14. Navigation

14.1 Introduction

14.1.1 The Environment Agency, in partnership with District Council, are delivering the Tidal Barrier (BTB) Scheme to reduce tidal flood risk to Bridgwater and surrounding areas.

14.1.2 This chapter considers the potential impacts of the BTB Scheme on the right of navigation in the and thus the navigation receptors (commercial and recreational users). It provides an assessment of the navigation impacts resulting from the construction and operation of the BTB Scheme. It assesses potential significant impacts identified through the scoping and subsequent detailed assessment processes and where necessary, proposes mitigation measures to avoid, reduce or minimise the significance of these effects. The assessment of likely significant effects on the upstream fish and eel passage sites is provided in Chapter 18 (Upstream fish and eel passage).

14.1.3 The following appendices have been prepared in support of this chapter:

. Appendix 14A – Bridgwater Tidal Barrier Navigation Risk Assessment

. Appendix 14B – Bridgwater Tidal Barrier Gate Width Assessment Technical Note

. Appendix 14C – Bridgwater Tidal Barrier Temporary Bypass Channel

14.1.4 This chapter is supported by the following figure:

. Figure 14.1 – Navigation Features and Proposed Scheme Sheets 1 and 2

14.2 Legislation and policy

14.2.1 The following legislation and policy has been considered in developing the scheme design. The lower reaches of the River Parrett are tidal and a right of navigation exists at common law, meaning the public have the right of passage in boats. The Transport and Works Act 1992 (TWA) enables orders to be made authorising the construction and operation of works interfering with rights of navigation.

14.2.2 Construction within coastal waters requires a marine licence under the Marine and Coastal Access Act 2009. A marine licence is required for works associated with the BTB Scheme that fall below the mean high water spring (MHWS) level.

14.2.3 Sedgemoor District Council is the Statutory Harbour Authority under the Bridgwater Navigation and Quays Act 1845 as amended by subsequent Acts and Regulations of 1904, 1908, and 1923 making amendments, additions and revocation to sections of the 1845 Act.

14.2.4 Sedgemoor District Council is also the Competent Harbour Authority for the , with duties and powers to manage the provision of a Pilotage service, as empowered and detailed within the Pilotage Act 1987.

14.2.5 There are two sets of Byelaws that relate to activities within the Port:

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. SDC Byelaws under the Public Health Amendment Act 1907 relate to the “seashore” at Burnham on Sea, Berrow and Brean, and are concerned with activities on the beaches and intertidal areas when exposed at low water; and

. SDC Byelaws under the Public Health Act 1961 relate to use of “pleasure boats” on the seashore and to a distance of 200m from the low water mark at Burnham on Sea, Berrow and Brean, and amongst other considerations impose a speed limit of 8 knots through the water (Sedgemoor District Council, 2019).

14.3 Assessment methodology

14.3.1 This section describes how potential impacts upon navigation have been assessed.

14.3.2 The scoping assessment carried out at the scoping stage, documented in the PEIR (Environment Agency, 2018a), has been reviewed at this detailed assessment stage in light of scheme changes that have occurred in the interim, and the subsequent scoping responses received. Full details of the scoping opinion and our subsequent responses are included in Appendix 1A.

14.3.3 Table 14.1 summarises the impacts that have been scoped in and out of further assessment, along with the rationale. Where this differs from the scoping stage, it is clearly stated and the reasons for this are also provided. Where the potential for significant effects can be clearly discounted, these have not been assessed further.

14.3.4 The assessment of navigation impacts has involved a combination of desk top study, consultation, numerical modelling and closed-circuit television (CCTV) monitoring of the River Parrett at Wharf and Reedmoor Rhyne.

Study area

14.3.5 The study area for the navigation assessment lies within the Port of Bridgwater limits, and for the consideration of navigation impacts comprises the River Parrett from Stert Point at the downstream extent (in the north) to Bridgwater Docks at the upstream extent (in the south). This study area is deemed appropriate based on professional judgement and the likely areas/receptors with the potential to be impacted by the proposed BTB Scheme. Figure 14.1 shows the study area and features relevant to this chapter.

Desk top study

14.3.6 A desk top study has been undertaken to inform the baseline against which the impacts of the proposed BTB Scheme are assessed. The main sources of information used for the desk top study include:

. Construction information as detailed in Chapter 6 (Proposed scheme), Section 6.3 of this Environmental Statement (ES)

. Operational information as detailed in Chapter 6, Section 6.4 of this ES

. Consultation with the Harbour Authority and Harbour Master, river users, operating authorities and other interested stakeholders (refer also to Section 14.4)

. Information collected, derived and assessed during the scheme’s appraisal stage

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. Hydraulic modelling results (water levels and flow velocities) (refer also to Chapter 6, Section 6.5

. Regime modelling results (channel morphology) (refer also to Chapter 6, Section 6.5 and Appendix 6H)

. Assessment of short term siltation following closure, HR Wallingford, 2019, refer to Appendix 6J

. Port of Bridgwater Operations Manual (Sedgemoor District Council, 2019) – available publicly

. Other publicly available information

Modelling

14.3.7 A regime modelling approach (regime theory) was adopted to investigate the effects of the barrier on channel morphology in the River Parrett (refer to Chapter 6 (Proposed scheme), Section 6.5). This high-level assessment makes use of outputs (cross-section area and discharge) from a one-dimensional (1-D) Flood Modeller hydraulic model.

14.3.8 Regime theory assumes that:

. The estuary is in equilibrium (net sediment movement, over a long period of time at any place is negligible, disregarding seasonal variation)

. The equilibrium estuary cross section can be characterised by a (“regime”) relationship with peak discharge (specifically peak discharge or tidal prism)

14.3.9 These two assumptions lead, using standard equations of sediment transport, to the following conclusion:

. An estuary changed from the equilibrium will respond by moving back towards its equilibrium “regime” state.

14.3.10 The regime modelling has been used to develop an understanding of the implications of the BTB Scheme on geomorphological processes within the study area. Further details about this assessment are documented in Appendix 6H.

Navigation Risk Assessment

14.3.11 A Navigation Risk Assessment (NRA) (see Appendix 14A) has been carried out to assess the hazards, risks and mitigating measures associated with the construction and operation of the BTB Scheme. The assessment follows a commonly used process using As Low As Reasonably Practicable (ALARP) principles.

14.3.12 The aims of the NRA are to:

. Assess the risk to users navigating the river adjacent to and through the barrier

. Assess the impact on the navigation of vessels during construction of the barrier

. Assess the impact on the navigation of vessels during operation of the barrier

. Assess the requirements for aids to navigation

. Assess the need for operational procedures required for safe navigation

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14.3.13 The NRA process has included a workshop with the Sedgemoor District Council Harbour Master.

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Table 14.1 Summary of navigation impacts scoped in, and out, of the assessment

Scheme Impact Scoped Justification component IN or OUT Barrier Impacts on navigation between IN The A39 Drove Bridge, 1.7 km upstream of the barrier location, is the first existing Dunball and the Drove Bridge vertical clearance restriction after the overhead power lines at and due to the barrier structure Dunball, therefore Drove Bridge is considered the appropriate upstream limit for during construction and consideration within the impact assessment. operation There is currently very little navigation upstream of Dunball, however consultation undertaken during this detailed assessment stage has identified that recreational and small commercial vessel use could increase in the future. The number of barrier closures is also anticipated to increase in the future, restricting navigation at certain times in this reach (refer also to Chapter 6 (Proposed scheme), Section 6.4). The overall impact of barrier closures on navigation in this stretch of the river is therefore likely to increase in the future. Therefore, the potential for impacts on navigation between Dunball and Drove Bridge as a result of barrier closures are scoped in. A number of consultees have expressed concern that there could be an impact on navigation rights at the barrier location during construction and therefore this aspect has also been scoped in for further consideration. Impacts on navigation between IN Construction and operational phase impacts have been scoped in to the assessment Dunball and Drove Bridge due as there is potential for the temporary works for construction and operation of the to changes in river bed levels barrier to result in changes to flow velocity and flow patterns and hence erosion and and hydraulic conditions during deposition of sediment close to the barrier. construction and operation. Barrier closures could cause changes to patterns of sediment transport, affecting estuary morphology. These changes in river bed level could affect navigation close to the barrier and between Dunball and Drove Bridge.

