Bridgwater Tidal Barrier Scheme WFD Compliance Assessment

Environmental Statement Appendix 8A

November 2019 General description of works and key assumptions

Barrier description: 1 The tidal barrier will be a surge barrier (not a tidal exclusion sluice) with a design life of 100 years. During normal conditions it will be fully open and flows will not be impounded in either direction. The tidal barrier will be located at NGR ST302390, the general arrangement of the barrier is provided on Drawing Number ENVIMSW002039-CH2-FBS-T5-DR-C-7020 and the site location plan on Drawing Number ENVIMSW002039-CH2-FDE-T5-DR-LP-7010.

The barrier type will be a vertical lift gate. There will be 2 gates, supported by towers on each bank and with one central pier in the channel. Both gates will be operated together except for gate operation checks. The towers will be concrete, 2 supported on mass concrete or piled foundations to 19-27m below ground level. The detailed design of the central pier will minimise its footprint and hydraulic influences. The gates will close on to a concrete cill in the river bed, which will be retained by sheet piles in the bed. Cill flushing (open gates) to clear sediment is expected to be required monthly if the barrier has not operated for flood risk, potentially more 3 frequently during times of the year when sediment loads are high. In addition to the gate closure cill, the river bed will be protected from erosion by a concrete stilling basin extending 13.5m downstream and upstream from the gates. Rock rip-rap will be used to protect the bed for 15m upstream and downstream 4 of the cill and stilling basin. The complete length of built structures along the channel is approximately 60m and this extends across the channel width. A 20m long sheet pile retaining wall with concrete capping beam will extend upstream and downstream from the pier on the left and right hand banks and will tie into the existing flood embankment.

Barrier operation - when the barrier first becomes operational: Barrier operation will be triggered by a projected surge tide with a pre-determined height trigger. It would close at low water and remain closed for a maximum of 6 hours during a high tide, re-opening once tidal and fluvial levels either side are equal as the tide ebbs. The frequency of barrier operations when the barrier first becomes operational is anticipated to be as follows: - An average of 1 to 5 barrier closures per year for tidal flood risk management (anticipated maximum of 6 hours duration). - Up to 5 barrier closures per year for testing and training (approx. 4-5 hour duration), timing dependent on previous closures of greater than 4 hours. The main assumption here is that sufficient time is allowed between full closures for testing or 5 training purposes, or between a closure for tidal flood risk management and a full closure for testing or training purposes, to allow for some self-regulation of the channel. A conservative period of 12 weeks is considered to be appropriate until monitoring data is available to refine this estimate. - An average of 12 to 24 barrier closures per year for maintenance purposes (approx. 1 hour duration).

It is anticipated that the greatest number of barrier closures are likely to be during the months of February and March. Some of the barrier closures for tidal flood risk management will be over consecutive tides. Analysis of records indicates that this could be for between 2 and 4 consecutive tides and that overall approximately 50% of the barrier closures will occur on consecutive tides.

Barrier operation - future (climate change) scenarios: In the future, the frequency of operation for flood risk management is predicted to increase due to sea level rise. As operations for tidal flood risk management increase, the number of barrier closures for maintenance purposes decreases (as maintenance checks can be completed during the flood risk management operations). The frequency of barrier operations by 2055 is anticipated to be as follows: - An average of 2 to 8 barrier closures per year for tidal flood risk management (maximum of 6 hours duration). - Up to 3 barrier closures per year for testing and training (approximately 4-5 hour duration), timing dependent on previous closures of greater than 4 hours. The period between closures will be informed by the monitoring undertaken during the earlier epoch and may be shortened from 12 weeks if it is shown that there is no detrimental impact to the estuary regime. 6 - An average of 10 to 20 barrier closures per year for maintenance purposes (approximately 1 hour duration). The frequency of barrier operations by 2124 is anticipated to be as follows: - An average of 20 to 34 barrier closures per year for tidal flood risk management (maximum of 6 hours duration). - An average of 6 to 12 barrier closures per year for maintenance purposes (approximately 1 hour duration).

The number of barrier closures for tidal flood risk management across consecutive tides is anticipated to increase in the future. By 2055, it is anticipated that approximately 67% of barrier closures will be across consecutive tides and this could be for between 2 and 8 consecutive tides. By 2124, it is anticipated that approximately 85% of barrier closures will be across consecutive tides and this could be for up to 9 consecutive tides.

Tab 1. WFD Assumptions Fish Passage: An options appraisal has considered the viability of various fish passage design options within the setting of the barrier (APEM, 2018). This found that there was only one possible multi-species design solution, comprising fixed orifices in the barrer gates/structure. All other designs were unviable due to operational, environmental or construction related constraints which could not be overcome. The fixed orifice design was further investigated, and after detailed consideration by the Environment Agency's Fisheries Technical Sub-group (set up for this BTB Scheme) this option was discounted due to low confidence in the effectiveness of this type of measure, as follows: - Environment Agency fisheries technical specialists have noted that it is an untested approach in a tidal situation such as this, and there is concern that the fresh water indicator will not be achieved and therefore will be unattractive to fish. - Environment Agency research into this type of solution/ evidence from other penstock/ orifice installations shows mixed reports on efficiency. - There is concern that the orifice sizing is problematic. The larger the orifices become the greater the potential of increased flood risk upstream and increased leakage. 7 - The Environment Agency operational team have also raised a range of concerns over the operation and maintenance difficulties this solution would present, based on their understanding and experience of working in this challenging estuarine environment, i.e. high turbidity levels/ large tidal ranges, which have the potential to cause high rates of failure/ high maintenance needs and fundamentally health and safety concerns for operatives. The option of providing a single species solution for eels was also discounted for similar reasons.

The Environment Agency has instead identified suitable works to improve 12 sites upstream of the barrier where there are existing delays in migration. These improvement works will mitigate for the temporary delays in fish/eel migration when the tidal barrier is closed.

The proposed works at these 12 sites have been shown to be WFD compliant; refer Chapter 18 of the Environmental Statement for this scheme.

Downstream Flood Defences: The existing downstream flood embankments will continue to be maintained, as a separate project(s), in accordance with the Parrett Estuary Flood Risk Management Strategy (PEFRMS). 8 Raised primary and new secondary flood defences will be built as part of this project to protect local communities around Combwich, Chilton Trinity and Pawlett. The site plan for the downstream defences is provided in Drawing Number 122507- BVL-FEV-SW-DR-C-00017. They will be raised/built landward of the estuary so as not to impact on the intertidal zone. Alignments of the secondary defences have sought to minimise impacts on drainage.

Construction: The construction process will involve the following in-channel components over a period of approximately 3 years: - A temporary bypass channel consisting of a 20m wide, 200m long, rectangular cross-section and flow guidance bunds upstream and downstream of the diversion, to bypass flows around the barrier site (approximately half of the capacity of the Parrett). 9 - Rip rap will be placed to provide scour protection through the narrower sections of the bypass channel and in the transition areas (it is assumed that this will include areas in the River Parrett as appropriate). - During reinstatement the river bypass channel will be filled with stockpiled excavated material and the temporary stabilisation / scour protection piles will be removed. It is not known at this stage how the River Parrett banks will be reinstated however, for the purposes of this assessment, it is assumed that this will ensure structural stability, whilst promoting sediment accretion and avoiding hard/soft transitions with adjacent river banks. (For example, reinstated banks to be composed from a mix of both hard (rip-rap) overlain with a soft (e.g. brushwood mattress) engineered solution.) - Construction of jetties and a cofferdam to allow the barrier to be constructed in the dry.

Temporary construction impacts: Consideration has been given to permanent impacts of the scheme. Temporary construction impacts are not usually considered to present a risk to WFD compliance due to their temporary nature, however it is acknowledged that working within the 10 estuary bed and banks for four years during construction may cause permanent disturbance of sediment and use of the bypass channel will alter flows which could have a permanent impact on estuary morphology, therefore effects relation to potential permanent effects from these specific construction related issues is included.

Upstream designated sites: These are located upstream of the proposed works and are hydraulically linked via the River Parrett and Kings Sedgemoor Drain. Water in the Somerset Levels and Moors is regulated under WLMPs whereby water levels are controlled through a network of water level control structures. In some moors, where gravity drainage is not possible during floods, pumps operate when water levels are above the agreed pen levels within the WLMP area. At present, during high fluvial flows which 11 coincide with storm/surge tides, some pumps will briefly shut off at peak tide to avoid water in the river channel overtopping the flood banks. In this situation the closed barrier will prevent spring major storm tides reaching the Somerset Levels and therefore the pumps will continue to operate as agreed to remove excess floodwater from the moors. Under such conditions, pumps could in theory be allowed to operate for longer. However, in practice, whether or not this occurs will be determined by the operating protocol on the moors (i.e. the WLMP). Any impact on the frequency of overtopping into the moors, which is driven by fluvial flow and not tide levels, will be negligible.

Tab 1. WFD Assumptions Downstream designated sites: The tidal barrier is included within the overarching Parrett Estuary Flood Risk Management Strategy (PEFRMS), for which a Habitat Regulations Assessment (HRA) has been undertaken to explain how the integrity of the designated sites within and adjacent to the strategy area will be maintained, or where affected, mitigation and compensation will be made. The PEFRMS HRA notes that further assessments will be made at Scheme Appraisal stage through Environmental Impact Assessments (EIA). The PEFRMS HRA found there will be an adverse effect on the internationally designated sites, where 7ha of Atlantic salt meadow habitat fronting the defences will be lost in the short to medium term and a further 39ha will be lost in the long 12 term due to coastal squeeze (i.e. the process whereby coastal habitats are trapped between a fixed landward boundary and the sea as a result of rising sea levels or increased storminess). A Statement of Case (SoC) was prepared to demonstrate Imperative Reasons of Overriding Public Interest (IROPI), which Defra and Natural England supported. This concluded that none of the alternative solutions identified in the PEFRMS could be considered to be sustainable over the long term, or were likely to produce lesser adverse effects than the preferred Strategy. The SoC also concluded that the preferred strategy is required for IROPI as it addresses the unacceptable risk to human health and public safety from an uncontrolled breach of the existing flood defences. The SoC outlines the compensation requirements for coastal squeeze as a result of the schemes within the strategy and within the PEFRMS has made provision for compensatory measures, including the provision of at least 46ha of Atlantic salt meadow before losses from coastal squeeze are incurred. Compensatory Atlantic Salt meadows have been created at Steart and are due to be delivered at Pawlett Hams.

Invasive and Non-Native Species (INNS): In over 7 different habitat surveys for the BTB Scheme no aquatic invasive species have been identified at the site of the barrier or upstream and downstream within the surveyed areas. Himalayan balsam and Japanese knotweed are known in the 13 catchment but not recorded during surveys. Because of the tendency for both these species to spread along watercourses, there is potential for them to be present in future. Due to the location on the barrier in the river, there is a risk of INNS colonizing in the period between the baseline and construction. Therefore, an INNS survey and a management plan will be implemented during construction to manage risk to ensure no spread. As there is no pathway for effects, INNS are not considered further within this assessment. Fish species assessed: The Severn Estuary SAC lists three Annex II species as features, i.e. sea lamprey, river lamprey and twaite shad. Seven species are listed as features of the Severn Estuary Ramsar site, i.e. sea and river lamprey, allis and twaite shad, Atlantic salmon, European eel and sea trout. In addition, the wider estuarine fish assemblage is covered as a ‘notable species assemblage’ sub feature of the SAC and Ramsar ‘Estuaries’ features.

There is no record of a spawning population of twaite shad within the River Parrett (Hillman, 2003). Available EA data indicate sea and river lamprey are not likely to be present within the Parrett catchment (any lamprey individuals recorded are 14 considered likely to be brook lamprey; Pang M, Environment Agency, pers. comm .) and so we have not assessed further. There is no spawning population of shads in the Parrett / Tone catchments and so the project will not inhibit migration of this species. Shads are also not species which comprise either the river fish classification (FCS2) or the transitional fish classification (TFCI) under the WFD. We have appropriately considered shad within the HRA for the project.

Therefore, the only species requiring consideration in this assessment are Atlantic salmon, sea trout and European eel. The potential impact of the barrier on upstream and downstream passage of salmon, sea trout and eel has been assessed in detail within Chapter 9 (Biodiversity) of the Environmental Statement.

Modelling: 15 Hydraulic modelling and a high level assessment using a regime modelling approach (regime theory) has been undertaken to provide an assessment of the potential effects of the BTB Scheme on the flows, sediment processes and channel morphology in the River Parrett. The outcomes of the modelling work is provided in Tab 1a of this assessment.

Impacts on eels from control structures required for the downstream defences: The design of control structures at locations where the secondary flood defences cross drainage ditches has been reviewed to ensure that there is no impact on any future use by eels. Proposed control structures comprise either penstocks or flap valves depending on the normal flow direction compared with flood flow direction, with a preference for passive controls (flap valves) to minimise operational requirements. Penstocks will be closed in times of flood and at other times will remain 16 open. There will therefore be no impact on the ability of eel to pass these structures as eels will not be migrating during floods (Pang, M, Environment Agency Fisheries Pers Comm). Flap valves will close automatically in times of flood and at other times will allow normal flows to pass. Under very low flow conditions, which will often be the case in these locations, flap valves may not open sufficiently to allow eels to pass. However these structures are located towards the edge of the floodplain, i.e. generally only drain 1 or 2 fields. Therefore, due to the limited quantity (length) and quality (low flow, poor water quality) of habitat available upstream of the flap valves further measures to allow passage for eel are not necessary as it is not desirable to encourage eels into these areas (Pang, M, Environment Agency Fisheries pers. comm) .

Reference: APEM (2018), Bridgwater Tidal Barrier: Fish Passage Options Appraisal APEM Technical Note.

