
Scenario Geometry Fluvial/tidal Climate no. boundaries change reference year
1 As existing “Upper bound fluvial flow” 2024 and 50% AEP tidal 2 As existing “Upper bound fluvial flow” 2125 and 50% AEP tidal 3 Post-scheme (normal “Upper bound fluvial flow” 2024 operation of barrier) and 50% AEP tidal 4 Post-scheme (normal “Upper bound fluvial flow” 2125 operation of barrier) and 50% AEP tidal
11 Jacobs, 2019. Bridgwater Tidal Barrier Appraisal Hydraulic Modelling report (Doc ref: ENVIMSW002039-CH2-FEV-SW-RP-HY-00105) 28 Flood Risk Assessment
4.4.5 Further scenarios have also been considered in order to assess the residual risk from the scheme – these are discussed separately in Section 4.4.9.
4.4.6 Baseline flood risk
2024 fluvial event (“upper bound fluvial flow” with 50% AEP tide)
4.4.6.1 In this event, no fluvial flooding out of the river channel is shown (see drawing 2513 in Appendix A). The in-channel levels are shown in Figure 4-3. This shows that in channel water levels in this event are typically considerably lower than adjacent bank levels in the majority of locations.
Figure 4-3: In-channel water levels in “upper bound fluvial flow” 2024 event
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2125 fluvial event (“upper bound fluvial flow” with 50% AEP tide)
4.4.6.2 In the 2125 “upper bound fluvial flow” event there is inundation from the floodplain resulting from increased tide levels at the downstream boundary (see drawing 2514 in Appendix A). This can also be seen in the Figure 4-4 where in-channel water levels are shown to exceed bank levels downstream of the tidal barrier. Within Bridgwater, water levels are at around bank level in this event resulting in some overtopping of banks at low spots.
Figure 4-4: In-channel water levels in “upper bound fluvial flow” 2125 event
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4.4.7 Development impact on flood risk
4.4.7.1 When the barrier is closed, water will be prevented from flowing out of the River Parrett until the barrier reopens on the receding tide. A comparison between in-channel flood levels in the baseline and “with scheme” scenarios for the 2024 “upper bound fluvial flow” event is shown in Figure 4-5. It can be seen from Figure 4-5 that implementation of the scheme significantly reduces flood levels throughout Bridgwater in this event. This is due to the tidal influence on the water levels being removed. In the 2125 scenario (Figure 4-6) this reduction in water levels in the River Parrett can also be seen although there is a small localised increase immediately downstream of the barrier.
4.4.7.2 At times when the barrier is closed, these results indicate that the BTB Scheme reduces fluvial water levels upstream as the tidal influence is removed. The scheme is therefore considered to have a beneficial impact on fluvial flood risk.
Figure 4-5: Comparison of in-channel water levels in “upper bound fluvial flow” 2024 event (baseline and with scheme)
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Figure 4-6: Comparison of in-channel water levels in “upper bound fluvial flow” 2125 event (baseline and with scheme)
4.4.8 Fish and eel pass sites
4.4.8.1 All of the proposed fish and eel pass improvements except at Bradford-on-Tone, are located on existing structures within watercourses. The proposals are typically for minor works which are unlikely to have significant impacts on flood risk. A fluvial flood risk assessment for each site is shown in Table 4-8. This assessment has been made based on the Environment Agency’s Flood Map for Planning.
Table 4-8: Fluvial flood risk assessment for fish pass sites
Site Watercourse Flood risk assessment affected
Ablake Weir River Yeo Works proposed are to Ablake Weir structure, with proposed works to replace the existing reinforced concrete sidewall with a new structure incorporating a gravity fed eel pass. The structure is surrounded by a large floodplain, indicating bypassing of the structure during a flood event, with a flood storage area immediately upstream. It is anticipated that the sidewall will be replaced with a structure of similar dimensions to existing meaning that there should be no significant impacts on flood risk. Flow through the eel pass is expected to be minimal compared to flood flows on the river and therefore is not expected to increase risk downstream.
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Site Watercourse Flood risk assessment affected
Ham Weir River Parrett Works proposed are to Ham Weir structure, with proposed works to attach eel stud tiles to the weir face and make minor modifications to the weir. The structure is surrounded by a large area of floodplain indicating bypassing of the weir in flood events. Modifications made to the weir are not expected to have a significant effect on flood risk as there is significant bypassing of the structure during a flood event. The type of modifications proposed are minor which means minimal effect on in-channel flows is expected.