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Scheme Impact Scoped Justification component IN or OUT Impacts on navigation IN Construction and operational phase impacts on the ability of vessels to navigate the downstream of Dunball during channel downstream of Dunball due to river bed level changes and changes in construction and operation. hydraulic conditions have been scoped in since the scoping stage due to concerns raised by stakeholders during and after the scoping process (see Table 14.4). Impacts on navigation through OUT The expected vertical clearance of the barrier when the gates are open is to the barrier when gates in open 9.5mAOD as is the expected vertical clearance of the proposed foot and cycle position. bridge. The clear opening width is expected to be 15m per gate. Upstream of the barrier, clearances for navigation are restricted by Drove Bridge with a vertical clearance to 8.6mAOD (2.5 m above MHWS) (Source: County Council, 1995) and The Clink bridge the clear width of central bridge opening is 13.3m. The proposed barrier clearance restrictions are therefore greater than those upstream and are considered sufficient for the size of vessel that could practically navigate the channel to the barrier location and beyond. Refer to Appendix 14B for the gate width assessment undertaken in 2017 to inform the options appraisal stage and Appendix 6B for details of the barrier and bridge. Significant effects on navigation are therefore not anticipated, and operational effects on navigation as a result of the barrier structure itself have been scoped out. Assessment of the requirements for safe passage through the barrier structure, including navigational aids and operational procedures has been considered in the NRA (Appendix 14A). Downstream Construction phase impacts OUT There are no works to the River Parrett channel (including the navigation channel) flood defences proposed as part of the downstream flood defence works therefore, there are no impacts during the construction phase.

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Scheme Impact Scoped Justification component IN or OUT Operational impacts OUT There are no works to the River Parrett channel (including the navigation channel) proposed as part of the downstream flood defence works therefore, there are no impacts during the operational phase.

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Defining significance of effects

14.3.14 Navigation receptors are defined as users of the navigable channel. Impacts on navigation receptors have been assessed using a consistent scale of sensitivity and magnitude. The general process as detailed in Chapter 7 (Environmental assessment methodology) has been broadly followed to assess the significance of effects, however, topic-specific descriptors are provided in Tables 14.2 and 14.3 below.

14.3.15 Sensitivity has been scored using criterion ranging from low sensitivity to high sensitivity as described in Table 14.2. The greater the business/operational impact, and/or the lower the ability to adapt to the impact, the greater the sensitivity. A receptor can only be affected if there is a pathway through which an impact can be transmitted between the source activity and the receptor.

Table 14.2 Criteria for classifying the sensitivity of navigation receptors

Sensitivity Criteria High Low tolerance to permanent or temporary change; High level of commercial impacts potentially resulting in permanent effects on business operations; or Limited ability to adapt to new effect. Moderate Medium tolerance to permanent or temporary change; Medium level of commercial impacts potentially resulting in permanent effects on business operations; or Ability to adapt to the majority of new effect. Low High tolerance to permanent or temporary change; Low level of commercial impact; or Ability to adapt to new effect.

14.3.16 The magnitude of navigation impact incorporates a degree of subjectivity as there are no defined parameters and it has been assessed based on a combination of baseline data, consultation feedback from stakeholders and expert judgement. When assessing the magnitude of an impact, the geographical extent, the duration, the number of transits of specific vessel and/or vessel type and the likelihood of occurrence of the impact have been considered. Consideration has been given to the general criteria for classifying magnitude as defined in Chapter 7 (Environmental assessment methodology), Table 7.4 and this has been developed into a specific description of magnitude of impact for navigation receptors, as defined in Table 14.3.

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Table 14.3 Description of magnitude of impact

Magnitude Description of impact High Stopping up or permanent lengthy deviation of navigable channel; Permanent impact on receptors in the River Parrett; Very likely or likely probability of occurrence; or High degree of change relative to the baseline. Moderate Permanent deviation of navigable channel; Lengthy temporary deviation of navigable channel; Moderate permanent impact on receptors in the River Parrett; Probability of occurrence is about as likely as not; or Medium degree of change relative to the baseline. Low Some temporary deviation of navigable channel; Temporary impact on receptors in the River Parrett; Unlikely or very unlikely probability of occurrence; Low degree of change relative to the baseline. Very low Minor temporary deviation of navigable channel; Slight temporary impact on area of construction or limited scheme area; Exceptionally unlikely probability of occurrence; or Very low degree of change relative to the baseline. None No change to baseline.

14.3.17 The significance of the effect has been evaluated by combining the assessment of both sensitivity of receptor and magnitude of the potential impact as defined in Chapter 7 (Environmental assessment methodology) Table 7.5.

14.3.18 Significant effects are those that are major and moderate. Effects identified as minor or negligible are not considered significant.

14.3.19 The criteria for defining the duration of effect are set out in Chapter 7, Table 7.7.

14.4 Consultation

14.4.1 Consultation was undertaken to seek views of stakeholders on potential impacts to navigation during construction and in operation of the BTB Scheme. This consultation included an EIA scoping exercise which proposed the scope of the assessment to be included in the ES and which sought the views of the consenting authorities. A summary of the comments received with the Scoping Opinions is included in Appendix 1A.

14.4.2 During the development of the outline design, further consultation has been undertaken with parties including Sedgemoor District Council and the Harbour Master. Table 14.4 provides a summary of the key navigation related comments that have been made during the EIA process, but which were not necessarily made as part of the scoping exercise. The table also details how they have been considered in this assessment.

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Table 14.4 Consultee comments

Consultee Comment Response Sedgemoor There have been a number of Potential for impacts on District commercial vessels operating navigation between Dunball Council upstream of Dunball in the last few and the Drove Bridge are (Officer years, and Sedgemoor District Council scoped in to this comments) have been informed that commercial assessment. It is understood vessels are planned to operate that the commercial vessel(s) upstream of Dunball on a yearly basis referred to is the Internal for the foreseeable future. Drainage Board (IDB) dredger(s). Impact to river bed changes need to be As detailed in Section 14.6 of looked at, together with measures to this chapter, potential for dredge areas that are critical to river bed changes to impact navigation and berthing and passing navigation has been through the barrier with vessels. considered and no significant changes are expected. There is no requirement for operational currently identified. Can the reasons for ‘scoped out’ Following receipt of impacts during construction be comments from detailed? Construction will stop stakeholders, and in a vessels navigating upstream of change to the proposed Dunball and may cause a light ‘scoped in topics’ presented nuisance (lighting to light the site) to in the PEIR (Environment other vessels operating in the area. Agency, 2018a) construction phase impacts are scoped in to this assessment. Navigation aids/lighting will likely be A NRA has been undertaken required for the completed structure. to identify and assess the The requirement for this should be hazards and risks affecting determined with consultation with the navigation (refer to Appendix Harbour Master and . 14A). This has included There would be a need to maintain consideration of controls these navigation aids/lights for the life such as navigation aids and of the barrier. lighting that could be adopted to make risk as low as reasonably practicable. There is vehicle access to navigation Access to the markers will be lights and markers that are currently maintained. along the river edge, by use of the track on the top of the flood defence bank. With regard to provision of moorings A NRA has been undertaken upstream and downstream of the to identify and assess the barrier, depending on the state of the hazards and risks affecting tide and the duration of the delay, it navigation. Moorings were may be required that vessels take not considered necessary as Bridgwater Tidal Barrier Environmental Statement 14- 10

Consultee Comment Response bottom, as the river ‘dries out’ and warning signs will be returns to river flow. The site for safe installed upstream and mooring site should be assessed and if downstream of the barrier. there is a need to dredge the area to maintain a safe mooring site. EDF are going to refurbish the The project team and EDF Combwich facility starting next year to have liaised to understand allow North Sea barges to access the potential interfaces between terminal - this may require the two projects. Impacts consideration. downstream of Dunball have been scoped in to this assessment and there are not anticipated to be any impacts at Combwich. Harbour There is a particularly sensitive area in Modelling and regime theory Master the region of Ninestreams Light some calculations have been 1.5 to 2.0 nautical miles (2.8 to 3.7 km) undertaken and there are no downstream of the proposed site. We significant changes to the do not know what impact the barrier river bed levels expected. will have on that site or indeed other Monitoring of the estuary will areas of the Harbour, however it would be undertaken to record be prudent to have some contingency geomorphological baseline plans and dredging licences in place to change and also change counter any adverse impacts should during and after construction they arise. of the barrier. The monitoring data will be used to make decisions about whether the barrier is causing change that is affecting navigation and how the change can be managed. Recreational use could increase, as Construction and operational could small commercial vessel use. phase impacts on navigation This year we have seen two water rights / ability to navigate the injection dredgers launching at or channel have been scoped in downstream of Dunball and transiting to this assessment. upstream beyond the Drove Bridge. A formal NRA should be conducted A NRA has been undertaken including Sedgemoor District Council including a workshop with staff. the Harbour Master which was held in August 2019. Potential impacts on existing The scheme will not have navigation aids on the river banks any impact on access to, or need to be considered, both in terms of the function of the existing access and function. beacons, markers and signs.