Tab 1. WFD Assumptions In-combination effects The table below includes a list of projects identified within the screened-in water body, and considered for potential in combination effects for WFD. Projects listed below have been scoped out if there are no potential in-combination effects. Screened in projects (shown in bold below) are considered in more detail in the In-combination Assessment Table (tab 6). EA projects Projects by others Parrett Internal Drainage Board (IDB) Water Injection dredging (WID) upstream of Bridgwater. This 5-year protocol will be developed into the Bridgwater Tidal Barrier (this project) Somerset Rivers Authority Dredging Programme. EDF Energy Hinkley Point C Nuclear Reactor & associated projects. Taunton Flood Defence Scheme (scoped out as only smaller elements of the scheme may be constructed in the next 10 years, larger elements (e.g. potential storage reservoir) may not be constructed for another 30 years. Furthermore, any WFD in- combination effects considered unlikely given distance from the BTB Scheme). Tidal Lagoon Power Projects (scoped out as project is on hold with no known date of implementation). Potential maintenance of existing defences along the River Parrett, as outlined in the PEFRMS (scoped out as details about the locations, scale and programme of these potential works are not known and so it is not possible to include them within this assessment. Any future projects to raise and strengthen defences will be subject to their own assessment which will need to take into account BTB Scheme construction and/or operation as appropriate). Potential options for maintenance and timing of works should be spread over a number of years if not emergency works.

River Sowy and Kings Sedgemoor Drain Capacity Improvements (scoped out as desilting works along the Kings Sedgemoor Drain are complete and the Cannington Flood Alleviation Scheme (scoped out as scheme will be in place at the time of the BTB Scheme construction so forms remainder of any works must be completed by 2021 due to funding constraints; works will therefore be complete before construction commences on part of the baseline environment) the BTB Scheme).

Cannington Bends Flood Risk Management Scheme (scoped out as measures will be in place at the time of the BTB Scheme Avonmouth Severnside Enterprise Area (ASEA) Ecology and Mitigation Flood Defence Project. construction so forms part of the baseline environment)

Bristol Port Company Deep Sea Container Terminal (scoped out as works have been delayed until global economic conditions improve; it is therefore assumed that construction would commence after the completion of the BTB Scheme).

Colley Lane Southern Access Road project (scoped out as road will be in place at the time of the BTB Scheme construction so forms part of the baseline environment).

Land at former Royal Ordnance Factory (BAE Systems), Puriton (now known as Gravity) (scoped out as it is an application for an energy park >1km from the Parrett Transitional water body, and unlikely to have in-combination effects with the BTB Scheme).

Land off A38 Bristol Road and A39 Bath Road, Bridgwater (scoped out as it is an application for residential development >0.5km from the Parrett Transitional water body, and unlikely to have in-combination effects with the BTB Scheme).

Land to the East of Kings Road, Bridgwater (scoped out as it is an application for residential development >1km from the Parrett Transitional water body, and unlikely to have in-combination effects with the BTB Scheme).

Land to the West of Kings Road, Bridgwater (scoped out as it is an application for a mixed use development >0.5km from the Parrett Transitional water body, and unlikely to have in-combination effects with the BTB Scheme).

Land South of Little Sydenham Farm, Bridgwater (scoped out as it is an application for residential development >1km from the Parrett Transitional water body, and unlikely to have in-combination effects with the BTB Scheme).

Land to South East of Bridgwater Community Hospital (scoped out as it is an application for mixed use development >1km from the Parrett Transitional water body, and unlikely to have in-combination effects with the BTB Scheme).

Land at Cokerhurst Farm, Bridgwater (scoped out as it is an application for mixed use development >2km from the Parrett Transitional water body, and unlikely to have in-combination effects with the BTB Scheme).

Land to the South of Quantock Road, Bridgwater (scoped out as it is an application for residential development >2km from the Parrett Transitional water body, and unlikely to have in-combination effects with the BTB Scheme).

Land North of Grange Farm, Cannington (scoped out as it is an application for residential development >2km from the Parrett Transitional water body, and unlikely to have in-combination effects with the BTB Scheme).

Tab 1. In-combination effects HIGH LEVEL ASSESSMENT OF POTENTIAL EFFECTS ON MORPHOLOGY

Hydraulic modelling and a high-level assessment using a regime modelling approach (regime theory) have been undertaken to provide an assessment of the potential effects of the BTB Scheme on the flows, sediment processes and channel morphology in the River Parrett. The regime theory approach was adopted to investigate the effects of the barrier on channel morphology in the River Parrett. This high-level assessment makes use of outputs (cross-section area and discharge) from a one-dimensional (1- D) Flood Modeller hydraulic model. The flow model was used to simulate a 14-month period of hydrodynamics with and without the proposed barrier scheme in place and the high-level assessment using regime theory was undertaken using the model outputs. Regime theory assumes that the estuary channel morphology (specifically cross-section area) is in quasi-equilibrium with the long-term hydrodynamic forces (specifically peak discharge or tidal prism). A regime relationship between peak discharge and cross-section area was developed for the Parrett Estuary based on the results of the 1D hydraulic model. This work has been used to develop a preliminary understanding of the implications of the scheme on geomorphological processes, which has informed this WFD assessment for consideration of hydromorphological effects. Further details about this assessment are documented in Chapter 6 of the ES and Barrier Morphological Impact using Regime Theory (Jacobs, 2019).

River flows When the barrier is closed, tidally driven flows up the River Parrett upstream of the barrier will be prevented. Current tidal velocities in the vicinity of the barrier are 1 metre per second (m/s) to 2m/s over a tide cycle, and this will be reduced to zero 1 immediately upstream and downstream of the barrier when it is closed. Reduced velocities will occur during both the barrier closure period and during the subsequent ebb tide after the barrier has re-opened and will extend approximately 5.5 km upstream of the barrier (just upstream of the M5), and approximately 5 km downstream of the barrier (near Pawlett). During current conditions, the River Parrett upstream of the M5 (approximately 5.5km upstream of the barrier) is rarely influenced by salt water intrusion. Only during extreme tides does some weak salt water intrusion extend as far upstream as 2 Burrowbridge (13km upstream of the barrier). During a barrier closure the upstream movement of salt water from the incoming tide is prevented. This will temporarily reduce the salinity upstream of the barrier. During those extreme tides when the barrier is closed, the frequency and extent of this weak salt water intrusion will be reduced. During current baseline conditions, freshwater flows downstream but is slowed by the tide and, during high tides, can reverse upstream on an incoming tide. When the barrier is closed, freshwater flows moving downstream of the barrier will be 3 temporarily stopped. There is a reduction in flow velocities both upstream and downstream of the barrier, as discussed above. Freshwater flows will still move downstream but flow velocities are temporarily close to zero in the vicinity of the barrier. At Burrowbridge, 13km upstream of the barrier, closure of the barrier will prevent reversal of the freshwater flow direction that occurs on the flood tide under current (baseline) conditions. With a barrier closure, average velocities are reduced over 4 a greater length of channel and by a greater degree upstream of the barrier than downstream, resulting in an increased tendency for settlement of sediment, although the tidally driven transport of sediment into the river upstream of the barrier is prevented on closure (refer also to ‘Sediment processes’, below). The River Parrett also experiences a small tidal wave, known as a bore, which currently passes through Bridgwater on the highest spring tides. Operation of the barrier would stop the bore passing upstream of the structure on some of these high 5 tides. Detailed designs will seek to ensure that, when open, the impact of the barrier on river flows is minimal. This will be achieved by ensuring the dimensions of the new structure reflect as closely as possible the existing channel size and flow 6 characteristics. Sediment processes High spring flood tides are particularly important for sediment transport, with peak suspended sediment concentrations (SSC) occurring on these extreme tides due to increased mobilisation of sediment stored within Bridgwater Bay. This makes the 7 timing of closure of the barrier particularly important with regard to the overall sediment budget of the Parrett upstream of the barrier. The barrier is most likely to be closed for FRM closures during the more extreme high spring tides, therefore there is likely to be a reduction in peak SSC up estuary of the barrier during closure and a potential increase down estuary; impacting on the 8 overall sediment budget of the Parrett. During current baseline conditions, sediment is deposited in an estuary during periods of slack tide and is mobilised during periods of high flow. The flood tide is relatively rapid in the River Parrett estuary and tends to mobilise sediment and carry it up estuary. Conversely the ebb tide is slower and so less deposited silt is mobilised and carried down estuary. Because of the tidal asymmetry there tends to be a landward movement of tidally driven sediment. This process is referred to as ‘tidal pumping’. However, high fluvial flows in the winter carry sediment downs estuary and it is this balance between tidal pumping and fluvial movement of sediment which drives the sediment regime. Up and down estuary flows converge at some 9 point, with the down estuary flows having a significant fluvial component. Sediment concentrations tend to increase at this convergence, causing a turbidity maximum (TM) and increased sedimentation (due to the low velocities and high sediment concentrations). Sedimentation is directly related to the concentration of suspended sediments, which is highest near to the TM. The geographical location of the TM is dependent on river discharge and tidal conditions at any given time. It can vary on a daily basis but a significant pattern has been observed seasonally in the Parrett (typically summer vs winter), Up estuary of the barrier, the channel is increasingly dependent on high fluvial flow events to maintain the artificially constrained equilibrium cross-sectional area and monitoring data has shown that ebb flow entrainment combined with the 10 scouring effect that occurs during larger fluvial flows is important in maintaining the overall estuary regime morphology and depth. Barrier closure will lead to a reduction in the incoming flood tide sediment load (SSC) up estuary, due to the blockage of the highest incoming tides. In addition, sediment carried in suspension up estuary of the barrier and within freshwater flows will deposit particularly within the location of the TM zone, which precedes closure due to reduced outgoing ebb flow velocities up estuary of the barrier. In the long-term, this would give rise to a reduction in cross sectional area in the reach immediately upstream of the barrier that is proportionate to the reduction in average peak flood discharge. The predicted reduction in cross sectional area is 4% for 13 closures in a 14 month period (typical of the average number of closures 11 expected in the period up to 2055) and up to 5% for 32 closures (typical of the average number of closures expected during the period 2055 to 2124)). The SSC that is cut off by the incoming flood tide is expected to be much larger than the river sediment load, which will temporarily reduce the available SSC volume wihin the TM zone up estuary of the barrier during closure. As the incoming flood tide sediment transport is more dominant, the overall impact on channel cross-section area upstream of the barrier will be dominated by the reduction in the incoming flood tide SSC. 12 Down estuary of barrier closure, the overall effect on sediment processes is anticipated to be an increase in deposition, due to the reduction in flow velocities (refer also to ‘River flows’ above).

Tab 1a. Assessment of effects At approximately 5 km 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 is negligible. Closer to the barrier, as flow discharge reduces, the incoming sediment load is deposited at a rate that is higher than the existing situation. Furthermore, the deposited material will have longer to consolidate, and ebb velocities are lower following barrier closure, so that when the barrier is open, the 13 rate of resuspension will be expected to be less than the existing situation. This leads to a tendency for deposition downstream of the barrier until new equilibrium cross-section area is reached. The increase in deposition is estimated to result in approximately 5% reduction in the equilibrium cross-section area close to the barrier, reducing to less than 0.5% about 5 km downstream. For the barrier closures investigated (13 closures and 32 closures over 14 months), the increase in deposition and therefore reduction in average river cross-section areas is predicted to extend approximately 5 km downstream of the barrier. 14 Furthermore, the channel cross-section area is expected to experience a small proportionate reduction in accretion over approximately 5.5 km upstream of the barrier (to the M5) in the long term. This is due to blockage of the sediment load with the highest incoming tides, which is much larger than the fluvial sediment load. Further upstream and downstream, the effect of closures on river cross-section areas is expected to be negligible. As well as depositing material on the river bed, the River Parrett demonstrates cycles of channel narrowing as sediment builds up on the river banks within the intertidal areas. This material is known to be very cohesive and is not easily eroded away. 15 It can therefore accumulate over time on the river banks and can stabilise further through vegetation growth. However, once this accumulated bank material becomes too steep, it will collapse and a dynamic equilibrium state is maintained. Empirical evidence indicates that zones of very high sediment concentration (fluid mud) can develop close to the bed, and the local effect of the barrier on such processes may be greater, especially with higher frequency of operation in the future 16 (beyond 2055). However, the current understanding of locations, processes and risks posed by changes in fluid mud on the Parrett Estuary is very limited and such changes may fall within the realms of natural variability. Seasonal average natural river bed level changes between 9km and 13km upstream of the barrier have been found to be in the order of up to 300mm (derived from Parrett Internal Drainage Board, 2017). Approximately 14km downstream of the 17 barrier in the vicinity of the Steart Marshes managed realignment scheme, bed levels have been observed to vary by 500mm to 2000mm, with the larger changes being due to the movement of the low water channel (refer also to ‘Channel morphology’ below) (Halcrow, 2015). A precautionary approach has been taken for this assessment, which assumes that natural variation in river bed levels within the area influenced by the barrier closures is in the order of 300mm. This may, however, be greater due to the influence of 18 tidal waters with higher sediment concentrations; this approach is therefore conservative. Thus, the predicted long-term change in bed level is less than the natural variation in bed levels of 300 mm. By 2055 and 2124, the increased number of barrier closures required for flood risk management, and the potential for closures to be consecutive over up to 8 or 9 tides respectively, is anticipated to result in greater increases in deposition. The 19 predicted increase in deposition is less than the natural variation in bed levels of 300 mm, as discussed above. Expert judgment suggests that the geographical scale of influence in these future scenarios would not significantly change from that anticipated when the barrier first becomes operational. Closure frequencies are anticipated to increase in the 20 future in accordance with the rate of sea level rise and it is likely that the number of consecutive tides when the barrier would be closed will also increase (refer Tab 1). There would also be changes in geomorphology associated with sea level rise which is also uncertain. 21 Detailed design of the barrier will seek to ensure that, when open, the impact of the barrier on hydraulic and sediment processes is minimal. Channel morphology The Parrett Estuary has a high tidal range (>10 m), which means that extensive areas are exposed at low water. It also has a meandering low water channel, whose movement causes localised erosion and deposition. This low water channel is 22 thought to be sensitive to changes in the interaction of the tidal and freshwater flows and resulting sediment transport; further movements in the low water channel should therefore be expected in the future and could be triggered by the operation of the barrier. Surveys have however, shown that the position of the low water channel changes significantly over time due to natural processes. Construction effects The construction of the barrier has the potential to influence channel flows and channel morphology. The general construction methodology is provided in Chapter 6 of the ES and will include in-channel temporary works including construction of a 23 cofferdam and a temporary bypass channel. It is likely that these activities will create some temporary localised effects on flow, although these impacts can be managed as discussed below. Modelling of the temporary works indicates that the changes in flow will cause a localised increase in velocity which could cause local scour and deposition within approximately 500 m upstream and downstream of the barrier. The wider impacts are 24 likely to be within the range of natural variation and would be expected to return to natural levels following construction. This process is likely to be quite slow (i.e. in the order of months) (as explained in ‘Sediment processes’, above). The detailed construction methodology and design of temporary works will aim to minimise the impact on channel flow, sediment processes and morphology. Hydraulic modelling will consider all phases of construction to minimise the impact, 25 building on the modelling that has been undertaken. Local scour and increased sediment deposition will be managed as part of construction and will include local bed and bank protection and potentially localised channel reprofiling to return the channel to its pre-construction condition. Reduced surface water flooding (operation) 26 The scheme has been designed to provide a 1 in 200 year standard of protection to the built environment until 2125. The barrier will be closed in times where surge tides are forecast. The Somerset Levels and Moors Natura 2000 site is located upstream of the proposed works and is hydraulically linked via the River Parrett and Kings Sedgemoor Drain. Water in the Somerset Levels and Moors is regulated under Water Level 27 Management Plans (WLMPs) whereby water levels are controlled through a network of water level control structures. Water levels and associated habitats in the site are not reliant on river flooding for water. In some moors, where gravity drainage is not possible during floods, pumps operate when water levels are above the agreed pen levels within the WLMP area. At present, during high fluvial flows which coincide with spring tides, some pumps will briefly shut off at peak tide to avoid water in the river channel overtopping the flood banks. Modelling has shown that when the barrier is closed, pumping 28 from the moors will be slightly more effective (refer to ES Appendix 9D, HRA Appendix F). However whether pumping is undertaken is driven by the WLMP and associated protocols. Therefore the barrier may provide some additional water control flexibility but does not impact on the Somerset Levels and Moors Natura 2000 site. 29 As flooding into the moors is mainly due to high fluvial flows the impact on flood water onto the moors associated with the barrier is negligible. Refer also to ES Appendix 9D, HRA Appendix F. 30 The improvements to the downstream flood defences will increase the standard of protection provided against tidal flooding to communities and properties downstream of the barrier, whilst maintaining the existing standard of protection to land.