Long Load River Yeo Works proposed are to Long Load Sluice structure, Sluice with proposed works to replace the existing reinforced concrete sidewall with a new structure incorporating a gravity fed eel pass. The structure is surrounded by a large area of floodplain indicating bypassing of the weir in flood events. It is anticipated that the sidewall will be replaced with a structure of similar dimensions to existing meaning that there should be no significant impacts on flood risk. Flow through the eel pass is expected to be minimal compared to flood flows on the river and therefore is not expected to increase risk downstream. Monk’s Leaze River Parrett/River Works proposed are to Monk’s Leaze Clyce structure, Clyce Sowy with proposed works to install a pumped eel bristle pass on the side wall of the existing sluice structure. The structure is surrounded by a large floodplain, indicating bypassing of the structure during a flood event, with a flood storage area immediately upstream. The dimensions of the proposed works are expected to be small so are not expected to have a significant effect on overall channel capacity. Furthermore, the large amount of bypassing of the structure in a flood event indicates that any works to the structure are not expected to have a significant impact on flood risk.
Midelney River Isle Works proposed are to the weir at Midelney Pumping Pumping Station, with proposed works to replace the existing Station reinforced concrete sidewall with a new structure incorporating a gravity fed eel pass. The River Isle splits into two channels at the structure with the left hand channel going to the pumping station and the right hand channel flowing over the weir. The channels rejoin immediately downstream of the structure. There is a large floodplain adjacent to the left bank of the watercourse at the structure, indicating bypassing in flood events, and a flood storage area in the right bank floodplain. It is anticipated that the sidewall will be replaced with a structure of similar dimensions to existing meaning that there should be no significant impacts on flood
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Site Watercourse Flood risk assessment affected
Midelney risk. Flow through the eel pass is expected to be Pumping minimal compared to flood flows on the river and Station cont’ therefore is not expected to increase risk downstream.
Thorney Mill River Parrett Works proposed are to the tilting weir structure at Sluices Thorney Mill Sluices, with works proposed to install an eel bristle trough fixed to the channel side wall and make minor modifications to the weir crest to fit the new pass. The River Parrett in the vicinity of the structure has raised defences on both sides although these are exceeded in the 1% AEP event (Flood Zone 3), with significant ponding in the floodplain. The dimensions of the proposed works are expected to be small so are not expected to have a significant effect on overall channel capacity. Witcombe Witcombe Bottom Main Works proposed are to the tilting weir structure at Bottom Tilting Drain/ River Yeo Witcombe Bottom, with works proposed to install an Weir eel bristle trough on the existing weir. The structure is surrounded by a large area of floodplain indicating bypassing of the weir in flood events. The dimensions of the proposed works are expected to be small so are not expected to have a significant effect on overall channel capacity. Furthermore, the large amount of bypassing of the structure in a flood event indicates that any works to the structure are not expected to have a significant impact on flood risk.
Bishop’s Hull River Tone Works proposed are to the weir at Bishop’s Hull Gauging Gauging Station, with works proposed to lower part of Station Weir the weir crest and install eel tiles on the existing river wall. The structure has a large area of floodplain on the left bank, indicating significant bypassing of the structure during flood events. The National River Flow Archive (NRFA) information for the site indicates that out-of- bank flow occurs before bankfull and during extreme flood events the weir is drowned by backwater from the downstream bridge resulting in floodplain inundation. This indicates that works to the weir are not expected to have a significant effect on flood risk. However, it is important to note that this gauging station forms an important part of the flood warning system for Taunton and work may be required to the gauging station rating following construction to re- calibrate the system.
Bradford-on- River Tone Works proposed are adjacent to the existing weir Tone Weir structure, within the left bank floodplain of the watercourse. The proposed works are to install a rock ramp fish pass which bypasses the structure. The Flood Map for Planning shows a large left bank floodplain in this area.
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Site Watercourse Flood risk assessment affected
Bradford-on- The proposals are not expected to have a significant Tone Weir cont’ effect on flood risk due to the small size of the rock ramp bypass channel compared to the overall floodplain.