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Consultee Comment Response Inland In their response to the Preliminary The barrier will be open most Waterways Environmental Information Report of the time. When open, Association (PEIR), IWA requested that reference there will not be an (IWA) was included in the PEIR to making obstruction to navigation. passive provision for the construction Design considerations mean of a lock and approach channels in the that the temporary bypass footprint of the bypass channel in the channel could not practically future and for the relevant land to be be converted to a lock in the formally reserved for this purpose. future, due to the position of As regards the height of any vessel equipment/infrastructure navigating the river downstream of (specifically the need to allow Drove Bridge, any bridges that are access for a crane and a low proposed to be built across the River loader to the west side of the Parrett downstream of Drove Bridge Tidal Barrier) (for example a maintenance bridge or During the construction a public access bridge), must enable works, there will be a tall-masted yachts (without stepping temporary bridge installed their masts) and other vessels to pass across the bypass channel. underneath the proposed bridges that The minimum soffit level of are built and must in addition allow the temporary bridge will be such craft to pass underneath the 8.6mAOD, which is the same vertical lift gates when they are in a soffit level as the A39 Drove raised position. Bridge. The Drove Bridge has a vertical clearance of 2.5m above MHWS which is equivalent to 8.6mAOD. During operation, under normal conditions, the barrier gates will be held in the open position above the river channel this is expected to be at a height of 9.5mAOD. The soffit level of the foot and cycle bridge over the navigation fairway is expected to be 9.5mAOD). Navigation rights and safety need to be Potential for impacts on C Harbour protected. A small effect of siltation in navigation downstream of Master an area can impact the use of port. Dunball are scoped in to this assessment.

14.5 Baseline

The Port of Bridgwater

14.5.1 The Port of Bridgwater (hereafter referred to as ‘the Port’) lies on the north coast of Somerset in the Channel. It has been a statutory port since 1500 and a parliamentary Act of 1845 established its present boundaries and structure.

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14.5.2 The Port extends from upstream of Bridgwater Docks to the Severn Estuary. It covers 25 square nautical miles of mainly intertidal sand and mud banks off the coast ( to Hinkley Point), and over 14 square nautical miles of tidal estuary and river (Parrett, Brue and Axe)1. The limits of the Port are shown on Plate 14.1. They can be summarised as:

. from Brean Down to east of Hinkley Point

. The River Parrett as far as Bridgwater town centre, 0.96km upstream of Bridgwater Docks

. The as far as Highbridge Clyse Sluice

. A small part of the tidal River Axe2

14.5.3 The Gore Buoy, situated in the approaches to Burnham on Sea, is a safe water mark. Used in conjunction with the Western end of Brean Down it marks the line of shallow water and provides vessels bound for Bridgwater with a useful reference for their approach over the sands.

14.5.4 The main channel of the Port runs parallel with Burnham seafront and is marked with beacons, flashing red lights to port (left), and green to starboard (right). Coming up the river, the navigation fairway3 is marked by further lighted beacons to port and one to starboard opposite the berths at Dunball.

14.5.5 The Port of Bridgwater Operations Manual, (Sedgemoor District Council, 2019) details that freight wharves are located at: Dunball, Combwich and Bibby’s Wharves (See Paragraphs 14.5.27 to 14.5.36 for further details of commercial operations within the Port). All freight and passenger facilities within the port are operated by third parties. There are no commercial fishing vessels based at the Port of Bridgwater (Sedgemoor District Council, 2019).

14.5.6 Within the Port, leisure activity takes place from the beach at Burnham on Sea and there are leisure craft moorings at Combwich Pill and in the River Brue (Sedgemoor District Council, 2019).

14.5.7 There are no speed limits for piloted4 vessels using the main navigable channel within the Port as timings needed for safe passage dictate the speed required on the river from sea to Dunball.

Existing environmental conditions within the Port

14.5.8 The ability to navigate the river section of the Port (from Stert Point to the upstream limit of the Port) is constrained by channel width, tidal variation, air draft (vertical clearance) and under keel clearance. Account must also be taken of the mud banks within the river channel, which can vary in depth and location due to seasonal geomorphological variation.

1 http://www.ports.org.uk/port.asp?id=215 2 https://www.sedgemoor.gov.uk/article/972/General-Information-about-the-Port 3 A ‘fairway’ is a channel for vessels that, by virtue of sufficient depth and lack of obstacles, allows safe navigation across an expanse of water such as a river. 4 Pilotage in the River Parrett is compulsory, for all vessels over 30m in length Bridgwater Tidal Barrier Environmental Statement 14- 13

Channel dimensions

14.5.9 The river channel (and navigable channel) within the Port reduces in width as it goes upstream. At Combwich (approximately 6km from the mouth of the estuary) the channel is approximately 250m wide, further upstream, at Dunball it is approximately 100m wide. At the barrier site the channel is approximately 50m wide and at Bridgwater Docks approximately 25m wide. The navigable width is restricted by some of the bridges across the Parrett, for example The Clink bridge near the centre of Bridgwater has a central opening of 13.3m.

14.5.10 The barrier site is located between Dunball and Bridgwater Docks, just downstream of a tight meandering section of the estuary. The channel width at the barrier site is approximately 55m, with a central low flow channel and a river bed level of approximately 0.3mAOD. On the east side of the channel, the bank level is approximately 7.0mAOD and there is a wide berm at approximately 7.5mAOD. The berm is bounded by a flood bank with a crest level of 9.4mAOD. On the west side of the channel the bank level is approximately 7.0mAOD and there is a flood embankment close to the channel with a crest level of 8.4mAOD (refer to Appendix 14C).

14.5.11 Surveys of the Port, including those undertaken regularly by the Harbour Master, have shown that the position of the low water channel changes over time due to natural processes and, as detailed in Chapter 6 (Proposed scheme), Section 6.5, the movement of the meandering low water channel causes localised erosion and deposition.

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Plate 14.1 Port of Bridgwater limits5

5 Source: Port of Bridgwater Port Operations Manual, Revision 7, (Sedgemoor District Council, 2019) Bridgwater Tidal Barrier Environmental Statement 14- 15

Water levels

14.5.12 The fairway between Burnham on Sea at the seaward limit of the Port and the wharves at Dunball is generally self-scouring with established seasonal variation.

14.5.13 The Port has a high tidal range (up to 10m on spring tides); and navigation on the river is tidally dependant. In low tide conditions, the Port dries out completely, apart from river water, from above Stockland Reach to the head of navigation at Bridgwater town.

14.5.14 Navigation in the Port is only possible with tides of 3.0m or more on Bridgwater predictions (Sedgemoor District Council, 2019). Some vessels with a shallower draft, have been known to navigate within the Port on tides below 3.0m, in discussion with the Harbour Master, on a case by case basis. There is a tide board at Combwich that gives an advanced direct reading of the tide level at Dunball Wharf.

14.5.15 At the location of the barrier, the River Parrett has a tidal range of between 7m and 8m, the MHWS level at the barrier location is calculated to be approximately 5.9mAOD. The tide levels at two locations near to the barrier location are summarised in Table 14.5.

Table 14.5 Tide levels6

Hinkley Point Bridgwater, West Quay (mAOD) (mAOD) Highest Astronomical Tide – HAT 7.1 8.17 Mean High Water Spring – MHWS 5.6 6.1 Mean High Water Neap – MHWN 2.8 3.2 Mean Low Water Neap – MLWN -2.3 No tidal influence, river flows only Mean Low Water Spring – MLWS -5.0 No tidal influence, river flows only

14.5.16 When low freshwater flows coincide with spring tides, rising water in the River Parrett is funnelled into a low height wave known as the Parrett Bore (Plate 14.2). The bore usually occurs around 1 hour 40 minutes before high water, but this may be earlier or later depending on meteorological conditions.

6 https://www.ntslf.org/tides/hilo 7 https://flood-warning-information.service.gov.uk/river-and-sea-levels?lat=51.1298992240254&lng=- 3.00162488831435 Bridgwater Tidal Barrier Environmental Statement 14- 16

Plate 14.2 Parrett Bore at Bridgwater (Source: Environment Agency)

Existing physical features

14.5.17 The current vertical clearance limit for the Port is 32m above Highest Astronomical Tide (approximately 40mAOD) under overhead power lines at Combwich (Port of Bridgwater Harbour Master, pers. comms.).