Reference: Jacobs, 2019a. Bridgwater Tidal Barrier: Barrier Morphological Impact using Regime Theory, Memorandum, March 2019

Tab 1a. Assessment of effects STEP 1 - Identify WFD water bodies in study area (including upstream and downstream) and confirm which require detailed assessment.

Current A/ HMWB Water body Name Water body ID WFD status Screening and reasoning Designation (2015)*

Screened in: The construction and operation of a tidal barrier within the waterbody has potential for direct and indirect effects on all BQEs and supporting elements presenting a risk of deterioration, and also on delivering any required WFD mitigation measures to meet future objectives.

There is a potential effect on the Severn Estuary Natura 2000 site (which is approximately 4.3km downstream from proposed barrier location). Effects on the designated site have been assessed in detail as part of a Habitats Regulations Assessment for the BTB Scheme.

Moderate Possibility to affect fish passage depending on the frequency of operation of the tidal barrier as a result of habitat loss, increased turbidity, disturbance and changes to water Parrett (Transitional) GB540805210900 Yes - HMWB Potential chemistry.

Linked protected areas include Bathing Water Directive, Conservation of Wild Birds Directive, Habitats Directive, Nitrates Directive and Urban Waste Water Treatment Directive.

Risks of failures in WFD compliance are in relation to Chemicals and Metals (risk of non-compliance post-2015), Invasives (risk of deterioration by 2015 and 2050); Abstraction and flow (risk of deterioration in status at full licensed pressure and risk of not supporting Good status at full licensed pressure), and eutrophication (risk of becoming less than good ecological status post 2015).

Screened out: Operation of the barrier will not measurably increase the peak tide level nor extend the duration of high tides downstream of the barrier. The barrier is located well upstream in the estuary where the impact of preventing tides passing upstream has a negligible impact on tide levels downstream which are instead dominated by the natural estuary hydraulics. Pims Pill is the nearest downstream water body, approximately 0.5km downstream but it is not designated under the WFD and is not passable to fish from the estuary due to the presence of a penstock. The next nearest downstream water body, and water body potentially passable to fish, is King’s Sedgemoor Drain, approximately 2.25km River water bodies downstream downstream of the barrier. of the barrier (King's Sedgemoor Yes - Moderate Drain - Henley Sluice to mouth GB108052021150 Artificial Potential Compared to the no barrier situation, when the barrier is in operation, for some tides there will be a period between 3-4 hours into the flood tide when water levels are and all further downstream higher downstream of the barrier by up to 200mm . By this point in the tide any flap valves would have already closed. water bodies) This difference becomes negligible near the peak of the tide when, without the barrier, the highest levels would have equalised. On the ebb tide the difference in water levels downstream of the barrier is none.

The changes in water levels downstream of the barrier during operation will therefore have no effect upon the ability of fish to migrate into or out of the downstream water bodies. Therefore, effects on fish are scoped out of the EcIA.

Tab 2. Identify Waterbodies Current A/ HMWB Water body Name Water body ID WFD status Screening and reasoning Designation (2015)* Screened in for fish passage only: Water body is located upstream of the Parrett Transitional water body extent and extends up to Langport. The water body is classified as HMWB and currently classified as moderate potential. The water body is heavily embanked (for flood risk purposes) and there are a number of controlled offtakes and spillways which connect the channel to the adjacent moors (i.e. West Sedgemoor and Aller Moor, which are part of the Somerset Levels and Moors Natura 2000 site). As flooding into the moors is mainly due to high fluvial flows, the impact on flood water on the moors associated with the barrier is negligible. The barrier when in operation for flood risk management may allow for Moderate more effective pumping from the moors following a flood event, but this will be controlled by operating protocols which will take into account requirements of water level Potential Parrett - R Yeo to West management plans for the moors. GB108052015470 Yes - HMWB and Sedgemoor Drain (River) Chemical Upstream fish migration through the tidal barrier may affect fish populations within the water body, eels in particular, during periods when the barrier is closed. There are Failure no significant hydromorphological or physico-chemical impacts predicted on the upstream water bodies as the extent of influence of the barrier becomes less further upstream of the M5, therefore any effects will be indirect. Indirect impacts on WFD status in this water body in terms of risk of changing supporting conditions for BQEs is likely to be negligible and short-term, however, there is the potential for indirect effects on fish in terms of effects on migration, or loss of population by mortality. As a result effects on fish have been screened in for further assessment. Indirect effects on BQEs will be considered in the detailed assessment for the Parrett (Transitional) water body (Tab 5). Screened in for fish passage only: This water body is part of the River Tone, upstream of the tidal section of the Parrett and extending upstream through Taunton. The water body is classified as HMWB and is managed for flood risk and environmental purposes (to support habitat within the Somerset Levels and Moors Natura 2000 site). The river is heavily embanked and there are a number of controlled offtakes and spillways which connect the channel to the adjacent moors. The tidal barrier at Bridgwater is not envisaged to significantly affect water levels on the moors due to their location upstream of the tidal section of river, with water levels able to be controlled by alternative water level management measures. Moderate Tone DS Taunton (River) GB108052015482 Yes - HMWB Potential Upstream fish migration through the tidal barrier may affect fish populations within the water body, eels in particular, during periods when the barrier is closed. There are no significant hydromorphological or physico-chemical impacts predicted on the upstream water bodies as the extent of influence of the barrier becomes less further upstream of the M5, therefore any effects will be indirect. Indirect impacts on WFD status in this water body in terms of risk of changing supporting conditions for BQEs is likely to be negligible and short-term, however, there is the potential for indirect effects on fish in terms of effects on migration, or loss of population by mortality. As a result effects on fish have been screened in for further assessment. Indirect effects on BQEs will be considered in the detailed assessment for the Parrett (Transitional) water body.

Screened out: Water body flows into the Parrett (transitional) approximately 8km downstream of the barrier site. The water body flows into the Parrett (transitional) through a tidal sluice Moderate Cannington Bk - Lower (River) GB108052021310 Yes - HMWB located at Stallingtons Clyce. The sluice consists of a tide flap which closes when the tide rises within the Parrett Estuary to prevent any tidal ingress along the Cannington Potential Brook water body, which occurs at every high tide. Closure of the barrier will therefore have negligible impact on the frequency of closure and given the distance from the proposed tidal barrier no effects (direct or indirect) are envisaged.

Screened out for all QE's except fish: Water body flows into the Parrett (transitional) water body upstream of the barrier site. The water body flows into the Parrett (transitional) through a tidal sluice located in Poor Durleigh Brook (River) GB108052021270 Yes - HMWB Bridgwater. The sluice consists of a tide flap which closes when the tide rises within the Parrett Estuary to prevent any tidal ingress along the Durleigh Brook water body, Potential which occurs at every high tide. Closure of the barrier will therefore have negligible impact on the frequency of closure and given the distance from the proposed tidal barrier no direct or indirect effects are envisaged. Screened out: Kings Sedgemoor Drain - Henley Yes - Moderate Modelling (Jacobs, 2019) was completed to understand the impact of a closed barrier on Kings Sedgemoor Drain at the short-list site assessment stage, this evidence GB108052021150 Sluice to mouth (River) Artificial Potential indicated that for barrier locations upstream of the confluence there is little impact on water level in the Parrett downstream, and no impact on discharge or water level in Kings Sedgemoor Drain. No direct or indirect impacts are envisaged for the preferred barrier location.

Tab 2. Identify Waterbodies Current A/ HMWB Water body Name Water body ID WFD status Screening and reasoning Designation (2015)* Screened in for fish passage only: Petherton Stream discharges into the tidal section of the Parrett through an outfall structure near Fordgate (approx. 6km upstream of the barrier site). Closure of the barrier Petherton Stream (River) GB108052021360 No Poor will have negligible impact on the frequency of discharge and given the distance from the proposed tidal barrier no direct or indirect effects are envisaged, therefore this water body is screened out from further assessment. Yes - Screened out: Bridgwater and Taunton Canal Moderate GB708100069 Artificial The Bridgwater and Taunton Canal joins the Parrett (Transitional) in Bridgwater. Water control structures in the form of navigable locks are present where the canal meets (Canal) Potential (canal) the River Parrett. No effect from proposed scheme is anticipated therefore this water body has been screened out from further assessment. Screened out: Waterbody underlies the barrier location. Limited borehole logs confirm anticipated geology. Bedrock is Mercia Mudstone (Mudstone and Halite Stone) which is not an aquifer. Above the bedrock, superficial geology include tidal flat deposits consisting of clay, silt and sand. As with other parts of the Somerset Levels, these materials are Poor (good unlikely to provide any more than a very minor aquifer. Past borehole logs of the area have encountered groundwater at depths of approximately 5m. quantitative Tone and Somerset Streams GB40802GB06400 N/A and poor Hydrogeology is unlikely to be at risk based on lack of predicted connectivity between river bed/banks and aquifer. (Groundwater body) chemical quality) The contaminated land assessment (chapter in draft) states that there are no significant contamination sources around the barrier or bypass structure which the piling work and excavation of the bypass channel could mobilise. Following further GI methods of working can be specified to ensure no release of contaminants via any pathway resulting in a neutral residual effect on groundwater.

Screened out: Waterbody is located approximately 16km downstream of the barrier, within the wider estuary. The barrier could impact on sediment circulation within the wider sediment Moderate Bridgwater Bay (Coastal) GB670807410000 No system, however, as the barrier is to be operated on surge tides only, this reduces the frequency and duration of closures. The impact on the Bridgwater Bay coastal Status waterbody is therefore considered to be negligible based on understanding of the sediment cell in Bridgwater Bay and within the scale of natural variation. This water body has therefore been screened out from further assessment. * WFD screening based on data from the published 2015 South-West RBMP (cycle 2), as shown on the Environment Agency's Catchment Data Explorer. http://environment.data.gov.uk/catchment-planning/ and an extract from the Environment Agency's Catchment Planning System received 08/5/19 Key: Transitional water body River water body Canal water body Groundwater water body Coastal water body