Firepool Weir River Tone Works proposed are improvements to the existing fish pass at the site to replace the timber stop boards with new boards and fit a gravity eel pass to the side wall of the fish pass. There is a narrow corridor of open space floodplain adjacent to the structure but there are also urban areas of Taunton nearby which are at flood risk. As the proposed works are to the separate fish pass channel and not to the main channel of the watercourse, the proposals are not expected to have a significant impact on flood risk. French Weir River Tone Works proposed are improvements to the existing fish pass at the site to replace the timber stop boards with new boards, fit a gravity eel pass to the side wall of the fish pass and install a debris deflector at the fish pass exit. The structure is located within an open space floodplain area but is close to urban areas of Taunton, some of which are at flood risk. As the proposed works are to the separate fish pass channel and not to the main channel of the watercourse, the proposals are not expected to have a significant impact on flood risk. Longaller Weir River Tone Works proposed are to the existing weir structure with works proposed to install an eel board onto the weir face, make minor modifications to the weir crest and install a rock pre-barrage at the base of the existing fish pass. There is a large floodplain on either side of the structure, bounded by the railway line on the left bank floodplain and high ground on the right bank floodplain. There are several properties located within the floodplain downstream of the structure. The modifications to the weir are minor and not expected to have a significant effect on channel capacity. The works downstream of the weir are also not expected to have a significant effect on flood risk as the upstream weir controls water levels. Furthermore, the proposals are small in comparison to the size of the floodplain and are therefore not expected to have a significant effect on flood risk.
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4.4.9 Residual risks – failed closed
4.4.9.1 There is a residual risk of fluvial flooding if the tidal barrier were to become stuck in the gates closed position as this would mean water would be unable to drain out of the River Parrett on the receding tide. This scenario has been tested on both the 2024 and 2125 “upper bound fluvial flow” scenarios by allowing the model to run with the tidal barrier stuck closed until the upstream water level reaches equilibrium. The results of this are shown in Figure 4-7 and 4- 8.
Figure 4-7: Comparison of in-channel water levels in “upper bound fluvial flow” 2024 failed closed scenario
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Figure 4-8: Comparison of in-channel water levels in “upper bound fluvial flow” 2125 failed closed scenario
4.4.9.2 The results show that whilst the tidal barrier failing closed does result in elevated upstream water levels compared to the gates opening on the receding tide, these are lower than the baseline flood levels for this event throughout Bridgwater. This is due to the water levels being regulated by upstream infrastructure on the River Parrett. This is because various structures upstream of Bridgwater allow flows from the Parrett to spill into adjacent systems. Further information on this can be found in the hydraulic modelling report12.
4.4.9.3 This indicates that the residual risk of barrier failure would not have a significant impact on fluvial flood risk to Bridgwater.
4.4.10 Temporary bypass channel
4.4.10.1 During construction a 20m wide bypass channel will be constructed at the barrier site to allow navigation on the River Parrett and maintain flows around the temporary cofferdam. The final design for the bypass channel will need to be agreed and an application made for a Flood Risk Activity Permit under the Environmental Permitting Regulations (England and Wales) 2016.
4.4.10.2 Modelling of the following scenarios with the bypass channel and for the baseline has been undertaken: · MHWS tide and “average” fluvial flow (~50% of “maximum fluvial flow”) · 100% AEP tide and “average” fluvial flow (~50% of “maximum fluvial flow”)
12 Jacobs, 2019. Bridgwater Tidal Barrier Appraisal Hydraulic Modelling report (Doc ref: ENVIMSW002039-CH2-FEV-SW-RP-HY-00105)
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· 50% AEP tide (onset of overtopping downstream of the barrier) and “maximum fluvial flow”
4.4.10.3 The modelling showed no significant change in water levels due to the bypass channel and the potential for an increase in flow velocity in the bypass channel of up to 0.7m/s at some states of the tide. This indicates that the bypass channel will not have a significant impact on flood risk.
4.5 Flood Risk from surface water
4.5.1 Surface water runoff is defined as water flowing over the ground that has not yet entered a watercourse or sewer. It usually occurs as a result of an intense period of rainfall which exceeds the infiltration capacity of the ground. Typically, runoff occurs on sloping land or where the ground surface is relatively impermeable. The ground can be impermeable either naturally due to the soil type or geology, or due to development which places impervious material over the ground surface (e.g. paving and roads).