14.5.18 Some 1.7km upstream of the barrier location, the A39 Drove Bridge poses the next existing vertical clearance restriction. The Drove Bridge has a vertical clearance of 2.5m above MHWS (, 1995), or to 8.6mAOD.

14.5.19 Bridgwater Docks is located some 2km upstream of the barrier site. There are two locks between the docks and the River Parrett, the Barge Lock is approximately 4m wide and the Ship Lock is approximately 12m wide. Both locks are sealed and currently not in operation, so there is no access between the river channel and the docks.

River users

Recreational navigation

14.5.20 At the upstream extent of the study area, there is a marina at Bridgwater Docks. This marina is accessed from the Bridgwater and Canal. There is currently no access between the Bridgwater Docks and the River Parrett, due to the sealed locks between the Parrett and the Docks (see Paragraph 14.5.19); this has been the status since the 1980’s. The Inland Waterways Association (IWA) are currently promoting the case for reopening the Barge Lock and Bascule Bridge in order that navigation can once again take place between the River Parrett and the Bridgwater and Taunton Canal via Bridgwater Docks, thus creating opportunities for leisure and recreational

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boating.8 However, at the time of writing there is no navigable route linking the River Parrett and the Bridgwater and Taunton Canal.

14.5.21 The day-trip passenger vessel ‘Balmoral’ has previously used Dunball Wharf to embark passengers for a cruise. However, this has not happened since 2012.

14.5.22 Combwich Pill towards the downstream extent of the study area has historically been used as a berth for vessels using the River Parrett to travel to and from Bridgwater. It is now home to the Combwich Motor Boat and Sailing Club who have been established at Combwich since 1966. Facilities associated with the club include moorings and mud berths on the Pill and a club house. Recreational activities associated with the Combwich Motor Boat and Sailing Club include leisure sailing, motor boating and gig rowing.

14.5.23 Motion sensitive CCTV camera equipment has been put in place by the Environment Agency at Dunball, recording since 3rd April 2019, and at Reedmoor Rhyne (upstream of barrier site) since the 15th May 2019, in order to inform the baseline position regarding navigation use of the channel.

14.5.24 Table 14.6 details the movements recorded at each of these locations since installation of the equipment. Figure 14.1 shows the locations of the cameras and Plates 14.3 and 14.4 show examples of the images captured.

14.5.25 The results of the camera survey undertaken in Spring/ Summer 2019 indicate there is only very occasional navigation along the River Parrett upstream of Dunball. Movements during the periods of monitoring are limited to small craft including jet skis, gigs and an inshore lifeboat.

14.5.26 Recreational navigation users are considered to be of moderate sensitivity as they are likely to have a medium tolerance to any temporary delay in transit during disruption to the navigable channel and are likely to be able to adapt to any new effect (refer to Table 14.2).

8 https://www.waterways.org.uk/westcountry/pdfs/bascule_bridge_vision_report Bridgwater Tidal Barrier Environmental Statement 14- 18

Table 14.6 Navigation movements recorded by motion sensitive camera equipment

Date Time* Vessel type (Camera) Direction 05/05/2019 18:16 Jet ski x 2 (Dunball) Upstream 18:29 Jet ski x 2 (Dunball) Downstream 06/05/2019 06:37 Gig rowing boat x 2 (Dunball) Upstream 07:56 Gig rowing boat x 2 (Dunball) Downstream 12/05/2019 11:11 Jet ski x 2 (Dunball) Upstream 11:23 Jet ski x 2 (Dunball) Downstream 08/06/2019 08:43 Gig rowing boat x1 (Dunball) Upstream 08:51 Gig rowing boat x1 (Dunball) Upstream 08:55 Gig rowing boat x1 (Dunball) Upstream 10:02 Gig rowing boat x1 (Dunball) Downstream 10:08 Gig rowing boat x1 (Dunball) Downstream 10:16 Gig rowing boat x1 (Dunball) Downstream 09:07 Gig rowing boat x1 (Reedmoor) Upstream 09:14 Gig rowing boat x1 (Reedmoor) Upstream 09:36 Gig rowing boat x1 (Reedmoor) Downstream 09:37 Gig rowing boat x1 (Reedmoor) Downstream 09:45 Gig rowing boat x1 (Reedmoor) Downstream 23/06/2019 08:37 Orange rib lifeboat x1 (Dunball) Upstream 09:04 Orange rib lifeboat x1 (Dunball) Downstream 08:44 Orange rib lifeboat x1 (Reedmoor) Upstream 09:04 Orange rib lifeboat x1 (Reedmoor) Downstream 21/07/2019 08:30 Orange rib lifeboat x1 (Dunball) Upstream 08:55 Orange rib lifeboat x1 (Dunball) Downstream 08:34 Orange rib lifeboat x1 (Reedmoor) Upstream 08:49 Orange rib lifeboat x1 (Reedmoor) Downstream 23/08/2019 12:57 2 person canoe x1 (Reedmoor) Downstream 13:23 2 person canoe x1 (Dunball) Downstream 14:24 Gig rowing boat x1 (Dunball) Upstream 14:27 Gig rowing boat x1 (Dunball) Downstream 18:10 Gig rowing boat x1 (Dunball) Upstream 18:23 Gig rowing boat x1 (Dunball) Downstream 31/08/2019 06:46 Gig rowing boat x1 (Dunball) Upstream 07:06 Gig rowing boat x1 (Reedmoor) Upstream 07:23 Gig rowing boat x1 (Reedmoor) Downstream 07:49 Gig rowing boat x1 (Dunball) Downstream 29/09/2019 18:45 Small motored vessel x 1 (Reedmoor) Upstream 18:45 Small motored vessel x 1 (Reedmoor) Downstream 05/10/2019 10:10 Grey rib x1 (Reedmoor) Upstream 10:17 Grey rib x1 (Reedmoor) Downstream 20/10/2019 10:23 Orange rib lifeboat x2 (Dunball) Upstream 10:47 Orange rib lifeboat x2 (Dunball) Downstream 1033 Orange rib lifeboat x2 (Reedmoor) Upstream 1036 Orange rib lifeboat x2 (Reedmoor) Downstream 21/10/2019 11:24 Small motored vessel x 1 (Dunball) Upstream 1136 Small motored vessel x 1 (Dunball) Downstream (*Times recorded Greenwich Mean Time)

Bridgwater Tidal Barrier Environmental Statement 14- 19

Plate 14.3 Image from motion sensitive camera equipment installed at Dunball (Source: Environment Agency)

Plate 14.4 Image from motion sensitive camera equipment installed at Reedmoor Rhyne (Source: Environment Agency)

Bridgwater Tidal Barrier Environmental Statement 14- 20

Commercial navigation

14.5.27 Commercial navigation on the river is generally only possible for vessels on tides of 3.0m or more above Bridgwater datum (i.e. on tides of approx. 4.5mAOD or more) and is generally limited to the waters from Dunball Wharf to Bridgwater Bay (downstream of the barrier location). Vessels with a shallower draft can, however, navigate within the Port on lower tides, on a case by case basis, in discussion with the Harbour Master and occasionally commercial vessels do go further upstream than the commercial wharfs (Plate 14.5 shows a small commercial vessel moored in Bridgwater town).

14.5.28 The commercial wharves within the Port are situated as follows9:

. Combwich - roll-on roll-off (Ro-Ro) facility catering for Hinkley Point power stations. This wharf was used to bring in material for the construction of Hinkley Point power stations and is occasionally used for maintenance operations and for importing abnormal loads associated with the Hinkley Point works. There is a planned refurbishment to allow larger barges to use the facility in the construction of Hinkley Point C power station. The Ro-Ro berth is normally useable by ships up to a maximum length of 100m and beam of 19m.10

. Bibby’s Wharf – not currently operational.

. Dunball Wharf – a general wharf historically used by passenger vessels and cargo. More recently, an aggregates berth is in regular use. This berth is able to take modern, manoeuvrable vessels up to 80m in length. A water injection dredger is launched from Dunball Wharf annually by the Parrett IDB, it is understood this vessel navigates upstream to above the M5 to undertake dredging work (refer to Plate 14.5 and 14.6).

14.5.29 All active commercial berths within the Port are located downstream of the barrier location.

14.5.30 There is a former berth at Bristol Road (Silveys Wharf) upstream of the barrier location. This has not been operational since the 1970’s and is not included in the Port of Bridgwater Operations Manual. There are no known plans to use this wharf in the future. However, it is recognised that Sedgemoor District Council have a duty under Government Guidance, to safeguard the Port for future use and Silveys Wharf forms part of the Port, therefore vessels should not be excluded from use of this wharf.