Reference: Jacobs, 2019b. Hydraulic Modelling Report

Tab 2. Identify Waterbodies STEP 1: Identification of upstream waterbodies in Parrett and Tone catchments - WFD screening in relation to fish passage effects. The WFD assessment has screened in WFD water bodies upstream of the Parrett (Transitional) that could be potentially affected by the construction and operation phases of the Bridgwater Tidal Barrier (BTB). The key consideration for screening in upstream water bodies is the potential presence of migratory fish species (salmon, sea trout and European eel), whose passage upstream or downstream could be affected by construction of the barrier, or closure of the barrier during operation. The specific consideration is whether effects could be sufficient to cause a deterioration of Fish status from “pass” to “fail” (i.e. reduce from Good or High status) or to prevent the water body achieving Good Ecological Status (that is, prevent an otherwise anticipated improvement from “fail” to “pass”) for each screened in water body. Refer table below. Overall WFD Assessment Screened Waterbody ID Water body Fish WFD Status Reason for not achieving Good status Justification for screening out Status year in? Flood protection - structures, Industrial discharge (EPR), GB108052015600 Dodham Brook (Yeo (Somset)) Bad Bad Urbanisation - urban development, Trading/industrial 2015 ✓ NA estates, Other (not in list) GB108052021270 Durleigh Brook Poor Bad Physical modification (Barriers to fish migration) 2015 ✓ NA GB108052015290 Merriot Stream Poor Poor Physical modification (Barriers to fish migration) 2015 ✓ NA GB108052015370 Parrett - R Isle to R Yeo Moderate Poor Morphology (suspect data) 2015 ✓ NA Physical modification (Barriers to fish migration, Land GB108052021360 Petherton Stream Poor Poor 2015 ✓ NA drainage - structures) Physical modification (Barriers to fish migration, Flood GB108052015310 Chinnock BK Moderate Moderate 2015 ✓ NA protection - water level management) Flow (unknown), Diffuse source pollution (mixed GB108052015170 Dowlish Brook Poor Moderate 2015 ✓ NA agricultural) Parrett - headwaters to Broad GB108052015260 Moderate Moderate Physical modification (Barriers to fish migration) 2015 ✓ NA River GB108052015450 West Sedgemoor Main Drain Moderate Moderate Physical modification (Barriers to fish migration) 2015 ✓ NA GB108052015650Cam - Lower Moderate Good NA 2015 ✓ NA GB108052015140 Cary - source to conf with KSD Moderate Good NA 2015 ✓ NA GB108052015220 Isle - Cad Bk to Fivehead River Moderate Good NA 2014 ✓ NA GB108052015160Isle - Upper Moderate Good NA 2014 ✓ NA GB108052015360 Parrett - Lopen Bk to R Isle Poor Good NA 2014 ✓ NA GB108052015570Wriggle Moderate Good NA 2014 ✓ NA GB108052015540Wriggle River Poor Good NA 2015 ✓ NA GB108052015630Yeo Moderate Good NA 2015 ✓ NA GB108052015682Yeo Ds Over Compton Moderate Good NA 2015 ✓ NA GB108052015681Yeo Us Over Compton Moderate Good NA 2014 ✓ NA GB108052015241Fivehead River Poor High NA 2015 ✓ NA GB108052015250Isle Poor High NA 2014 ✓ NA GB108052015190 Isle - Upper to conf Cad Bk Moderate High NA 2014 ✓ NA GB108052021310 Cannington Bk - Lower Moderate Good NA 2015 x Confluence with Parrett is downstream of BTB - no impact on fish migration

Confluence with Parrett is downstream of BTB - no impact on fish migration. GB108052021290 Cannington Bk - Upper Moderate Good NA 2014 x Ashford reservoir dam is currently a complete barrier to migratory fish species

Tab 2a. FISH passage waterbodies Overall WFD Assessment Screened Waterbody ID Water body Fish WFD Status Reason for not achieving Good status Justification for screening out Status year in? Physical modification (Barriers to fish migration), GB108052021320 Fiddington Brook Bad Poor Pollution (Point source) , Other pressures (Ecological 2015 x Confluence with Parrett is downstream of BTB - no impact on fish migration recovery time - surface waters) King's Sedgemoor Drain - Henley GB108052021150 Moderate Moderate Suspect data, pending further investigation 2015 Confluence with Parrett is downstream of BTB - no impact on fish migration Sluice to mouth x Confluence with Parrett is downstream of BTB - no impact on fish migration. Sutton Bingham Stream - East GB108052015520 Poor Good NA 2014 Sutton Bingham reservoir located downstream of the waterbody is currently a Arm x complete barrier to migratory fish species

GB108052021390 Back Stream Poor Unknown (pending investigation) 2015 ✓ NA GB108052015410 Sherford Stream Moderate Unknown (pending investigation) 2015 ✓ NA Diffuse source pollution (mixed agricultural), Physical GB108052015510 Hillfarrance Bk Moderate 2015 ✓ NA modification (Barriers to fish migration)

GB108052015380Westford Stream Good NA 2015 ✓ NA GB108052015390Haywards Water Good NA 2015 ✓ NA GB108052015482Tone Ds Taunton Good NA 2015 ✓ NA GB108052021370Tone - Upper Good NA 2014 ✓ NA GB108052021380Halse WTR Good NA 2015 ✓ NA GB108052015460Allen Brook(Maiden brook) High NA 2015 ✓ NA GB108052015481Tone - Wellington to Taunton High NA 2014 ✓ NA

Notes: Waterbodies listed are only those that have fish as a classified biological quality element.

Tab 2a. FISH passage waterbodies Legend k Barrier Location k Tidal limit WFD Water Bodies Rivers Canal Coastal

PARRETT Transitional (TRANSITIONAL) GB540805210900 Groundwater Bodies BRIDGWATER BAY (COASTAL) River Waterbody Catchment GB670807410000 Screened in for fish only Screened out Screened in

FIDDINGTON BROOK KING'S SEDGEMOOR DRAIN - HENLEY SLUICE TO MOUTH (RIVER) (RIVER) GB108052021320 GB108052021150

CANNINGTON BK - LOWER (RIVER) GB108052021310 PREFERRED k BARRIER LOCATION

CANNINGTON BK - UPPER (RIVER) HALSE WTR BACK STREAM GB108052021290 PETHERTON STREAM (RIVER) (RIVER) (RIVER) GB108052021380 GB108052021390 CARY - SOURCE TO CONF WITH KSD GB108052021360 (RIVER) DURLEIGH BROOK GB108052015140 (RIVER) GB108052021270 BRIDGWATER & TAUNTON CANAL (CANAL) WEST SEDGEMOOR MAIN DRAIN The limits, including the height and depths of the Works, shown in this drawing are not to be taken as limiting the GB70810069 (RIVER) GB108052015450 obligations of the contractor under Contract. SAFETY, HEALTH AND ENVIRONMENTAL INFORMATION TONE AND NORTH SOMERSET STREAMS IN ADDITION TO THE HAZARDS OR RISKS NORMALLY ASSOCIATED (GROUNDWATER) PARRETT - R YEO TO WEST SEDGEMOOR DRAIN WITH THE TYPES OF WORK DETAILED ON THIS DRAWING, THE GB40802G806400 (RIVER) FOLLOWING SIGNIFICANT RESIDUAL RISKS SHOULD BE NOTED. GB108052015470 FURTHER DETAILS ARE INCLUDED IN THE CDM DESIGN RISK MANAGEMENT REGISTER

ALLEN BROOK(MAIDEN BROOK) CONSTRUCTION : (RIVER) CAM - LOWER NOT APPLICABLE GB108052015460 k YEO DS OVER COMPTON (RIVER WATERBODY CATCHMENT) (RIVER) GB108052015650 GB108052015682 MAINTENANCE, CLEANING AND OPERATION : NOT APPLICABLE HILLFARRANCE BK TONE DS TAUNTON (RIVER) (RIVER) YEO GB108052015510 GB108052015482 (RIVER) DECOMMISSIONING OR DEMOLITION : GB108052015630 NOT APPLICABLE 9 1

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W behalf of ENVIRONMENT AGENCY with the permission of the Controller of Her Majesty's (RIVER) Drawn by: AJS Date: 30/09/2019 G Stationery Office, © Crown copyright. Unauthorised reproduction infringes Crown copyright and D I GB108052015540 Checked by: R may lead to prosecution or civil proceedings.Licence Number: 100026380 DOWLISH BROOK WH Date: 30/09/2019 B _

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: Drawing Scale: 1:200,000 AS SHOWN AT A3 R © Copyright Black & Veatch Ltd. 2019 Print Version: Print Quality (300dpi @ A3) Tab 3. Water body Map STEP 2 - Collate baseline WFD classification data for water bodies plus additional baseline information from other sources. WFD Quality Elements (for transitional water body)

Hydromorphological 2015 and 2016 EA Source for baseline and evidence for assessment of potential Supporting Catchment Data Explorer Additional baseline information effects Conditions classification data

The Parrett has a mean spring tidal range at the mouth of about 11m (this is very high). The upstream tidal limit along the River Parrett is at Oath Lock.

The seasonal balance between the (scouring) fluvial/ebb tidal influence and the less frequent high spring flood tide supply of marine sediment (accumulation) dominates the hydraulic regime of the estuary, which in turn dictates the estuary sediment regime. The strong flood speeds (due to the estuaries flood dominance) carry large quantities of marine sediment into and up the estuary, whilst the lower speeds on the ebb (combined with fluvial flow) only carry more limited amount seawards, leading to a net deposition of sediment along the Observation/ local knowledge bed and banks of the River Parrett (and adjoining water bodies, such as the Tone). There is significant inter-annual variability in this balance due principally to different peak river discharge conditions between the years. Sediment influx from marine sources at times of high spring Sediment sampling undertaken during River Parrett and Tone tides and low river discharge (i.e. during summer months) is replaced by effective seaward scouring of this material at times of high river Channel Monitoring Project (Feb 2011) discharge (typically during the winter months). Supports Good (2015 and Hydrological regime EA Marine Monitoring Service Assessment & Reporting Request 2016) An annual cycle between these conflicting processes exists and gives rise to an equilibrium channel profile of the estuary in terms of Ref. 05SWR (Feb 2014) dimension and shape. It is important to note that recent dredging at the upstream end of the water body will have taken the estuary cross- section area out of ‘regime’ (equilibrium) in this area, so that it is able to convey the highest occurring fluvial floods. Dredging Trials Monitoring Programme Report (Ambios/Somerset Drainage Boards Consortium, Mar 2017) The body of water in which the saline and freshwaters meet is known as the mixing zone or Turbidity Maxima (TM). The mixing zone has important consequences on the mobility and transport of suspended particles so its geographical position is important. The geographical location of the TM is dependent on the river discharge and tidal conditions at any given time so varies during the seasons (summer v winter); being further downstream in the estuary during the winter and spring (in Bridgwater or potentially further downstream) and higher up the estuary in the summer and autumn months. Suspended solid concentrations are usually highest at the TM. During current conditions, the River Parrett upstream of the M5 (approximately 5.5km upstream of the barrier) is rarely influenced by salt water intrusion. Only during extreme tides does some weak salt water intrusion extend as far upstream as Burrowbridge (13km upstream of the barrier).

The downstream end of the water body (within Bridgwater Bay) is exposed to wave action generated by the dominant south-westerly winds that funnel up through the Severn Estuary. Further upstream away from the coast wave exposure reduces, due to the sheltering effect of Steart Peninsula, so the majority of the estuary is relatively sheltered with limited influence from waves and wind. The high tidal range and Wave exposure No classification data Observation/ local knowledge 'funnel' shape of the estuary means a tidal bore wave develops a few times a year during high spring tides. This can result in a 3ft high standing wave, which travels upstream a far as Bridgwater travelling at up to 10 km/h. Bores can generate temporary but intense turbulence and mixing and the potential for higher sediment mobilisation than in normal tidal conditions.

Tab 4. Baseline Data TRAC Hydromorphological 2015 and 2016 EA Source for baseline and evidence for assessment of potential Supporting Catchment Data Explorer Additional baseline information effects Conditions classification data

The tidal range in the Severn Estuary is the second highest in the world and can exceed 14.5m.

During current baseline conditions, some of the sediment load in the incoming tide is deposited during the slack tide. The deposited sediments are resuspended and carried downstream during outgoing tides. Assuming the cross-section is nearly in “equilibrium”, the deposited sediments during incoming tides are balanced by the resuspended sediment during the outgoing flow in the long term (after several months or years). In addition, the River Parrett demonstrates cycles of channel narrowing as sediment builds up on the river banks River Parrett Dredge - Northmoor Pump Station to Linden within the intertidal areas. This material is known to be very cohesive and is not easily eroded away. It can therefore accumulate over time Farm: Environmental Statement Appendix E: Water Framework on the river banks and can stabilise further through vegetation growth. However, once this accumulated bank material becomes too steep, it Directive Assessment and River Parrett Dredge Phase 2 WFD will collapse and slump into the bed of the channel and a dynamic equilibrium state is maintained. Assessment (Environment Agency, 2015) These processes help to maintain a tidal prism relationship (e.g. the relationship between cross-sectional area, width and depth and the Parrett Barrier: Geomorphology Assessment and Peer Review volume of water passing through it). In general, the long term trend is that the Parrett Estuary prism relationship is generally considered to (CH2M, 2015) Morphological be close to the ideal, neither strongly erosional nor depositional (i.e. in equilibrium). This regime relationship, however, is at odds with some conditions - Depth No classification data conclusions stated elsewhere that the estuary is flood dominant and accreting, but this could be explained by short term variations around a Bridgwater Tidal Barrier - Barrier Morphological Impacts variation longer term equilibrium (Parrett Barrier: Geomorphology Assessment and Peer Review (CH2M, 2015)).This scenario does deviate from this Modelling Technical Note (draft) (CH2M, Mar 2017) ideal in some areas of the water body, particularly where the channel cross-section has been artificially modified and dredged. Several dredging projects have been undertaken by the Environment Agency, Internal Drainage Board and Somerset Rivers Authority upstream of Bridgwater Tidal Barrier - Geomorphological Baseline Report Bridgwater and the M5 crossing since major flooding of the Somerset Levels in winter 2013/14, including water injection dredging (WID) (CH2M/B&V, 2017) trials, which are now likely to be undertaken as an annual event. The Environment Agency has recently provided a 5-year protocol to follow to ensure WID works are legally compliant with environmental legislation. No dredging is undertaken downstream of Bridgwater. Dredging Dredging Trials Monitoring Programme Report at the upstream end of the water body will have taken the estuary cross-section area out of ‘regime’ (equilibrium) in this area (increasing the (Ambios/Somerset Drainage Boards Consortium, Mar 2017) depth/width channel variability), so that it is able to convey the highest occurring fluvial floods.

Existing seasonal average natural river bed level changes between 9km and 13km upstream of the barrier have been found to be in the order of up to 300mm (derived from Parrett Internal Drainage Board, 2017). Approximately 14km downstream of the barrier in the vicinity of the Steart Marshes managed realignment scheme, bed levels have been observed to vary by 500mm to 2000mm, with the larger changes being due to the movement of the low water channel (refer also to ‘Channel morphology’ below) (Halcrow, 2015).