4.5.2 Baseline flood risk
4.5.2.1 The floodplain downstream of Bridgwater is served by an extensive land and field drainage network managed by the Internal Drainage Boards (Axe, Brue, Parrett and North Somerset Levels). The Environment Agency has powers over main rivers in the area including the River Parrett, King’s Sedgmoor Drain, Cannington Brook and Combwich Brook. The land drainage network is displayed in Figure 4-9 There are numerous outfall structures beneath the existing defences connecting the land drainage network to the Parrett.
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Figure 4-9: Map of Somerset Drainage Boards Consortium, main rivers, drains and structures (http://www.arcgis.com/apps/View/index.html?appid=22497f115856472eb9e58fdba3023191)
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4.5.2.2 The Environment Agency’s updated Flood Map for Surface Water can be used as a general indicator of an areas flood risk including that arising from the extensive network of land drainage in the surrounding area. The data available from these maps is presented in Figure 4-10.
4.5.2.3 Within the study area there is very little risk of surface water flooding. The current flood risk around Chilton Trinity is categorised as Low Risk Depth and Low Risk Velocity with very little risk in the village itself.
Figure 4-10: Environment Agency’s Updated Flood Maps for Surface Water.
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4.5.3 Development impact on flood risk
4.5.3.1 There are a number of points along the both the primary and secondary defence alignments where the existing surface water flood risk may interact with the proposed works. The areas of interest are mainly around Chilton Trinity where the new secondary embankment will cross areas of surface water flood risk and may therefore block surface water drainage routes. However, construction of secondary defences will be undertaken with consideration to the operation of existing drainage networks. All new embankments will incorporate culverts with penstock or flap valve controls to ensure that existing drainage arrangements can be maintained, whilst minimising the need for new control structures. There will be no change to the operation or management of existing outfalls and therefore no change to flood risk associated with these structures.
4.5.3.2 At the tidal barrier site, there is a risk that any increase in area of hardstanding could increase surface water runoff. The landscaping proposals for the operational site have not yet been confirmed but it is expected that there will be a mixture of hardstanding and permeable surfaces such as areas of vegetation/grass. It should be noted that there is already a large impermeable surface area on the operational site due to the former building slab so there may be no increase in surface water runoff resulting from the proposals.
4.5.3.3 The drainage proposals for the site will be confirmed at detailed design stage but it is proposed that Sustainable Drainage Systems (SuDS) will be used where possible to provide surface water drainage. Any increase in runoff will be addressed either through storage in SuDS features or through discharge to the tidal side of the defence where there will be no influence on storage capacity. The proposed scheme is therefore not considered to have a significant effect on surface water flood risk.
4.6 Flood Risk from groundwater
4.6.1 Existing flood risk
4.6.1.1 Groundwater flooding is caused by the emergence of water from beneath the ground at either point or diffuse locations when the natural levels of the water table rises above ground level. This type of flooding usually occurs in response to a combination of already high groundwater levels and intense or unusually lengthy storm events which can result in deep and long-lasting flooding of low-lying or below-ground infrastructure. Groundwater flooding can cause significant damage to property, especially in urban areas, and can pose further risks to the environment and ground stability.
4.6.1.2 The Parrett Catchment Flood Management Plan and the North and Mid Somerset Catchment Flood Management Plans do not identify groundwater as being a significant source of flooding. Information on groundwater flooding is limited within Sedgemoor District. The Sedgemoor District Level 2 SFRA concludes that the risk from groundwater flooding is relatively minor in Sedgemoor when considered in proportion to other risks such as fluvial and tidal flooding.
4.6.2 Development impact on flood risk
4.6.2.1 The BTB Scheme will not require deep excavations and significant below ground construction. The foundation of the tidal barrier will require deep permanent piles and a piled cut-off to prevent tidal flows passing under the structure, but these will be installed from the surface. Therefore, it is thought that the below ground construction will not significantly impede groundwater flows.
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4.6.2.2 The existing and proposed ground water flood risk will remain unchanged and therefore result in no change in flood risk.
4.7 Flood risk from reservoirs
4.7.1 Reservoir failure can be a particularly dangerous form of flooding as it results in the sudden release of large volumes of water which can travel at high velocity. This can result in deep and widespread flooding, potentially resulting in significant damage. The likelihood of reservoir flooding occurring is generally extremely low given that all large reservoirs are managed in accordance with the Reservoirs Act 1975. Under the Reservoirs Act 1975, a large raised reservoir is defined as one that holds over 25,000 cubic metres of water above the level of the surrounding land. The Environment Agency’s online reservoir inundation map illustrates the maximum flood extents that could potentially occur in the event of a reservoir failure.