9 Information taken from the Port of Bridgwater Operations Manual Version 7 (Port of Bridgwater, amendment September 2019) and anecdotal information from SDC. 10 At the time of preparing this chapter, the Combwich Ro-Ro refurbishment has not started, but plans are being developed to progress the refurbishment. The BTB Scheme project team and Combwich refurbishment teams are aware of each other’s planned works. Bridgwater Tidal Barrier Environmental Statement 14- 21

Plate 14.5 Commercial vessel moored on the River Parrett at West Quay, Bridgwater (Source: Environment Agency)

Plate 14.6 Water injection dredger, The Borr (Source: Somerset Drainage Boards Consortium, 2017)

Bridgwater Tidal Barrier Environmental Statement 14- 22

14.5.31 Table 14.7 indicates the annual throughput of the Port for the period 2000-201811. The data shows a decline in the period 2006 to 2014 in the total number of vessels and overall cargo handled through the Port. Since 2015, the number of vessels and overall cargo has risen.

14.5.32 The figures also show the primary commercial operation within the Port is the transportation of dredged sand and gravel (unloaded at Dunball Wharf). The aggregate is typically discharged from a mid-size marine aggregate dredging vessel (refer to Plate 14.7).

Table 14.7 Annual throughput at the Port of Bridgwater (Source: Sedgemoor Annual Air Quality Report, 2017; and Sedgemoor District Council)

Year Number of Passengers Sand/gravel Other cargo vessels numbers cargo (tonnes) (tonnes) 2000 67 514 32,583 51,886 2001 76 377 40,400 63,213 2002 65 472 45,718 40,325 2003 80 446 56,403 44,749 2004 79 314 67,633 37,492 2005 87 170 69,516 36,961 2006 80 340 55,754 34,961 2007 42 192 49,880 0 2008 38 200 46,389 0 2009 27 270 30,576 470 2010 37 260 40,333 173 2011 36 0 45,243 0 2012 38 119 45,693 0 2013 30 0 36,579 0 2014 31 0 38,016 0 2015 50 0 58,942 332 2016 60 0 72,438 0 2017 73 0 86,235 0 2018 68 0 69,892 926

14.5.33 There are understood to be no commercial fishing vessels currently based at the Port of Bridgwater.

14.5.34 All operational commercial wharves are located downstream of the barrier, and therefore do not necessitate passage past the barrier location. It is therefore considered that construction and operation of the barrier is likely to have a low/negligible level of impact on commercial vessels using these wharves, if any at all.

14.5.35 A commercial dredger does operate upstream of the barrier on an annual basis and it is understood dredging activities are planned to continue for the foreseeable future. One of the dredgers used, The Borr, is 19m long and 5m wide (refer to Plate 14.5). Commercial vessels that operate upstream of the barrier could be subject to impacts from closures of the barrier.

11 At the time of preparing this chapter, 2019 data is not available Bridgwater Tidal Barrier Environmental Statement 14- 23

14.5.36 With reference to the criteria for classifying sensitivity of navigation receptors (Table 14.1), commercial navigation users within the study area are considered to be of moderate sensitivity as they are likely to have a medium tolerance to any temporary delay in transit during disruption to the navigable channel and are likely to be able to adapt to any new effect.

Plate 14.7 Marine aggregate dredger on the River Parrett in transit to Dunball Wharf (Source: Environment Agency)

14.6 Impact assessment

14.6.1 This section details the predicted effects of the BTB Scheme on navigation receptors.

Impacts on navigation between Dunball and the Drove Bridge due to the barrier structure

Construction impacts

14.6.2 The barrier will be constructed in a temporary cofferdam in the River Parrett. A temporary bypass channel will be constructed around the west side of the cofferdam. The bypass channel will be approximately 20m wide and will allow tidal and fluvial flows to pass around the cofferdam and maintain a navigable channel. The bypass channel will be constructed prior to the cofferdam and will remain in operation while the river channel is obstructed by the cofferdam. The existing river channel at this location is approximately 50m wide.

14.6.3 An initial assessment to define the required barrier opening width for navigation was carried out in accordance with ‘Harbour Approach Channels Design Guidelines’ (PIANC Report 121, 2014) in April 2017. The assessment showed a 15m wide clear opening between towers will not restrict current or foreseeable future navigation access (Appendix 14B). Therefore, a 20m wide bypass channel is considered suitable to allow navigation past the site during the construction phase.

14.6.4 There will be a temporary bridge installed across the bypass channel to allow construction access. The minimum soffit level of the temporary bridge will be not less than the soffit level of the A39 Drove Bridge 1.7km upstream of the barrier location. Bridgwater Tidal Barrier Environmental Statement 14- 24

This means that the temporary bridge will have the same vertical clearance as Drove Bridge for vessels navigating the river.

14.6.5 The temporary cofferdam across the full width of the River Parrett will result in a temporary disruption to the transit of vessels within the defined limits of the Port at the barrier location. However, the construction of the bypass channel will permit transit of vessels throughout the construction stage.

14.6.6 A limited number of small recreational vessels have been known to use the channel between Dunball and Drove Bridge. This number may increase in the future if aspirations such as opening up access from the Bridgwater Docks to the River Parrett are realised, however this is unlikely to occur whilst the barrier is under construction.

14.6.7 Even taking into account the potential for a future increase in the numbers of recreational users of the river between Dunball and Drove Bridge and the potential for an increase in flow velocity in the bypass channel at some states of the tide (up to 0.7m/s), the fact that the temporary bypass channel will permit transit of vessels throughout the construction stage means that the magnitude of any impact on recreational users is considered to be low (refer to Table 14.3).

14.6.8 The effect on recreational river users (moderate sensitivity, refer to Table 14.2) between Dunball and Drove Bridge is therefore considered to be minor negative (not significant) (refer to Table 7.5) before mitigation. Given the associated timeframes of the effect, it is considered as temporary (medium-term) (refer to Table 7.6) and direct.

14.6.9 All currently active commercial berths associated with the Port are located downstream of the barrier. With the exception of the IDB dredger, which is understood to be launched annually at Dunball before travelling upstream to beyond the upstream limits of the Port, no commercial vessels are currently understood to use the section of the river between Dunball and Drove Bridge. This assumption is supported by the results of the CCTV cameras that were installed in 2019 to monitor river traffic (refer to Table 14.6). Should any commercial vessels wish to transit the Port and beyond upstream of the barrier, the bypass channel will permit this throughout the construction stage in similar water conditions to those that the main channel currently allows.

14.6.10 Based upon the current use of the river between Dunball and Drove Bridge by commercial vessels, the fact that there are no active commercial wharves upstream of the barrier and no known plans to open any commercial wharves upstream of the barrier, and having regard to the fact that the bypass channel will be operational for the duration of the construction works, the magnitude of impact on commercial users is considered to be low.

14.6.11 The effect on commercial river users (moderate sensitivity) between Dunball and Drove Bridge is therefore considered to be minor negative (not significant) before mitigation. The effect is considered temporary (medium-term) and direct.

14.6.12 Although these effects are not assessed as significant, mitigation will be implemented during the construction works to ensure any navigation risks are ALARP. The principle of ALARP involves weighing a risk against the trouble, time and money needed to control it. The mitigation will consist of:

Bridgwater Tidal Barrier Environmental Statement 14- 25

. Warning signs advising of the bypass channel situated at locations remote from, and at, the barrier site

. Aids to navigation such as signs, lights and buoys at and on approaches to the barrier site

. Ladders at the barrier site to allow escape from the channel

. Methods to minimise creation of waves during construction

. Correspondence with the Harbour Authority and advertisements to advise of construction programme

. Ongoing consultation with the Port of Bridgwater Harbour Master, owners and operators of Dunball Wharf, and potentially other groups such as owners and operators of Bridgwater Docks during construction

. Prevention of flotsam and jetsam from the construction works entering the water

. Inspection of the river bed at low tide and removal of submerged jetsam that could cause a hazard

. Maintenance of visibility of the cofferdam and bypass channel from the channel by storing plant and materials away from the river banks.

14.6.13 The contractor will be required by the construction contract to prepare a Navigation Management Plan that will detail the mitigation measures outlined above.

14.6.14 Following implementation of this mitigation, the residual effects will remain as minor negative (not significant) for recreational and commercial river users.

Operational impacts

14.6.15 During normal conditions, the barrier gates will be held in the open position above the river channel at an expected minimum level of 9.5mAOD. The soffit level of the proposed foot and cycle bridge over the river channel is also expected to be a minimum of 9.5mAOD, refer to Plate 14.7. The two vertical lift gates will be closed for flood risk management purposes when tidal levels at Bridgwater are forecast to exceed a level of 7.3mAOD when the barrier first becomes operational, increasing to 7.5mAOD by 2125. The barrier will also be closed for maintenance, training and testing purposes, as described in Chapter 6 (Proposed scheme), Section 6.4.