Tab 4. Baseline Data TRAC Hydromorphological 2015 and 2016 EA Source for baseline and evidence for assessment of potential Supporting Catchment Data Explorer Additional baseline information effects Conditions classification data

Parrett Barrier: Geomorphology Assessment and Peer Review (CH2M, 2015)

Bridgwater Bay is a major sediment sink within the Severn Estuary and is considered to be a long term source of fine sediment for the bed River Parrett Dredge - Northmoor Pump Station to Linden within the Parrett water body. The main sediment source is believed to be marine although fluvial inputs are important in the upper reaches. Farm: Environmental Statement Appendix E: Water Framework Generally within the River Parrett the bed sediment is fine, with sandy and coarse silt and compositionally homogenous (i.e. bed sediment Directive Assessment and River Parrett Dredge Phase 2 WFD type does not vary significantly). Suspended solid concentrations in the River Parrett are also exceptionally high, in the region of 300-900mg/l Assessment (Environment Agency, 2015) on average during spring tide conditions, with maximum levels recorded as high as 10,000mg/l. During periods of slack water suspended sediment can sink to the bed forming fluid mud, a near-bed, high density cohesive sediment suspension layer. EA Marine Monitoring Service Assessment & Reporting Request Ref. 05SWR (Feb 2014) The barrier site is dominated by tidally deposited sediment (medium/fine silt); however, fluid mud is also known to be deposited on the bed Morphological during slack water. Bed sediment (particularly fluid mud) is known to be very mobile and easily entrained during high flow events. During Sediment sampling undertaken during River Parrett and Tone conditions - Bed No classification data summer/autumn typical baseline conditions, an equilibrium is maintained on the channel floor (thalweg) where prolonged ebb currents can Channel Monitoring Project (B&V/EA report, Feb 2011) substrate scour away accumulations on the bed of the channel. Historic Trends Analysis undertaken during River Parrett and There is significant inter-annual variability in the quality and quantity of the bed substrate due to the different hydrological regime Tone Channel Monitoring Project (B&V/EA report, June 2009) conditions. Sediment influx from marine sources (which is typically medium/fine silt) is deposited at times of high spring tides and low river discharge (i.e. typically during summer months) but is scoured during times of high river discharge (typically during the winter months). There Estuarine Transmission of Heavy Metals: A literature review. K is some comparative evidence that more recent fluctuations in channel section and erosion of the bed within the Parrett Estuary water body A Turner & T N Burt. (March 1985) has occurred in response to extreme fluvial flood events, as evidenced from comparison of the LiDAR and bathymetry data data from 2012 and 2013 cross-sections; following flooding in 2012, the 2013 section was substantially larger. Physical Processes in Estuaries, J Dronkers (1988)

Dredging Trials Monitoring Programme Report (Ambios/Somerset Drainage Boards Consortium, Mar 2017)

Tab 4. Baseline Data TRAC Hydromorphological 2015 and 2016 EA Source for baseline and evidence for assessment of potential Supporting Catchment Data Explorer Additional baseline information effects Conditions classification data

River Parrett Dredge - Northmoor Pump Station to Linden Farm: Environmental Statement Appendix E: Water Framework Directive Assessment and River Parrett Dredge Phase 2 WFD The narrow intertidal zone along the River Parrett is typically characterised by steep banks with extensive silted berms, which become Assessment (Environment Agency, 2015) exposed at low tide (and during low fluvial flows). Bank sediment is typically fine sand to coarse silt and recent sampling studies suggest it becomes finer and more cohesive in a seaward direction. The upper banks are vegetated, resulting in a very steep bank profile. There are Local knowledge and observations local areas of significant bank instability and erosion; often in association with vegetation control that does not leave enough of a margin to support the bank toe. So, locally, sediment supply from slumping of locally unstable banks can be high. Sediment sampling undertaken during River Parrett and Tone Channel Monitoring Project (report Feb 2011) Coastal grazing marsh, and to a lesser extent, saltmarsh, are generally the most extensive habitat types downstream of Bridgwater within the Morphological outer estuary. Saltmarsh habitat becomes more prevalent closer to the mouth of the estuary, and saltmarsh maps also show a small area of EA Saltmarsh maps (rec'd Feb 2014) conditions - No classification data saltmarsh close to Dunball. Other upstream areas of vegetation are generally characterised by reeds (Phragmites ). There are no known / Intertidal zone mapped saltmarsh areas upstream of Bridgwater. Flood embankments extend along the majority of the water body length, limiting the EA Marine Monitoring Service Assessment & Reporting Request extent of the intertidal zone (especially upstream of Bridgwater). A new area of intertidal habitat has recently been created at the Ref. 05SWR (Feb 2014) downstream end of the water body on the Steart Peninsula, which is located 14km from the barrier site. Historic Trends Analysis undertaken during River Parrett and During summer/autumn typical baseline conditions, accumulation is expected within the intertidal zone, particularly the lower side slopes Tone Channel Monitoring Project (B&V/EA report, June 2009) (inundated by the sediment-rich flood tides, which remain dry for much of the ebb and low river discharge periods). The deposits on the side slopes are periodically scoured by the severest winter river floods, to create a natural system where the cross-sectional area of the channel Dredging Trials Monitoring Programme Report changes seasonally. (Ambios/Somerset Drainage Boards Consortium, Mar 2017)

Environment Agency, Nuclear Regulatory Group 'Stop Hinkley' sediment report, 2017

Tab 4. Baseline Data TRAC 2015 and 2016 EA Physico-chemical Catchment Data Explorer Additional baseline information Source Supporting Elements classification data

The water body is highly turbid as a result of the strong tidal currents, which transport high concentrations of suspended solids into the River Observation/ Expert Judgement Parrett system. This is less so at the upstream end of the water body due to the greater influence of freshwater flow. Turbidity remains very Transparency Not assessed high along the Parrett all year; suspended solid concentrations in the River Parrett have been measured in the region of 50,000-180,000mg/l Estuarine Transmission of Heavy Metals: A literature review. K (Turner & Burt, 1985). Typically thorough mixing occurs within the water column particularly at the downstream end, due to the high tidal A Turner & T N Burt. (March 1985) range. However there is thought to be less mixing of the tidal and freshwater flows at the upstream end of the water body.

Tidal inflow along the water body means that temperature is generally kept relatively low. The upstream end of the water body is Thermal conditions Not assessed considered to be more vulnerable to high water temperatures during the summer months due to low fluvial flows (reduced water depths Observation/ Expert Judgement within the channel). The Parrett can experience extensive algal blooms during high water temperatures and elevated nutrient conditions.

Biological Oxygen Demand (BOD) of naturally occurring marine detritus in the Estuary, which is periodically re-suspended, is believed to be Expert judgement more than the BOD created from dissolved and particulate effluent discharges within the Estuary. Dissolved Oxygen levels naturally fall below 60% saturation during spring tides. The detritus is thought to be derived from the Severn Estuary and a major source is thought to be Bridgwater Tidal Barrier: Barrier Water Quality Impacts Note 1 dead phytoplankton from the more turbid parts of the Estuary. Oxygenation (CH2M, 2017) Not assessed conditions Should expect normal conditions to have over 70% Dissolved Oxygen. Characterisation of European Marine Sites: The Severn Estuary (possible) Special Area of Conservation Special Protection Area, UKTAG WFD standards to acheive 'good' status for transitional and coastal waters are 5-percentile standards for dissolved oxygen in the (April 2003) range 5-7mg/l, dependent on salinity.

Expert judgement Salinity in Bridgwater Bay ranges from 22 to 33% depending on the freshwater flow from the rivers; salinity within the River Parrett Estuary Salinity Not assessed would therefore be expected to be similar, or lower depending on the tidal state. Salinity is likely to be highest at the downstream extent of Bridgwater Tidal Barrier Preliminary Aquatic Ecology Baseline the waterbody due to tidal influence. Characterisation (APEM, 2016)

Nutrient enrichment of watercourses is known to be a problem in the catchment caused by discharges from sewage treatment works and diffuse pollution from agriculture and other sources. There are a number of large sewage treatment works within the water body catchment Observation/ expert judgement Nutrient conditions Not assessed at Bridgwater and further upstream, which may elevate nutrient levels, along with runoff from arable and pasture. Nutrient levels are likely to be elevated following flooding and pollution from septic tanks. The Parrett is known to suffer algal blooms. Wessex Water have plans in Parrett Catchment Information Pack (EA, Feb 2014) place to control pollution impacts from any of their operations.

Tab 4. Baseline Data TRAC 2015 and 2016 EA Physico-chemical Catchment Data Explorer Additional baseline information Source Supporting Elements classification data

There are a number of potential contamination sources present within the catchment (both point sources and diffuse sources) and historical data shows contamination of estuary sediments.

Sampling of sediments within the Parrett and laboratory testing of these samples was undertaken for the BTB Scheme to look for heavy metals, cyanide, TBTs, petroleum hydrocarbons, PAHs and PCBs. This testing did not find grossly elevated levels of the tested for contaminants within the sediments. The spatial distribution was relatively uniform, indicating a point source out-with the sampling area (either upstream or from downstream).

The Barrier is located approximately 200m north of the Saltlands Waste Disposal Site. The site operated between 1977 and 1992 and accepted a mix of inert, household, commercial and industrial waste. Investigations for a pipeline across the western area of landfill (Structural Soils, 2000) described up to 5m of clay or sandy clay fill with mixed domestic refuse, including bricks, plastic bags, concrete Sediment sampling and contamination testing by Partrac (2009) fragments, paper, plastic, wood and lignite. Tests on three samples did not show excessive contaminant levels although levels of mineral oils

and poly-aromatic hydrocarbons were elevated to action threshold levels in places and copper and zinc levels were elevated in one location River Parrett and Tone Channel Monitoring Project (Feb 2011) (CH2M, 2016b). As the landfill site is located upstream of the barrier site, the proposed works are not anticipated to impact upon it and it is not anticipated to impact on any proposed works. Squares Road landfill, a small historic landfill, comprising two areas of landfill, is located Parrett Catchment Information Pack (EA, Feb 2014) approximately 250m to the west (left bank) of the barrier site. The types of waste accepted by the landfill and the dates of operation are not High Status 2015 and 2016 recorded, although the landfill is located at a sufficient distance that the proposed works would not be expected to impact upon it; nor it on Specific pollutants (copper, iron, toluene and Bridgwater Tidal Barrier Downstream Flood Defences Desk the works. zinc listed - all high) Study (CH2M, 2018a) On the left bank, there was formerly a brick and tile works, now agricultural land. Contamination was reported on part of this site some Bridgwater Tidal Barrier (Option 5) - Ground Investigation 250m to the west of the barrier site during a recent housing development. It is possible that contaminants from the brick and tile works were Report (GIR) (CH2M, 2018b) mobilised via undetected drainage features linking to ditches and ponds around the site. The brick and tile works site therefore forms a potential source of contamination. Although a thin layer of Made Ground was recorded within the holes undertaken during the 2017 ground Expert judgement. investigation in this area, there were no visual or olfactory signs of contamination. Chemical analysis of the Made Ground did not record significant contamination. Numerous flooded clay pits are located in the wider area and the possibility of unrecorded backfilled clay pits cannot be discounted. If present, the nature of the backfill has the potential to also be contaminative. On the right bank, beyond the existing flood bank there is brownfield land forming part of the Express Business Park. Historic borehole logs note that this ground includes brick, glass, concrete and clinker. Whilst there was no evidence for significant contamination within these deposits, this cannot be ruled out.

There are a number of large sewage treatment works within the water body at Bridgwater and also upstream.

Water quality is considered to be at normal background levels. Burnham, Berrow Beach and Brean beaches are assessed for Bathing Water Quality. Burnham Jetty is considered to be potentially sensitive to elevated bacterial levels and is currently classified as poor according to 2015 standards.

Tab 4. Baseline Data TRAC 2015 and 2016 EA Biological Quality Catchment Data Explorer Additional baseline information Source Elements classification data

A BTB specific survey was undertaken in 2016 and no macroalgae was recorded around the location of the barrier and downstream to Combwich. The absence of macroalgal communities recorded during the survey also reflect the findings of the BTB desktop study (APEM, 2016) which reported no opportunistic macroalgae and only one species (Cladophora sp.) which was reported further upstream on the Rivers Parrett and Tone in low densities (Sibley & Jewell, 2015). The Environment Agency has also conducted a further survey on opportunistic macroalgal species from 2008-2011 at a site further upstream on the River Parrett (from the dredging locations) however no opportunistic macroalgal species were found at this site (refer to Apem, 2016 for further details). RBMP2 (unpublished data) and local knowledge

Macroalgae (high status) Angiosperms, Intertidal macrophyte surveys were conducted on behalf of the Environment Agency upstream of the M5 motorway (approximately 3.7 km EA Saltmarsh maps (rec'd Feb 2014) 2015 and 2016 macroalgae and upstream of the aquatic ecology study area) as part of the Environment Agency Rivers Parrett and Tone Dredge Environmental Monitoring phytobenthos Report (Sibley & Jewell, 2015). The macrophytes recorded at this site were water dropwort Oenanthe crocata with <0.1% total cover, reed Rivers Parrett and Tone Dredge Environmental Monitoring Others - no classification (Aquatic flora) canary grass Phalaris arundinacea with a total cover of between 0.1 to 1% and the common reed Phragmites australis with a total cover Report (Sibley & Jewell, 2015) data between 5 and 10%. APEM (2019a) Bridgwater Tidal Barrier Intertidal Survey Phase 1 Habitat Survey Mapping undertaken for the BTB Scheme (refer to Chapter 9 (Biodiversity)) shows that there is no saltmarsh close to the proposed tidal barrier. A thin (approx. 10m) strip of intertidal mud is present on both banks at the barrier location, with marginal reeds present on the right bank only. Generally the banks are dominated by coastal grazing marsh habitat. Approximately 1km downstream of the barrier, scattered saltmarsh was mapped during the Phase 1 Habtiat Survey with continous areas of saltmarsh present at Pawlett and Combwich.