4.7.2 There are four reservoirs in the District area used for water resource purposes, as well as others that have a flood storage function. The Environment Agency’s Flood Risk from Reservoirs mapping shows flood risk in the district from reservoirs.
Figure 4-11: Environment Agency’s flood mapping: Flood Risk from reservoirs
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4.7.3 The BTB Scheme is a considerable distance downstream from any of the reservoirs in the district. The permanent works boundaries at both Combwich and Pawlett fall outside the reservoir inundation mapping. The Barrier’s position in the River Parrett means that it is potentially in the path of any excess flow a reservoir failure upstream may cause. During such an event, the flood management regime upstream of Bridgwater would behave in the same way as during a high flow event and act to divert some of the water to adjacent systems.
4.7.4 The new defences at Chilton Trinity do cut across land which is shown to be within a flood risk area associated with Hawkridge Reservoir. Should the unlikely event of a reservoir failure occur, the relatively small footprint of the permanent works within the flood risk area would have an insignificant effect on flood risk to the adjacent agricultural land.
4.8 Flood risk from sewers
4.8.1 Flooding from sewers primary occurs when flow entering a system exceeds available capacity or if the network capacity has been reduced through blockage or collapse. In the case of surface water sewers that discharge to watercourses, the same effect can be caused as a result of high water levels in the received watercourse. As a result, water can begin to surcharge the sewer network, emerging at ground level through gullies, and manholes and potentially causing flooding to highways and properties. If this occurs flooding can represent a significant hazard to human health due to the potential for contaminants in the flood water.
4.8.2 The land use in the vicinity of the BTB Scheme is primarily agricultural with very low density of residential properties. The development will have no adverse impact upon sewer flood risk in these areas. Upstream of the barrier, as discussed in Section 4.4, the proposed scheme is expected to reduce in-channel water levels in the River Parrett. This will potentially improve the discharge capacity of any surface water sewers that drain in to this system as the effect of this downstream boundary condition will be reduced or removed.
4.8.3 The BTB Scheme is therefore not considered to have an adverse impact on sewer flood risk.
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5. Summary and Conclusions
5.1.1 The FRA has been performed in accordance with the revised NPPF, PPG and other relevant policies for the River Parrett flood risk management. The development comprises;
· Tidal barrier across the River Parrett between Chilton Trinity and the Express Park · Improvement to primary (existing) defences at Combwich, Pawlett and Chilton Trinity · Construction of secondary (new) defences at Combwich, Pawlett and Chilton Trinity · Installation of navigation signs at two locations · Fish passage and eel improvement measures at a number of locations throughout the River Parrett and its tributaries
5.1.2 As the development exceeds 1ha and lies within Flood Zone 3 it requires an FRA. The development is considered to pass the Sequential Test as it would not be appropriate to locate it in areas at lower flood risk. The development is classified as Water Compatible. This type of development is acceptable in Flood Zone 3b. There is no requirement for the Exception Test to be applied. However, the following is required:
In Flood Zone 3b (functional floodplain) essential infrastructure that has to be there and has passed the Exception Test, and water-compatible uses, should be designed and constructed to; · remain operational and safe for users in times of flood; · results in no net loss of floodplain storage; · not impede water flows and not increase flood risk elsewhere.
5.1.3 The proposed development is considered to meet the first of these requirements as the operational building will be set at a raised level which will allow it to remain operational and safe during times of flooding.
5.1.4 The requirement for no net loss of floodplain storage is met as the floodplain in this area is tidally dominated so is not required to provide storage. This is supported by the SFRA which does not designate these areas as Functional Floodplain (Flood Zone 3b). The proposed works are therefore not considered to impact on floodplain storage.
5.1.5 The requirement for the proposed works not to impede flows is met as the barrier under normal (non-flood) conditions is open and therefore does not impede flows in the channel. When the barrier is closed water temporarily will be prevented from flowing but this is also the case in the baseline where tide levels restricts fluvial flows. The comparison of the hydraulic model results between the baseline and with scheme scenarios for fluvial flows show a reduction in maximum water level upstream with the scheme in place indicating that it does not have an adverse impact on fluvial flows.