14.6.16 Each closure during an extreme surge tide event is expected to last a maximum of six hours. The barrier will be closed at low water ahead of the forecast surge and will remain closed until the tide level downstream and the fluvial water level upstream of the barrier are the same. It is considered unlikely that vessels would be navigating the river during extreme surge tides due to the high water levels, flow rates and flow velocities that would be experienced under these conditions.

14.6.17 Most closures for maintenance will be for approximately 1 hour, there will also be occasional testing/training exercises lasting 4-5 hours. Some maintenance closures will be of one gate at a time and therefore there will be no disruption to navigation as the other gate will remain open. It is likely some maintenance closures will be at low tide when navigation is not possible. Closures for maintenance and testing/training will be planned and will be advertised in advance

Bridgwater Tidal Barrier Environmental Statement 14- 26

Plate 14.7: Indicative visualisation of the in-channel approach to the barrier from the south (upstream)

14.6.18 The closure of the barrier will temporarily prevent the transit of vessels upstream and downstream. Thus, closure of the barrier introduces a risk of vessels being delayed in their journeys or journeys being cancelled altogether.

14.6.19 In the future there will be an increase in the number of barrier closures due to sea level rise. However, these additional closures will be for high and extreme surge tides when vessels are unlikely to be using the river due to difficult conditions for navigation. The number of closures for maintenance and testing will decrease with time, refer to Chapter 6 (Proposed scheme) Table 6.10.

14.6.20 The current usage of the river past the barrier site is very low. There may be a future increase in the numbers of recreational users of the channel between Dunball and Drove Bridge, however this is uncertain, and the magnitude is unknown.

14.6.21 At the times at which the barrier will be closed for a surge event, conditions are not likely to be favourable for recreational navigation. In the case of maintenance closures, these will be planned and advertised in advance. On the basis of the low usage and the pattern of barrier closures, the magnitude of any impact on recreational users is therefore considered to be low.

14.6.22 The potential effect of increasing barrier closures in the future, to take account of climate change impacts, affecting recreational river users (moderate sensitivity) between Dunball and Drove Bridge is considered to be minor negative (not significant) before mitigation. Although the potential effect is not expected until 2055, it is considered as permanent and direct.

14.6.23 All currently active commercial berths associated with the Port are located downstream of the barrier and therefore will not be directly affected by the closure of the barrier. With the exception of the IDB dredger, no commercial vessels are known to use the section of the channel between Dunball and Drove Bridge.

Bridgwater Tidal Barrier Environmental Statement 14- 27

14.6.24 Based on the current low usage of the channel between Dunball and Drove Bridge by commercial vessels, the fact that there are no active commercial wharves upstream of the barrier and no known plans to open any commercial wharves upstream of the barrier, the magnitude of any operational impact on commercial users is considered to be low.

14.6.25 The potential future effects on commercial river users (moderate sensitivity) between Dunball and Drove Bridge is therefore considered to be minor (not significant) before mitigation. Given the associated timeframes of the potential effect, it is considered permanent and direct.

14.6.26 Although these effects are not assessed as significant, mitigation will be incorporated into the detailed design and implemented to ensure navigation risks are as low as reasonably practical (ALARP). This will consist of:

. Signs advising of the status of the barrier at locations remote from, and at the barrier

. Aids to navigation lights at the barrier

. Maintenance lines of site to the barrier by managing vegetation

. Ladders on the barrier for escape from the channel

. CCTV to monitor the ladders and piers immediately adjacent to the ladders

. Visual monitoring of the river bed at low water for obstructions

. Regular surveys of the river channel for changes in channel alignment and notification of river users

. Correspondence with the Harbour Authority and advertisements to advise of planned maintenance closures

. The Harbour Master will be able to issue ‘Notices to Mariners’ to inform the public when the gates are being closed. Issuing ‘Notices to Mariners’ informs a number of bodies on issues affecting navigation within the Port and is circulated to commercial vessel bodies, nearby ports and the Royal National Lifeboat Institution

. Ensuring adequate warning time is provided to river users, the decision to close the barrier will usually be taken at least 12 hours prior to the closure and will be based on interpretation of the latest tide and surge predictions

. Development and maintenance of a published closure procedure plan prepared in consultation with the appropriate authorities

. Ongoing consultation with the Port of Bridgwater Harbour Master, owners and operators of Bridgwater Docks and all commercial wharves in the Port; and

. Update of the Bridgwater Harbour Operations Manual to include details of the barrier structure and operation.

14.6.27 Following implementation of this mitigation, residual effects will remain as minor negative (not significant) for recreational and commercial river users respectively.

Bridgwater Tidal Barrier Environmental Statement 14- 28

Impacts on navigation between Dunball and Drove Bridge due to changes in river bed levels and hydraulic conditions

14.6.28 Navigation upstream of Dunball includes commercial vessels on a very infrequent basis and occasional recreational vessels.

14.6.29 An assessment of the potential short-term effects of the operation of the barrier has been carried out based on the understanding of the sediment processes in the estuary. Refer to technical note in Appendix 6J.

14.6.30 An assessment of the potential long-term effects of the operation of the barrier on channel morphology has been carried out using a regime theory approach. This assessment is described in Chapter 8 (Water and geomorphology), Section 8.3 and in a technical note in Appendix 6H.

14.6.31 Hydraulic modelling has been undertaken to assess the impact of the temporary construction works and the operational barrier on flow velocities near to the barrier site. Refer to hydraulic modelling report in Appendix 6G.

Construction impacts

14.6.32 As discussed in Chapter 6 (Proposed scheme), Section 6.5, construction works associated with the barrier including the installation of the temporary bypass channel and the cofferdam have the potential to influence channel flows and channel morphology. Hydraulic modelling, using a 1-D model of the temporary works, including the 20m wide, 200m long temporary bypass channel has been undertaken. The modelling indicates the temporary works result in a local increase in flow velocity (the increase in velocity within the bypass channel is generally expected to be in the range of 0m/s to 0.7m/s over a tide cycle) which could result in temporary changed patterns of scour and deposition within approximately 500m upstream and downstream of the barrier location. The increased flow velocity through the bypass channel may result in increased scour of the river bed and banks near to the ends of the channel. The potential for scour in these locations will be minimised by stone scour protection or suitable alternative through the bypass channel and in the transition areas back to the main river channel. Material scoured through this process may be deposited on the banks upstream or downstream of the channel. This is anticipated to be small scale and is not likely to impact navigation.

14.6.33 Commercial and recreational river users are considered to be of moderate sensitivity (refer to Paragraphs 14.5.26 and 14.5.36), the magnitude of the potential impact (i.e. changes to river bed level) during construction is considered to be low, resulting in a minor negative (not significant) effect on both commercial and recreational users. The effect will be indirect, and due to the mobile nature of the sediment and likely maintenance of the temporary construction works and channel (refer also to Chapter 6 (Proposed scheme), Section 6.3), any localised sedimentation is expected to be temporary (short-term).

14.6.34 Although these effects are not assessed as significant, the following mitigation will be implemented to reduce this potential construction phase effect:

. Regular monitoring and survey of the temporary bypass channel and river bed levels during construction. The monitoring data will be used to make decisions about whether construction of the barrier is causing change that is affecting

Bridgwater Tidal Barrier Environmental Statement 14- 29

navigation and how the change can be managed. This will be discussed and agreed with the Harbour Authority.

. Following completion of the construction of the barrier, the temporary bypass channel will be infilled with the original material excavated and temporary sheet piles will be removed.

14.6.35 Following implementation of this mitigation, residual effects will remain as minor negative (not significant) for recreational and commercial river users respectively.

Operational impacts

14.6.36 Surveys of the Port, including those undertaken regularly by the Harbour Master, have shown that the position of the low water channel changes over time due to natural processes and, as detailed in Chapter 6 (Proposed scheme), Section 6.5, the movement of the meandering low water channel causes localised erosion and deposition. This low water channel is thought to be sensitive to changes in the interaction of the tidal and freshwater flows and resulting sediment transport and therefore movements in the low water channel within the main channel could be influenced by the operation of the barrier thus altering the location of the navigable channel.

14.6.37 Hydraulic modelling has indicated that the barrier structure (with the gates open) will cause small changes in water level (less than 0.05m) and peak flow velocity (less than 0.3m/s) near to the barrier. There may be impacts on accretion and erosion local to the barrier, but these are not expected to impact on navigation.