Observation / expert judgement The invertebrate assemblage in this part of the Parrett Estuary is expected to be typical for an estuarine setting in south west England. The intertidal sediments within the Parrett Estuary are dominated by mud and support an assemblage of polychaetes, bivalves and oligochaetes. Presence of notable species ascertained from NBN Gateway Habitat mapping undertaken by APEM in 2016 showed that the barrier location is dominated by intertidal and subtidal habitat, characterised primarily by muddy sediments and communities which are tolerant to variable salinities and no oligochaetes were found. Further GIS data of the broad scale intertidal and subtidal habitats were downstream, adjacent to Steart Marshes, the area is subtidal dominated by sublittoral mud in variable salinity, which typically supports obtained from the European Marine Observation and Data Good status (2015 and communities characterised by oligochaetes, and polychaetes such as bristleworm Aphelochaeta marioni . Invertebrates Network, using habitat maps compiled by the Joint Nature 2016) Conservation Committee (JNCC) There are several NERC priority invertebrate species of importance in the area. The depressed river mussel Pseudanodonta complanata is a bivalve mussel inhabiting the beds of lowland rivers; it is known to be present in the catchment. Other notable freshwater species are Apem (2019a) Bridgwater Tidal Barrier Intertidal Survey associated with the smaller ditches and pools rather than the main river channels: the mud snail Omphiscola glabra, shining ram's horn snail Segmentina nitida and larged mouthed valve snail Valvata macrostoma . These would be expected to occur further upstream in the APEM (2019b) Bridgwater Tidal Barrier Aquatic Ecology freshwater reaches of the waterbody or in other freshwater habitats (e.g. ponds and ditches) and not within the Parrett Estuary. Baseline Characterisation.

Tab 4. Baseline Data TRAC 2015 and 2016 EA Biological Quality Catchment Data Explorer Additional baseline information Source Elements classification data

There is no fish WFD classification data for the water body. Key migratory fish species transiting through the Parrett include European eel Anguilla anguilla , Atlantic salmon Salmo salar and sea trout Salmo trutta . Elver fishing season starts Feb through to end May, with glass eel migration from December to June. Both freshwater and estuarine species are known to occur within the water body. Upstream water bodies have some WFD classification data - the Tone d/s Taunton has a Good classification (freshwater). The Parrett has no formal WFD classification data except much further upstream (freshwater). The Parrett is an Index river for eels and is therefore monitored by the Environment Agency. The Parrett is included in the South West River Basin District (SWRBD) Eel Management Plan under the EC Eel Recovery Plan (Council Regulation No 1100/2007/EC). The SWRBD is currently regarded as failing to meet the 40% silver eel escapement management target stipulated by the regulations. Observations and expert judgement During 2016 and 2017, four seasonal fish surveys were conducted in the Parrett Estuary between the A39 road bridge in Bridgwater and River Parrett Dredge - Northmoor Pump Station to Linden Dunball Wharf (Appendix 9H of the Environmental Statement). The surveys involved beam trawling and intertidal seine netting at three Farm: Environmental Statement Appendix E: Water Framework locations. Across the four surveys, 16 taxa were recorded. Beam trawl samples were generally dominated by gobies, with the exception of Directive Assessment and River Parrett Dredge Phase 2 WFD winter 2016/2017 when only two fish were caught (one goby and one ruffe). Seine net surveys were dominated by gobies, three-spined Assessment (Environment Agency, 2015) stickleback and thin lipped grey mullet. Overall, abundances were greatest in autumn and spring and the lowest catches were in winter Fish fauna No classification data largely due to few gobies being captured in winter months. The only migratory species recorded was a single juvenile European eel, with no Hinkley Point data source: EA Marine Monitoring Service, (Feb Atlantic salmon or sea trout captured. Other species of conservation importance were single individuals of plaice Pleuronectes platess a, 2014) whiting and Dover sole. South West River Basin District (SWRBD) Eel Management Plan Static fyke nets were used to monitor the effects of the Environment Agency's Parrett and Tone Dredge at locations up and downstream of the proposed BTB works (Environmental Monitoring Report, Sibley and Jewell, 2015). At the site downstream of Huntspill, species consisted Bridgwater Tidal Barrier, Combined Fish Survey Field Report of estuarine residents (e.g. flounder and sand goby), marine migrants (e.g. whiting Merlangius merlangus , pout Trisopterus luscus , Dover APEM (2017) sole Solea solea , cod Gadus morhua , sprat Sprattus sprattus , herring Clupea harengus and sea bass Dicentrarchus labrax ) and the diadromous species European eel, three-spined stickleback Gasterosteus aculeatus and thin lipped mullet Liza ramada .

Further upstream, the surveys have recorded many more freshwater species including chub Squalius cephalus , bream Arramis brama , rudd Scardinius erythrophthalmus , dace Leuciscus leuciscus, gudgeon Gobio gobio , ruffe Gymnocephalus cernua , perch Perca, fluviatilis stone loach Barbatula barbatula , roach Rutilus rutilus , silver bream Blicca bjoerkna and pike Esox lucius. This is as expected in areas with increased freshwater influence.

Tab 4. Baseline Data TRAC 2015 and 2016 EA HMWB Mitigation Catchment Data Explorer Measures classification data

Overall Mitigation Moderate or less (2015 Measures and 2016) Assessment Realign flood Measure is not 'in place' defence Fish passes Measure is not 'in place' Enhance ecology Measure is not 'in place' Remove obsolete Measure is not 'in place' structure Changes to locks etc. Measure is not 'in place' Avoid the need to Measure is not 'in place' dredge Dredging disposal Measure is not 'in place' strategy Reduce impact of Measure is not 'in place' dredging Reduce sediment Measure is not 'in place' resuspension Retime dredging or Measure is not 'in place' disposal Sediment Measure is not 'in place' management Dredge disposal site Measure is not 'in place' selection Manage disturbance Measure is not 'in place' Retain habitats Measure is not 'in place' Remove or soften Measure is not 'in place' hard bank Indirect mitigation Measure is not 'in place' Preserve or restore Measure is not 'in place' habitats In channel morph Measure is not 'in place' diversity Bank rehabilitation Measure is not 'in place'

Key: Element classified at bad status Element classified at poor status Element classified at moderate status, or mitigation measure not 'in place' Element classified at good status, or mitigation measure 'in place' Element classified at high status

Tab 4. Baseline Data TRAC STEP 3 - Assess potential changes in quality elements (A - risk of deterioration) and contribution to mitigation measures (B - achieving GEP). NOTE: The WFD assessment is based on the anticipated frequency and duration of operation of the barrier as a tidal surge barrier alone (see Assumptions Tab 1). Any deviation from this will require a new WFD compliance assessment to ensure implications of a changed operating regime are properly considered. 3A - QUALITY ELEMENTS

Scale and Actions for WFD WFD Quality certainty Compliance (including Risk to WFD Elements PARRETT (TRANSITIONAL) Risk to WFD compliance (before of impact proposed mitigation compliance (transitional - Assessment of effects on quality elements mitigation) before during design and (after water body) mitigation implementation of mitigation) works):

Hydromorphological Supporting Conditions

The effect during construction is localised (not at the water body scale) and changes to flow are Temporary effects on flow during construction: Modelling of the temporary works indicates that the changes in flow will cause a localised increase likely to be within the range of Hydrological in velocity which could cause local scour and deposition within approximately 500 m upstream and downstream of the barrier. The wider impacts existing natural variation. The flow regime - Tidal are likely to be within the range of natural variation and would be expected to return to natural levels following construction. This process is likely regime would be expected to and to be quite slow (i.e. in the order of months). During construction a temporary by-pass channel will be constructed to one side of estuary to divert return to natural levels following freshwater Local flow to enable the barrier to be constructed in the dry. This may cause some localised effects on flow. A 1D hydraulic model has been used to construction, however, this No risk to flow scale, investigate the effect of the 20m wide (200m long) temporary bypass channel. Velocities through the bypass channel are predicted to be higher process is likely to be quite slow Not required. WFD moderate depending on the point in the tide cycle and the fluvial flow. The increase in velocity is generally in the range of 0m/s to 0.7m/s over a tide cycle. (i.e. in the order of months). Given compliance. (WFD certainty. This may result in localised erosion and scour of the bed and banks of the channel upstream and downstream of the bypass channel. It is the measures to be incorporated classification anticipated that rip-rap will be placed to provide scour protection through the narrower sections of the bypass channel to cope with the increased to control scour during 2015 = speed of the river. During reinstatement the bypass channel will be filled with stockpiled excavated material and the temporary stabilisation/scour construction, the risk of 'supports protection piles will be removed. deterioration to existing 'Good good') Status' is assessed as low.

No risk to WFD compliance.

Tab 5. WFD Assessment-Parrett TRAC For the predicted long The assessment of effects has Changes to peak tidal velocity from barrier closure: High spring tides (usually during the winter months) are particularly important for sediment term (post-2055) effects, a determined there is a risk to WFD transport, with peak suspended sediment concentrations occurring on these extreme tides due to increased mobilisation of sediment stored within formalised Adaptive compliance on existing Bridgwater Bay. This makes the timing of closure of the barrier particularly important with regard to the overall sediment budget of the Parrett Environmental hydrological regime associated upstream of the barrier. The operation of the barrier restricts the upstream movement of the tide and reduces peak ebb and flood velocities, in Management Plan will be with : particular upstream of the barrier. The barrier is most likely to be closed for FRM closures during the more extreme high spring tides, therefore followed – precedent for 1. Reduced ebb and flood there is likely to be a reduction in peak suspended sediment concentration (SSC) upstream of the barrier during closure and a potential increase this on other large velocities up and down estuary downstream; impacting on the overall sediment budget of the Parrett. Model results show that velocities of 1-2m/s on average (faster on flood schemes where there is resulting in a corresponding than ebb) in the baseline case would be slowed to zero immediately upstream and downstream of the barrier when it is closed. Reduced velocities uncertainty over effects or reduction in peak SSC up estuary will occur during both the barrier closure period and during the subsequent ebb tide after the barrier has re-opened and will extend approximately that impacts may be and increase down estuary 5.5 km upstream of the barrier (just upstream of the M5), and approximately 5 km downstream of the barrier (near Pawlett). The combined tidal outwith timescales of affecting channel morphology. ebb/fluvial flow velocity is a factor in maintaining regime form in the estuary, particularly during the winter months. Infrequent individual (and current WFD horizon of 2. Impounded flow conditions consecutive) closures in the period up to 2055 are likely to have limited effect, but with a higher expected frequency of barrier use in the long term 2027, as is the case for the affecting water quality and (particularly due to a higher frequency of consecutive closures during the winter months and a reduction in the frequency of sediment transporting BTB Scheme. The AEMP sediment processes. high spring tidal events) could change this process. sets out the Environment 3. Prevention of flow reversal. Agency’s commitment to 4. Reduction in effectiveness of Changes to peak freshwater flow velocity from barrier closure: During summer/autumn typical baseline conditions, normal downstream Pre 2055: collect data and monitor winter fluvial scouring processes. freshwater flow is reversed on flood tides when tidal water moves upstream and this effect can extend as far as Burrowbridge, 13km upstream of Water the River Parrett 5. Potential change in stratification Hydrological the barrier on the River Parrett and to Newbridge Sluice (along the River Tone). During winter/spring baseline conditions, high freshwater fluvial body Transitional waterbody so of flows, i.e. extent to which saline regime - Tidal flows play an increasingly important role in transferring accumulated sediment further downstream (causing fluvial scouring). The operation of the scale, as to be in an informed Risk addressed and freshwater flow in opposite and barrier will locally impound downstream freshwater flow, but will also prevent reversal of the freshwater flow direction that occurs on the flood moderate position from which to by delivery of directions due to density freshwater tide under current (baseline) conditions. During barrier operation freshwater flow will be slowed to zero in the immediate vicinity of the barrier certainty. ensure WFD compliance in Adaptive differences in the water column. flow and reduced further upstream, creating an upstream ‘pond’ of freshwater building up behind the barrier until it is opened, the main difference the future as required. It Environmental 6. Reduction in the mixing of being this will not reverse upstream by being pushed back by incoming tidal flow. Average velocities are reduced over a greater length of channel Post 2055: provides a mechanism for Management freshwater and tidal flows could (WFD and by a greater degree upstream of the barrier than downstream, resulting in an increased tendency for settlement of sediment, although the Water trends to be identified, Plan. change the location of the classification tidally driven transport of sediment into the river upstream of the barrier is also prevented by barrier closure. Infrequent individual (and body and avoidance and 'turbidity maxima' affecting 2015 = consecutive) closures in the period up to 2055 are likely to have limited effect, but with a higher expected frequency of barrier use in the long term scale, low mitigation measures turbidity and salinity. 'supports (particularly a higher frequency of consecutive closures during the winter months) could result in changes to freshwater flows, in particular certainty evaluated and 7. Reduced 'tidal pumping' effect. good') reducing the fluvial scouring effect which is important in maintaining the equilibrium shape and form of the estuary. implemented, so that significant negative effects Not expected to be a risk in the can be managed. The Changes to mixing of tidal and freshwater flows from barrier closure: When closed the barrier will reduce mixing of freshwater and tidal flows short-med term due to limited Adaptive Environmental (flow stratification) and therefore influence the location of the 'turbidity maxima' (TM). Monitoring survey data shows that the River Parrett frequency and duration of closure Management Plan is upstream of the M5 lies above the normal zone of saline water intrusion and significant mixing in current conditions is only likely to occur when effects. Risk of more significant intended to be a live high spring tides coincide with low fluvial flows, resulting in weak salt water intrusion as far upstream as Burrowbridge (13km upstream of the change in long term (post 2055) document sitting alongside barrier), with anecdotal evidence indicating stratification of saline and freshwater flow occurs further upstream on the Tone. The operation of the due to anticipated higher this WFD Assessment, with barrier will reduce the frequency and extent of this weak salt water intrusion as closures will occur most frequently on the highest tidal flows that frequency of barrier operation and an expert group set up to cause mixing further upstream, which will temporarily reduce the salinity upstream of the barrier. Immediately downstream of the barrier, water timing of closures affecting winter review at key stages in the will be temporarily more saline due to freshwater flow being held upstream and reduced freshwater mixing during barrier operation. Mixing will fluvial scour processes, tidal and project. It is essential for recommence once the barrier is opened. fluvial mixing and disrupting establishing legal turbidity maxima during high tides. compliance in the case of Summary: Hydraulic model results show effects 5.5km upstream and 5km downstream so a total of 10.5km of the water body is predicted to be having to meet the affected. Impact is temporary, during the 4-6 hour period when the barrier is closed. Frequency will depend on operational use, which is predicted Mitigation proposed to assure requirements of Article to increase in future (see assumptions). Permanent changes in flow dynamics around barrier pier structures will be highly localised. future WFD compliance. 4.7. Wave The barrier will not have an effect on normal wave exposure as the location in the inner estuary is not influenced by wave processes. Operation of exposure the barrier will stop a tidal bore wave that occurs on high spring tides from passing up the system, and instead the energy in the bore will deflect Local No risk to when it meets the closed barrier. The main effect of the tidal bore is to increase potential sediment erosion from the bed and adjacent mudflats scale, No risk to WFD compliance. Not required. WFD (No formal due to the temporary increase in velocity and concentrated scouring effect. When the barrier closes on high spring tides this effect will no longer moderate compliance. baseline WFD occur upstream of the barrier. It is considered unlikely to have a significant influence on the long term hydromorphology of the estuary due to its certainty. classification) infrequent occurrence and short duration.