5.1.6 In order to assess whether the proposed development increases flood risk elsewhere, the flood risk from all sources has been considered. As determined in the SFRA, the primary flood risk to the area is from tidal sources. To assess the effectiveness of the proposed scheme, modelling has been performed to compare existing flood risk against that with the scheme in place. To do this a linked 1D-2D hydrodynamic model (Flood Modeller – TUFLOW) was used to assess the impacts of scheme implementation on the main river and surrounding area. The results of this model demonstrated that the scheme would reduce
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tidal flood risk to a wide area. Hydraulic modelling has also been undertaken to identify the fluvial flood risk. Other sources of flood risk have been assessed qualitatively.
5.1.7 Table 5-2 below summarises the results of the assessment of the potential impact of flood risk to the development and the development impact on flood risk.
Table 5.2 - Summary of flood risk impacts Source of Flood Risk Mitigation flooding To the development Development impact Required on risk Sea High Beneficial n/a Fluvial Low Little or no change n/a Surface water Low Little or no change n/a Groundwater Low Little or no change n/a Reservoir Low Little or no change n/a Sewers Low Little or no change n/a (potential beneficial)
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Flood Risk Assessment
Appendix A. Mapping
Drawing BIM reference Description number
2500 ENVIMSW002039-CH2-FBS-T5- Baseline flood depth – 0.5% AEP tidal event (2024) DR-HY-2500
2501 ENVIMSW002039-CH2-FBS-T5- Baseline flood depth – 0.5% AEP tidal event (2125) DR-HY-2501
2502 ENVIMSW002039-CH2-FBS-T5- Baseline flood depth – 0.1% AEP tidal event (2024) DR-HY-2502
2503 ENVIMSW002039-CH2-FBS-T5- Comparison between scheme and baseline DR-HY-2503 scenarios – 0.5% AEP tidal event (2024)
2504 ENVIMSW002039-CH2-FBS-T5- Comparison between scheme and baseline DR-HY-2504 scenarios – 0.5% AEP tidal event (2125)
2505 ENVIMSW002039-CH2-FBS-T5- Barrier failed open flood depth – 0.5% AEP tidal DR-HY-2505 event (2024)
2506 ENVIMSW002039-CH2-FBS-T5- Barrier failed open flood depth – 0.5% AEP tidal DR-HY-2506 event (2125)
2507 ENVIMSW002039-CH2-FBS-T5- Breach scenario 1 Hazard to People – 0.5% AEP DR-HY-2507 tidal event (2024)
2508 ENVIMSW002039-CH2-FBS-T5- Breach scenario 2 Hazard to People – 0.5% AEP DR-HY-2508 tidal event (2024)
2509 ENVIMSW002039-CH2-FBS-T5- Breach scenario 3 Hazard to People – 0.5% AEP DR-HY-2509 tidal event (2024)
2510 ENVIMSW002039-CH2-FBS-T5- Breach scenario 4 Hazard to People – 0.5% AEP DR-HY-2510 tidal event (2024)
2511 ENVIMSW002039-CH2-FBS-T5- Breach scenario 5 Hazard to People – 0.5% AEP DR-HY-2511 tidal event (2024)
2512 ENVIMSW002039-CH2-FBS-T5- Breach scenario 6 Hazard to People – 0.5% AEP DR-HY-2512 tidal event (2024)
2513 ENVIMSW002039-CH2-FBS-T5- Baseline flood depth – Maximum fluvial flow (2024) DR-HY-2513
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Drawing BIM reference Description number
2514 ENVIMSW002039-CH2-FBS-T5- Baseline flood depth – Maximum fluvial flow (2125) DR-HY-2514
2515 ENVIMSW002039-CH2-FBS-T5- Comparison between scheme and baseline DR-HY-2515 scenarios – Maximum fluvial flow (2024)
2516 ENVIMSW002039-CH2-FBS-T5- Comparison between scheme and baseline DR-HY-2516 scenarios – Maximum fluvial flow (2125)
2517 ENVIMSW002039-CH2-FBS-T5- Barrier failed closed flood depth – Maximum fluvial DR-HY-2517 flow (2024)
2518 ENVIMSW002039-CH2-FBS-T5- Barrier failed closed flood depth – Maximum fluvial DR-HY-2518 flow (2125)
2519 ENVIMSW002039-CH2-FBS-T5- Environment Agency Flood Map for Planning DR-HY-2519
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