14.6.38 Silt may accrete in the barrier gate openings. If this occurs, it will be removed by flushing water under partially closed gates during the regular maintenance operations. This method has been successful at the Hull Barrier.

14.6.39 Upstream of the barrier to Drove Bridge, regime modelling indicates there will be a reduction in channel cross-sectional area due to an increase in sediment deposition on the bed and banks in the long term. The increase in deposition is due to the reduction in the average peak tidal flows in the estuary caused by the barrier closures. The reduction in channel cross-sectional area is estimated to be up to a 4% up to 2055 and up to 5% beyond 2055. The changes in cross section will typically be experienced as a narrowing and shallowing of the cross section. These changes correspond to a change in bed level of 0.1m or less in most places.

14.6.40 Downstream of the barrier to Dunball, the reduction in cross sectional area in the long term is expected to be between 2% and 4%, corresponding to a change in bed level of less than 0.1m in most places.

14.6.41 HR Wallingford concluded that any deposition of sediment during barrier closures is expected to be re-eroded in the short term on the next spring tide but that there may be a small net deposition effect due to the absence of erosion during the closure period of material deposited during neap tides. (HR Wallingford, 2019, see Appendix 6J).

14.6.42 The overall impact of this short-term effect over the longer term depends on the frequency of closures. The short-term effect may be removed over the longer term by further spring tides or may form part of the long-term morphological change as the estuary adjusts to the new hydrodynamic regime resulting from barrier closures. Bridgwater Tidal Barrier Environmental Statement 14- 30

14.6.43 The predicted reduction in channel cross-sectional area and associated changes in bed level are within the natural seasonal variation in bed levels in the estuary, refer to Chapter 8 (Water and geomorphology), Section 8.6, and are not expected to impact on navigation between Dunball and Drove Bridge.

14.6.44 Commercial and recreational river users are considered to be of moderate sensitivity (refer to Paragraphs 14.5.26 and 14.5.36). Up to 2055, the magnitude of the potential impact (i.e. changes to river bed level) during operation is considered to be low. This results in a minor negative (not significant) effect on both commercial and recreational users. The effect will be indirect and permanent.

14.6.45 Changes resulting from barrier closure need to be considered in the context of wider hydromorphological change associated with sea level rise. This, along with inherent uncertainties in the long-term regime modelling, means that there is some uncertainty associated with the predicted effects on sediment processes in the future, beyond 2055, when the frequency of barrier operation for tidal flood risk management will also be higher.

14.6.46 Given the greater uncertainty over the impact on sediment transport from barrier closures in the future (beyond 2055) the magnitude of the effect on navigation before mitigation is considered, on a precautionary basis, as moderate, resulting in a moderate negative effect, which is significant and will require mitigation. The effect will be indirect and permanent.

Monitoring, mitigation measures and residual effects

14.6.47 Monitoring of the system will be undertaken as detailed in the Bridgwater Tidal Barrier Estuary Monitoring Plan (Appendix 8D) with the following objectives:

Record baseline geomorphological change within the Parrett Estuary, i.e. natural change (downstream of Bridgwater)

Establish the influence of the barrier on estuary geomorphology during construction and operation

14.6.48 This monitoring data will be used to develop evidence-based trigger levels to establish where change in bed levels due to barrier operation would adversely impact on navigation. Should these trigger levels be exceeded, the Environment Agency will discuss with the Harbour Authority what remedial action could be taken.

14.6.49 Possible remedial actions to reduce the effects post 2055 include:

Localised channel reprofiling

Operation of barrier for short-term storage and discharge of water (i.e. local flushing to mobilise fine deposited material)

Review barrier operational trigger levels to reduce frequency of operation if appropriate

14.6.50 The AEMP sets out the framework within which the monitoring data will be reviewed so as to be in an informed position from which to take all reasonable steps with the aim of ensuring navigation is not adversely impacted. It will provide a mechanism for trends to be identified, and avoidance and mitigation measures evaluated and implemented, so that any potentially significant negative effects can be addressed Bridgwater Tidal Barrier Environmental Statement 14- 31

wherever possible. The first draft of the AEMP is provided in Appendix 8C. The AEMP is intended to be a live document, which will be updated throughout the lifetime of the BTB Scheme. It will form part of the Operations and Maintenance Manual for the barrier.

14.6.51 The AEMP provides a mechanism for enabling that the uncertainty over potential effects, in particular beyond 2055, to be kept under active review. However, the AEMP does not guarantee that negative effects (should they arise) can be mitigated as some of the possible measures identified at this stage may not always be feasible or effective.

14.6.52 Therefore, on a precautionary basis, the magnitude of the residual effect in the future (beyond 2055) remains moderate, resulting in a direct, moderate negative, permanent effect that is significant.

Impacts on navigation downstream of Dunball due to changes in bed levels

14.6.53 Navigation downstream of Dunball includes commercial navigation to Combwich and Dunball Wharves and recreational navigation largely based at Combwich.

Construction impacts

14.6.54 The temporary works associated with construction of the barrier, including the bypass channel and the cofferdam are 2.2km upstream of Dunball, there are no physical works in the channel near to, or downstream of Dunball. The temporary works are anticipated to have a negligible impact on flow velocities and water levels downstream of Dunball and therefore, a negligible impact on channel bed levels.

14.6.55 Commercial and recreational river users are considered to be of moderate sensitivity (refer to Paragraphs 14.5.26 and 14.5.36), the magnitude of the potential impact (i.e. changes to flow velocities and water levels downstream of Dunball) during construction is considered to be very low, resulting in a negligible (not significant) effect on both commercial and recreational users. The effect will be indirect; and due to the mobile nature of the sediment and likely maintenance of the structure (refer also to Chapter 6 (Proposed Scheme), Section 6.4), any localised sedimentation is expected to be temporary (short-term).

Operational impacts

14.6.56 Regime modelling indicates that in the long term, operation of the barrier will result in a reduction in the channel cross section area at Dunball of approximately 2% and a reduction of less than 0.5% 3km downstream of Dunball. There are negligible effects predicted further down estuary. The changes in cross section will typically be experienced as a narrowing and shallowing of the cross section. Long term changes in the depth of the channel are expected to 0.1m or less in most places, which is considerably less than the natural variation experienced in the estuary.

14.6.57 During a barrier closure, flood tide velocities will be largely unaffected at the estuary mouth, but the effect of closure will increase with proximity to the barrier due to the proportional loss of tidal prism. This reduction in the flood tide velocities could reduce the resuspension of sediment deposited during the tidal cycle. This effect is experienced on neap tides in the estuary and it is expected therefore that sediment would be resuspended on the next set of spring tides and the overall impact on the

Bridgwater Tidal Barrier Environmental Statement 14- 32

bed levels in the estuary in the short term would be negligible. Further details of this assessment are documented in Appendix 6J.

14.6.58 A precautionary approach has been taken, which assumes that natural variation in riverbed levels within the area influenced by the barrier is likely to be in the order of 0.3m (refer to Chapter 6 (Proposed scheme), Section 6.5). There is likely to be considerable spatial variability within this estimate with distance from the barrier site and at approximately 5km downstream of the barrier, the effect of the barrier closures on the incoming flow discharge is negligible, and hence the effect on the associated sediment load, transport and depth variability, outside of natural variability, is considered to be negligible.

14.6.59 Commercial and recreational river users are considered to be of moderate sensitivity (refer to Paragraphs 14.5.26 and 14.5.36). The magnitude of the potential impact during operation is considered to be low due to infrequent full closure of the barrier up to 2055. This results in a minor negative (not significant) effect on both commercial and recreational users. The effect will be indirect; and due to the resuspension of sediment on the next set of spring tides, is expected to be temporary (short-term).

14.6.60 Changes resulting from barrier closure need to be considered in the context of wider hydromorphological change associated with sea level rise. This, along with inherent uncertainties in longer term regime modelling, means that there is some uncertainty associated with the predicted effects on sediment processes in the future. The impact as a probability of occurrence must therefore be assessed as ‘as likely as not’ post 2055. This results in a moderate negative (significant) effect on both commercial and recreational users. The effect will be indirect and permanent.

Monitoring, mitigation measures and residual effects

14.6.61 Monitoring of the system will be undertaken as set out in the Bridgwater Tidal Barrier Estuary Monitoring Plan (Appendix 8D) and the data reviewed within the framework set out in the AEMP. However, whilst this ensures that the uncertainty over potential effects is kept under active review, it does not guarantee that negative effects (should they arise) can be mitigated (Refer to Paragraphs 14.6.47 to 14.6.51).