Tab 5. WFD Assessment-Parrett TRAC Changes to patterns of erosion and deposition from barrier closure, affecting estuary depth variation: A high-level assessment of the potential The assessment of effects has effects of the scheme on channel morphology has been carried out using a regime approach. This approach has been developed to supplement the determined there is a risk to WFD wider understanding of the River Parrett derived from sediment monitoring, salinity testing, gauge data, the Geomorphological Baseline Report compliance on morphological (refer Appendix 8B of the Environmental Statement) and expert judgement. This assessment cannot be used to quantify exact areas and volumes of conditions - depth variation channel change resulting from deposition and erosion. Instead the regime relationship is used to predict the impact of changed flow conditions associated with : (specifically changed average spring peak discharge on the incoming tide) on long-term channel cross-section area. 1. Short-term increase in accretion immediately behind the barrier Upstream of the barrier during closure: When the barrier is closed, the sediment load in the incoming tide is cut off from the area upstream, and and within the TM zone up estuary the river flow velocities reduce over about 5.5 km upstream of the barrier (to just upstream of the M5 crossing). Reduced velocities will cause the immediately following closure. existing (pre closure) sediment load to deposit up estuary of the barrier particularly within the location of the TM zone which precedes closure. 2. Reduction in peak SSC load When the barrier opens, deposited sediment caused by the impoundment may not be fully resuspended due to the reduced ebb flow velocity during barrier closure will result in immediately following closure. Leading to a short-term temporary increase in accretion immediately behind the barrier and within the TM zone up a small reduction in sediment estuary following closure. Regime modelling indicates that in the long-term, the effect in the reach immediately upstream of the barrier is a deposition on the bed of the reduction in cross sectional area that is proportionate to the reduction in average peak flood discharge. The predicted reduction in cross sectional channel up estuary in the long- area is 4% for 13 closures in a 14 month period (typical of the average number or closures expected in the period up to 2055) and up to 5% for 32 term. closures (typical of the average number of closures expected during the period 2055 to 2124). The SSC that is cut off by the incoming tide is 3. Reduction in the effectiveness of expected to be much larger than the river sediment load. Hence, the net effect is anticipated to be a small reduction in sediment deposition on the winter fluvial scouring processes bed and banks of the channel upstream of the barrier in the long term. Further upstream of the barrier, the channel is increasingly dependent on potentially leading to reduced Pre 2055: high fluvial flow events to maintain the equilibrium cross-sectional area and monitoring data has shown that ebb flow erosion combined with the depth within the inner estuary. Water scouring effect that occurs under larger fluvial flows is important in maintaining the overall estuary regime morphology and depth. Existing seasonal Morphological 4. Down estuary increase in body average natural river bed level changes recorded 9km and 13km upstream of the barrier have been found to be in the order of up to 300mm conditions - deposition due to a reduction in scale, Risk addressed (derived from Parrett Internal Drainage Board, 2017). Where the channel cross-section has been modified and widened by dredging and is likely to Development of an Depth flow velocities, though this moderate by delivery of be accretional (i.e. upstream of the M5); high fluvial flows help to scour the channel potentially reducing the frequency of further maintenance Adaptive Environmental variation become negligible 5km further certainty. Adaptive dredging. Analysis shows that the duration of high fluvial flows in the lower Parrett tends to be long (e.g. days to weeks) due to catchment response Management Plan (as down estuary (in the outer Environmental and the presence of artificial flow control structures upstream. The duration of barrier closure is relatively small; therefore, it is predicted that above). (No formal estuary). Post 2055: Management barrier closure in the short-medium term (which is of limited frequency and duration) will not significantly impact on high flow events which are of baseline WFD Water Plan. much longer duration. In the long-term, the higher frequency and duration of barrier operation may negatively effect winter fluvial scouring classification) Not expected to be a risk in the body processes but this may be counteracted to some extent by the reduction in marine sediment transport into the reach upstream of the barrier short-med term due to limited scale, low meaning that in the long term regime depth is maintained. The net effect is anticipated to be small reduction in the accretion upstream of the frequency and duration of closure certainty barrier in the long term but this long-term balance (post 2055) is an area of some uncertainty. effects. Risk of more significant Downstream of the barrier during closure: The overall effect on sediment processes downstream of the barrier is anticipated to be an increase in change in long term (post 2055) deposition due to a reduction in flow velocities. For all the barrier closures investigated; 13 closures in the short-medium term (up to 2055) and 32 due to anticipated higher closures in the long-term (post 2055), the increase in deposition and therefore reduction in average river cross-section areas (and channel depth) is frequency of barrier operation, estimated to result in approximately 5% reduction in the equilibrium cross-section area close to the barrier, reducing to less than 0.5% about 5 km however, the changes in downstream. There are negligible effects predicted even further downstream. To put this into context, seasonal average natural river bed level deposition may not be easily changes approximately 14km downstream of the barrier in the vicinity of the Steart Marshes managed realignment scheme, have been observed to separated from natural variation vary by 500mm to 2000mm, with the largest changes being due to the movement of the low water channel (Halcrow, 2015). A precautionary and changes in geomorphology approach has been taken for this assessment, which assumes that natural variation in river bed levels within the area influenced by the barrier associated with sea level rise closures is likely to be in the order of 300mm. There is, however, likely to be considerable spatial variability within this estimate with distance from which is also uncertain. Given the the barrier site. At approximately 5 km downstream of the barrier, the effect of the barrier closures on the incoming flow discharge is negligible, uncertainty over the potential and hence the effect on the associated sediment load and depth variability, outside of natural variability, is considered to be negligible. Closer to effects in the future (beyond 2055) the barrier, as flow discharge reduces, the incoming sediment load is deposited at a rate that is likely to be higher than the existing situation and limited data used to inform (<300mm). Furthermore, the deposited material will have longer to consolidate, and ebb velocities are lower following barrier closure, so that the assessment, mitigation is when the barrier is open, the rate of resuspension will be expected to be less than the existing situation. This leads to a tendency for deposition proposed to reduce uncertainty in downstream of the barrier until a new equilibrium cross-section area is reached. There is some uncertainty over the rate at which this change in the future and to assure future depth variation and cross-sectional area resulting from barrier closure may take effect and these changes may not be easily separated from natural WFD compliance. variation and changes in geomorphology associated with sea level rise which is also uncertain.

Tab 5. WFD Assessment-Parrett TRAC Direct permanent loss of bed substrate associated with the Changes to bed material due to scour protection: The estuary bed will be permanently altered during construction of the barrier and associated Local footprint of the barrier. However No risk to cill, stilling basin and bypass channel. An area of the bed adjacent to the barrier structure will require rock armour bed protection to be installed to scale, the changes are localised and not Not required. WFD reduce scour risk. It is anticipated that the total length of hard bed protection from the stilling basin and rock armour could extend for a maximum moderate at the water body scale. compliance. total length of approximately 60m along the channel and this extends across the channel width. certainty. No risk to WFD compliance.

The assessment of effects has determined there is a risk to WFD compliance on existing morphological conditions - bed substrate associated with the reduction in deposition of tidally derived marine silt up estuary due to the reduced 'tidal pumping' Footprint Morphological effect and cut off SSC during of conditions - barrier closure. structure: Local Bed substrate Changes in sediment transport/deposition from barrier closure: There is potential for reduced tidal penetration upstream of the barrier at high Not expected to be a risk in the scale, high tides, causing a reduction in the incoming marine sediment (medium/fine silt) due to the blockage of the highest incoming tides and the associated short-med term due to limited certainty. (No formal sediment load). In addition, the sediment carried in the freshwater flows (sand and silt) will deposit due to reduced outgoing flow velocities Risk addressed frequency and duration of closure Development of an baseline WFD upstream of the barrier. During current baseline conditions, sediment is deposited in the estuary during periods of slack tide and is mobilised during by delivery of effects. Risk of more significant Barrier Adaptive Environmental classification) periods of high flow. Due to tidal asymmetry of the estuary (more rapid flood tide than ebb tide velocities) there tends to be an up estuary Adaptive change in long term (post 2055) operation: Management Plan (as movement of tidally driven sediment. The marine sediment load with the incoming flood tide (SSC), that is obstructed during barrier closure, is Environmental due to anticipated higher Local above). expected to be much greater in volume than the fluvial sediment load. Hence, the net effect is anticipated to be a small reduction in the deposition Management frequency of barrier operation, scale, of fine silt up estuary. However this is uncertain. Monitoring data indicates that zones of very high sediment concentration (fluid mud) can Plan. however, the changes in bed moderate develop close to the bed, and the local effect of the barrier on such processes may be greater, however understanding of locations, processes and substrate composition may not be certainty. risks posed by changes in fluid mud in the Parrett Estuary is limited. easily separated from natural Frequency variation and changes associated increasing with sea level rise which is also in long uncertain. Given the uncertainty term. over the potential effects in the future (beyond 2055) and limited data used to inform the assessment, mitigation is proposed to reduce uncertainty in the future and to assure future WFD compliance.

Tab 5. WFD Assessment-Parrett TRAC The effect during construction is permanent resulting from direct loss of the intertidal zone structure, primarily mudflat. This Temporary effects on intertidal zone structure (extent of mudflats and saltmarsh) during construction: During construction a temporary bypass will be localised in scale, limited to channel will be constructed to one side of estuary to divert flow to enable the barrier to be constructed in the dry. Modelling of the temporary the barrier site and therefore not Local works indicates that the changes in flow will cause a localised increase in velocity which could cause localised scour and deposition. The change in at the water body scale. Where the scale, flow could potentially destabilise the adjacent intertidal banks so rip rap will be placed to provide scour protection through the narrower sections temporary bypass channel is to be moderate Not required. Not required. of the by-pass channel and in the transition areas between the river and the temporary channel to cope with the increased speed of the river. The formed, measures are to be certainty. channel will be reinstated following construction (using appropriate substrate to top the reinstated tidal areas to allow intertial habitat to establish) incorporated to reinstate the and it is anticipated that the intertidal zone lost temporarily during construction will be reinstated to a condition the same as previous. intertidal area back to a condition Morphological either the same as previous or conditions - better. Intertidal zone No risk to WFD compliance. (No formal baseline WFD classification) New embankment alignments are Changes to intertidal zone structure (extent of mudflats/saltmarsh) from the barrier, embankment raising and new embankments: The location located away from the estuary of the barrier site was selected in an area that minimises both the direct footprint impact on intertidal habitats and the impact on adjacent margins. Minor localised effects intertidal habitats, being located higher up the estuary where intertidal habitats are less extensive. The construction of physical structures of the which are not significant at the barrier, bank and bed protection will have a localised impact on the intertidal zone over approximately 60m length of the channel and banks, which water body scale. The scheme will Local will disturb an existing strip of narrow, steep intertidal habitat. This will be a permanent impact but localised within the water body. These not prevent managed realignment scale, high Not required. Not required. modifications are not expected to have impacts upstream or downstream, as the estuary is already confined in this location. Raising of some being delivered in future at certainty. existing embankments will be undertaken but this will not extend onto intertidal habitats. New embankment alignments are located away from the strategically allocated sites (i.e. estuary margins and will not impact on the intertidal zone. Provision of compensatory Atlantic salt meadow habitats for the potential cumulative Pawlett Hams) . impacts of flood risk management actions (across the overarching PEFRMS) is being managed and delivered at a strategic level (refer to Tab 1 for details). No risk to WFD compliance.

Tab 5. WFD Assessment-Parrett TRAC The footprint of the physical structures of the barrier, bank and bed protection will have a localised impact on the intertidal zone, permanently removing an existing strip of narrow, steep intertidal Changes to intertidal zone structure (extent of mudflats/saltmarsh) from changes in estuary processes and dynamics: habitat. This change will be a During barrier closure the marine sediment load that is cut off with the incoming tide is expected to be much larger than the river sediment load. permanent but localised within the Hence, the net effect is anticipated to be a small reduction in accretion upstream of the barrier in the long-term. The River Parrett demonstrates water body and therefore not cycles of channel narrowing as sediment builds up on the river banks within the intertidal areas. This material is known to be very cohesive and is significant at the water body scale. not easily eroded away. It can therefore accumulate over time on the river banks and can stabilise further through vegetation growth. However, once this accumulated bank material becomes too steep it collapses and a dynamic equilibrium state is maintained. The operation of the barrier Operation of the barrier: Barrier reducing accretion upstream could marginally slow this process, but it would not change the equilibrium being reached. closures up to 2055, will not Morphological Downstream of the barrier during closure, the overall effect on sediment processes is anticipated to result in an increase in deposition due to a significantly impact on fluvial conditions - reduction in flow velocities. The increase in deposition is estimated to reduce to less than 0.5% about 5 km downstream of the barrier site. scouring high flow events which Local Intertidal zone Designated intertidal sites as part of the downstream Severn Estuary Natura 2000 sites are located approximately 4.8km downstream of the barrier are of much longer duration and scale, site along the River Parrett. The effect of barrier closures on these designated intertidal areas is therefore likely to lie outside of any significant important in maintaining the form moderate Not required. Not required. (No formal change to the cross-sectional area and is therefore considered to be negligible. The potential effects on designated intertidal habitats has been of the intertidal zone. In the future certainty. baseline WFD assessed within the shadow HRA. (beyond 2055), the higher classification) The regime model concludes that consecutive closures of the barrier in the future could result in a 5% reduction in the equilibrium cross-section frequency of barrier operation area close to the barrier, potentially leading to increased accretion of cohesive material on the intertidal banks. This predicted effect reduces with may negatively affect the winter distance to less than a 0.5% reduction 5km downstream. The intertidal biotope present immediately upstream and downstream of the barrier is fluvial scouring process, which will A2.3 ‘Littoral mud’, further downstream it is classified as A2.322 ‘Hediste diversicolor in littoral mud’ (Apem, 2016). The increased accretion of reduce the process of cohesive materials may lead to localised changes in the distribution of these zones up to 5km from the barrier, however any potential effect from erosion/scour of the intertidal the change in the intertidal zone of the transitional water body is considered negligible for the short-term and long-term operational predictions for banks. This process currently barrier operation when viewed against the existing dynamic estuarine environment and any changes to communities which could occur with sea occurs within the intertidal zone level rise. and therefore the change caused by barrier closure is unlikely to be significant in either the short or long-term in regards to the overall structure of the intertidal zone.