14.6.62 Therefore, on a precautionary basis, the magnitude of the residual effect in the future (beyond 2055) remains moderate, resulting in a direct, moderate negative, permanent effect that is significant.

14.7 Uncertainties

14.7.1 The assessment of impacts on navigation has been undertaken based upon information available at the time of writing (October 2019).

14.7.2 There are inherent uncertainties regarding the hydromorphology of the river channel, in particular those associated with the long term regime modelling and the unknown effects of sea level rise on the Parrett Estuary system. These uncertainties will be managed through pre-construction and operational monitoring (to be agreed with the Harbour Authority and Sedgemoor District Council) and through the AEMP which provides a mechanism for ensuring that the uncertainty over potential effects, in particular in the future, is kept under active review. The AEMP will provide a mechanism for trends to be identified, and avoidance and mitigation measures Bridgwater Tidal Barrier Environmental Statement 14- 33

evaluated and implemented, so that significant negative effects can be addressed wherever possible.

14.8 Summary

14.8.1 A summary of the effects on navigation is provided in Table 14.8. It is considered that the BTB Scheme can be implemented without any significant environmental effects on the identified navigation receptors during construction. During operation, whilst it is considered likely that any changes to the channel morphology would be within the ranges of natural variation, due to the inherent uncertainties with modelling and predictions of future channel morphology, operational impacts are assessed to be potentially significant both with and without mitigation.

14.8.2 The assessment in this chapter demonstrates that the BTB Scheme proposals are in conformity with the requirements of the legislation and policy, as detailed in Section 14.2 of this Chapter.

Bridgwater Tidal Barrier Environmental Statement 14- 34

Table 14.8 Summary of effects on navigation

Scheme Impact Sensitivity Magnitude Significance Mitigation Magnitude Significance component of of impact and duration of impact and duration receptor(s) before before after of residual mitigation mitigation mitigation effect Barrier Direct effect on Moderate Low Minor Navigation Mitigation Plan developed to Low Minor recreational river negative (not ensure navigational risk through construction negative (not users between significant), are as low as reasonably practicable. significant), Dunball and Drove temporary Measures to include: temporary Bridge during (medium- (medium- construction due to term)  Warning signs advising of the bypass term) the barrier. channel situated at locations remote from, and at, the barrier Barrier Direct effect on Moderate Low Minor Low Minor commercial river negative (not  Aids to navigation such as signs, lights negative (not users between significant), and buoys at and on approaches to the significant), Dunball and Drove temporary barrier site temporary Bridge during (medium-  Ladders at the barrier site to allow (medium- construction due to term) escape from the channel term) the barrier.  Methods to minimise creation of waves during construction  Correspondence with the Harbour Authority and advertisements to advise of construction programme  Ongoing consultation with the Port of Bridgwater Harbour Master, owners and operators of Dunball Wharf, and potentially other groups including owners and operators of Bridgwater Docks during construction

Bridgwater Tidal Barrier Environmental Statement 14- 35

Scheme Impact Sensitivity Magnitude Significance Mitigation Magnitude Significance component of of impact and duration of impact and duration receptor(s) before before after of residual mitigation mitigation mitigation effect  Prevention of flotsam and jetsam from the construction works entering the water  Inspection of the river bed at low tide and removal of submerged jetsam that could cause a hazard  Maintenance of visibility of the cofferdam and bypass channel from the channel by storing plant and materials away from the river banks. Barrier Direct effect on Moderate Low Minor  Detailed design, or procedures Low Minor recreational river negative (not developed during detailed design, to negative (not users between significant), incorporate the following measures to significant), Dunball and Drove permanent ensure navigational risks through permanent Bridge due to the operation are as low as reasonably barrier once it is practicable: operational  Signs advising of the status of the barrier Barrier Direct effect on Moderate Low Minor at locations remote from, and at, the Low Minor commercial river negative (not barrier negative (not users between significant), significant),  Aids to navigation lights at the barrier Dunball and Drove permanent permanent Bridge due to the  Maintenance of lines of site to the barrier barrier once it is by managing vegetation operational  Ladders on the barrier for escape from the channel  CCTV to monitor the ladders and piers immediately adjacent to the ladders

Bridgwater Tidal Barrier Environmental Statement 14- 36

Scheme Impact Sensitivity Magnitude Significance Mitigation Magnitude Significance component of of impact and duration of impact and duration receptor(s) before before after of residual mitigation mitigation mitigation effect  Visual monitoring of the river bed at low water for obstructions  Regular surveys of the river channel for changes in channel alignment and notification of river users  Correspondence with the Harbour Authority and advertisements to advise of planned maintenance closures  The Harbour Master will be able to issue ‘Notices to Mariners’ to inform the public when the gates are being closed. Issuing ‘Notices to Mariners’ informs a number of bodies on issues affecting navigation within the Port and is circulated to commercial vessel bodies, nearby ports and the Royal National Lifeboat Institution  Ensuring adequate warning time is provided to river users, the decision to close the barrier will usually be taken at least 12 hours prior to the closure and will be based on interpretation of the latest tide and surge predictions  Development and maintenance of a published closure procedure plan

Bridgwater Tidal Barrier Environmental Statement 14- 37

Scheme Impact Sensitivity Magnitude Significance Mitigation Magnitude Significance component of of impact and duration of impact and duration receptor(s) before before after of residual mitigation mitigation mitigation effect prepared in consultation with the appropriate authorities  Ongoing consultation with the Port of Bridgwater Harbour Master, owners and operators of Bridgwater Docks and all commercial wharves in the Port; and  Update of the Bridgwater Harbour Operations Manual to include details of the barrier structure and operation. Barrier Indirect impact to Moderate Low Minor Contractors method statements to be Low Minor navigation between negative (not developed to incorporate the following negative (not Dunball and Drove significant), measures: significant), Bridge due to temporary  Regular monitoring and survey of the temporary changes in river bed (short-term) temporary bypass channel and river bed (short-term) levels and hydraulic levels. The monitoring data will be used to conditions during make decisions about whether construction construction of the barrier is causing (recreational and change that is affecting navigation and commercial users) how the change can be managed. This will be discussed and agreed with the Harbour Authority.  Temporary bypass channel to be reinstated with original excavated material, temporary scour protection and piles will be removed

Bridgwater Tidal Barrier Environmental Statement 14- 38

Scheme Impact Sensitivity Magnitude Significance Mitigation Magnitude Significance component of of impact and duration of impact and duration receptor(s) before before after of residual mitigation mitigation mitigation effect Barrier Indirect impact to Moderate Up to Up to 2055:  Monitoring of the system will be Up to 2055 Up to 2055 navigation between 2055: Low Minor undertaken as detailed in the BTB Estuary n/a n/a Dunball and Drove negative (not Monitoring Plan (Appendix 8D) and the Bridge due to Beyond significant), processes set out in the AEMP (Appendix changes in river bed 2055: permanent 8C) implemented. levels and hydraulic Moderate  This monitoring data will be used to conditions during Beyond 2055: establish where changes in bed levels may Beyond Beyond 2055: operation Moderate result in adverse impacts on navigation. If 2055: Moderate (recreational and negative adverse impacts occur the Environment Moderate negative commercial users)) (significant), Agency will discuss with the Harbour (significant), permanent Authority if remedial action is required and, permanent if so, what actions could be taken. Possible mitigation measures are identified in the AEMP. Barrier Indirect impact to Moderate Very low Negligible (not No mitigation proposed. - - navigation significant), downstream of temporary Dunball during (short-term) construction (recreational and commercial users) Barrier Indirect impact to Moderate Up to Up to 2055:  Monitoring of the system will be Up to 2055 Up to 2055 navigation 2055: Low Minor undertaken as detailed in the BTB n/a n/a downstream of negative (not Estuary Monitoring Plan (Appendix 8D) Dunball during significant), and the processes set out in the AEMP operation permanent (Appendix 8C) implemented.

Bridgwater Tidal Barrier Environmental Statement 14- 39

Scheme Impact Sensitivity Magnitude Significance Mitigation Magnitude Significance component of of impact and duration of impact and duration receptor(s) before before after of residual mitigation mitigation mitigation effect (recreational and  This monitoring data will be used to commercial users) Beyond Beyond 2055: establish where changes in bed levels Beyond Beyond 2055: 2055: Moderate may result in adverse impacts on 2055: Moderate Moderate negative navigation. If adverse impacts occur the Moderate negative (significant), Environment Agency will discuss with the (significant), permanent Harbour Authority if remedial action is permanent required and, if so, what actions could be taken. Possible mitigation measures are identified in the AEMP.

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