No risk to WFD compliance.

Tab 5. WFD Assessment-Parrett TRAC Physico-chemical Supporting Elements Construction of the tidal barrier may result in a slight temporary increase in turbidity, in particular following piling activities or other in-channel Pre-2055: works such as creation of cofferdams or temporary bypass channel. The temporary restriction of flows within the channel during construction may Local influence turbidity, and monitoring during works should be considered as good practice as part of the Environmental Action Plan. However, given impact; Transparency the temporary nature of construction works, this is not considered to require detailed consideration in relation to WFD compliance. Certainty - The River Parrett is a highly silty and turbid environment, with high suspended sediment concentrations recorded, in particular around the zone of moderate. No risk to (No formal high mixing of tidal and freshwater flows. See 'Hydrological Regime - Freshwater Flow' above. These hydrodynamic changes will reduce mixing, but No risk to WFD compliance. None required. WFD baseline WFD to counter this the slower velocities could result in increased suspended sediment concentrations and therefore turbidity in the vicinity of the Post-2055: compliance. classification) barrier on both upstream and downstream sides. Local Changes in transparency are likely to be short term and within the range of natural variation in both the short and long term, and it is not impact considered likely to have an impact at the waterbody scale. Mixing will recommence once the barrier is opened, therefore this will be a temporary Certainty - effect, increasing in frequency with a higher number of barrier closures in the future. moderate Thermal Impoundment during barrier operation could tend to cause water to become warmer, and separation of tidal and fluvial water could result in some conditions Local minor changes in temperature during operation. No risk to impact; There will be no change during low or normal flow conditions, which is when warm temperatures are more of a concern. No risk to WFD compliance. None required. WFD (No formal Certainty - Given the short-term and infrequent nature of barrier closures there are not considered likely to be any significant effects upon thermal conditions compliance. baseline WFD moderate. at the waterbody scale. classification) The most likely water quality impact of barrier closure will be on dissolved oxygen resulting from organic matter discharged from the nearby Chilton Trinity Sewage Treatment Works (and other inputs from upstream) exerting a high oxygen demand. This would be especially during periods of low flows or slow velocity, when dilution is low, re-aeration is limited and sedimentation can occur. A 1D hydraulic model has been used to investigate the effect of the barrier on flows in the river. Analysis has been completed to consider velocities in the Parrett to test whether the propensity and duration of low velocities is sensitive to the barrier being in place. Low velocities are indicators of where aeration is poor and the risk of low Oxygenation dissolved oxygen is heightened. The data showed the presence of the barrier will reduce velocity upstream during operation. Not conditions waterbody No risk to UKTAG WFD standards to achieve 'good' status for transitional and coastal waters are for dissolved oxygen in the range 5-7mg/l dependent on scale No risk to WFD compliance. None required. WFD (No formal salinity. This value should be exceeded (higher dissolved oxygen) for 95% of the time. Barrier operations are currently expected to be, on average, impact; compliance. baseline WFD up to 79 hours per year (in 2024) rising to potentially 342 hours per year (in 2124), which equates to 0.9% and 3.9% over an annual cycle. Despite Certainty - classification) there being no baseline WFD data for current oxygen levels, it is unlikely that there would be a deterioration of dissolved oxygen conditions high. resulting from the barrier operation impacting the 5-percentile water quality standard as the barrier will not be closed for sufficiently long periods to have an influence.

The impact of wet weather flows on combined sewer overflows (CSOs) in Bridgwater has also been considered; It is concluded that the likelihood of river flows, barrier closures and CSOs coinciding is sufficiently small to suggest no impact upon the 5-percentile water quality standard. Not When the barrier is closed, tidally driven flows up the River Parrett upstream of the barrier will be prevented. Therefore, the upstream movement Salinity waterbody of salt water from the incoming tide will be prevented, reducing the extent of salinity upstream when the barrier is closed. This will temporarily No risk to No risk to WFD compliance. scale None required. reduce salinity upstream of the barrier, and increase salinity downstream due to reduce freshwater dilution. See 'Hydrological Regime - Freshwater WFD (No formal impact; Flow' above. Mixing will recommence once the barrier is opened, therefore this will be a temporary effect, increasing in frequency with a higher compliance. baseline WFD Certainty - number of barrier closures in the future. Even by 2124, a 6-hour closure on average 55 times is unlikely to result in significant habitat changes classification) moderate. upstream. Nutrient Not conditions waterbody The barrier will not create any additional source of nutrients to the waterbody. Closure of the barrier will only result in impoundment of water for No risk to scale short periods of time when high tidal flows are anticipated, which could temporarily increase concentrations upstream of the barrier. Upon re- No risk to WFD compliance. None required. WFD (No formal impact; opening of the barrier any impounded nutrients will be quickly re-dispersed following the return to dynamic flow conditions. compliance. baseline WFD Certainty - classification) high.

Tab 5. WFD Assessment-Parrett TRAC Testing of sediment within the River Parrett was undertaken in advance of dredging in 2014. This analysis did not find any grossly elevated levels of contaminants to the extent that dredged material was deemed suitable to be spread on agricultural fields. The assessment of effects from contamination has noted that there are no significant risks identified from the data available to predict a significant impact during construction. There is planned to be more testing ahead of detailed design to inform the construction methodology so that any contamination identified will not be released. Contamination testing will include the areas where in-channel construction works are required including the proposed bypass channel Specific and cofferdam locations. pollutants The barrier will not introduce any specific pollutants in to the water course particularly if good practice (such as regular maintenance of the barrier) Not is implemented. Any sediment that is mobilised within the waterbody by the proposed works is unlikely to be in any significantly greater volumes, waterbody No risk to High Status or travel any greater distances, than that which would be expected by the normal flux of material caused by flow and tidal movements. No change scale No risk to WFD compliance. None required. WFD (only copper, is therefore anticipated at the water body scale. impact; compliance. iron, toluene The barrier has been designed to reduce the need for localised desilting as far as possible, although monitoring may identify a need for ad-hoc Certainty - and zinc listed - desilting operations; this would be supported by its own WFD assessment and assessment of contamination risks. high. all high) If there is an upstream pollution incident travelling down the river, it could be blocked from flushing out of the estuary when the gates are closed. During operational maintenance closures, the barrier can be lifted if there has been an upstream incident, allowing pollutants to flush downstream on the ebb tide. If the barrier has been operated for tidal flood risk management, pollution may need to remain to protect Bridgwater from tidal flooding and therefore incident plans as part of the operating protocols for the barrrier will need to include responses for such situations, e.g. fish rescues, absorbent mats, etc. After the high tide has passed, the pollution will be able to flush down the channel as before. Any effects from the presence of the pollutants will not cause water body scale effects.

Biological Quality Elements

Flow changes: During construction there will be some localised effects on flow. Velocities through the bypass channel are predicted to be higher depending on the point in the tide cycle and the fluvial flow. The increase in velocity is generally in the range of 0m/s to 0.7m/s over a tide cycle. Increased velocity can reduce the occurrence of phytoplankton algal blooms and therefore any changes during construction are not expected to have a significant effect on the assemblage of phytoplankton within the Parrett Estuary.

Turbidity: Changes in turbidity resulting from the operation of the barrier are likely to be within the range of natural variation in both the short and Pre 2055: long-term. However, closure of the barrier will result in impoundment of water for short periods of time (up to 6 hours) when high tidal flows are Local anticipated, which could temporarily increase concentrations of nutrients upstream of the barrier. Upon re-opening of the barrier any impounded scale; Certainty: Phytoplankton nutrients will be quickly re-dispersed following the return to dynamic flow conditions. It is considered that impounding water for up to 6 hours Moderate. would not allow enough time for impounded algae to stabilise to take advantage of any increased nutrients or sunlight before the barrier is re- No risk to (No formal opened. Another key point is that barrier closures will be expected to primarily occur in February/March when water temperatures are unlikely to No risk to WFD compliance. None required. WFD Post 2055: baseline WFD be high enough to result in phytoplankton blooms. Even considering predicted temperature rises in the future the changes during operation are not compliance. Local classification) expected to have a significant long-term effect on the assemblage of phytoplankton within the Parrett Estuary. scale; Certainty: Flushing: Infrequent individual (and consecutive) closures up to 2055 are likely to have a negligible effect on estuary flushing time. Changes would Moderate. be short term while the barrier is in operation, with a closure duration of up to 6 hours before opening again, which will allow tidal mixing.

In conclusion, any changes that could affect phytoplankton will be localised and short term. The higher expected frequency of barrier closure in the long term (particularly due to a higher frequency of consecutive closures during the winter months) could potentially impact on flushing time, however the current view is that phytoplankton growth is unlikely to be significantly affected under the predicted operational regime for the barrier (Becky Griffin, Environment Agency pers. comms).

Tab 5. WFD Assessment-Parrett TRAC No macroalgae were recorded during a project-specific intertidal survey in 2016 (Apem, 2016). The absence of macroalgal communities recorded during the survey also reflect the findings of the BTB desktop study (APEM, 2016). Opportunistic macroalgae were found to occupy less than 5% of soft sediment within the Parrett Estuary during a previous WFD survey. This meant it was not considered significant enough for monitoring further (i.e. via formal surveys and species identification) (Mahoney G., Environment Agency, pers. comm. ).

As assessed above for the physico-chemical supporting element Salinity, the closure of the barrier is unlikely influence the salinity of the Pre 2055: transitional waterbody as upon opening mixing of freshwater and tidal flows will recommence. Local Opportunistic scale; Macroalgae The construction of the tidal barrier may result in a temporary increase in Turbidity , in particular following piling activities, or other in-channel Certainty: works, such as the creation of the cofferdam or the temporary bypass channel. However, as the cofferdam will be constructed early in the High. No risk to (WFD construction programme this will ensure the vast majority of the construction work is isolated from the main river channel. Whilst there will No risk WFD compliance. None required. WFD classification = inevitably be some mobilisation of silt during the construction of the cofferdam and associated with the limited works outside of the cofferdam, Post 2055: compliance. high) this should be seen in the context of the existing highly turbid environment. Once operational, during barrier closures, there is likely to be a slight Local increase in turbidity downstream of the barrier due to reduced mixing of tidal waters with the less turbid freshwater flow. Upstream of the barrier scale; there is expected to be a slight decrease in turbidity due to the reduction in sediment-laden tidal waters moving upstream, with turbidity more Certainty: closely linked to the sediment load of the river following closure. Barrier closures are likely to be of short duration and therefore changes in Moderate. turbidity will be temporary. Based on this assessment and the fact that there are insignificant quantities of macroalgae in the River Parrett there is unlikely to be an effect under the predicted operational regime for the barrier.

The operation of the barrier will slow flow down as the gates are closed. Once opened, the tidal movement of the estuary will resume. If any growth around the piers and retaining walls of the barrier occurred, this would not affect the waterbody status.

Invertebrate sampling data are available from EA sampling conducted in Bridgwater Bay (2009-2016) and at three sites along the River Parrett, and from project-specific intertidal survey work conducted by APEM (refer to APEM, 2016 and APEM, 2016b for details).

Species recorded in the tidal flats of Bridgwater Bay and within intertidal sediments in the Parrett Estuary are typical of estuarine environments and are common and widespread. The communities recorded within historic datasets both within Bridgwater Bay and the Parrett Estuary reflect Pre 2055: communities which would characterise the biotopes mapped within the Parrett Estuary (APEM, 2016). The biotopes identified during the intertidal Water survey for the project were A2.322 ‘Hediste diversicolor in littoral mud’ (LS.LMu.UEst.Hed) and A2.3 ‘Littoral mud’ (LS.LMu). A2.322 biotopes were body found in the more downstream reaches surveys (approximately 1.5km downstream of the barrier). scale; Invertebrates Certainty: Environment Agency sampling upstream of the barrier site showed communities dominated by invertebrate taxa indicative of more freshwater High. No risk to environments, e.g. chironomid larvae and freshwater gammarid amphipods. The closest upstream Environment Agency sampling point was located No risk to WFD compliance. (WFD None required. WFD 3km upstream of the barrier indicating that composition of the samples in this area are not reliant on saline water from the tide. They are classification = Post 2055: compliance. therefore, unlikely to be affected by minor short-term changes in salinity in the estuary when the barrier is operated during times of extreme tidal good) Water surges in either the 2024 and 2055 scenarios. body scale; There will be a loss of 1,300m2 of intertidal muds due to the footprint of the barrier. The barrier is located within the litoral mud biotope which Certainty: when sampled as part of the intertidal survey (APEM, 2016) had a virtual absence of conspicious species. The loss of this small areas of mud within Moderate. the context of the waterbody would be negligible on the invertebrate population.

Invertebrates in areas affected by the barrier are well adapted to the dynamic estuarine regime and are unlikely to be affected by the short-term predicted changes in water quality, salinity and turbidity during barrier operations. It is anticipated that effects will not be significant for the predicted operational regime of the barrier and there will be no deterioration in status of the water body.

Tab 5. WFD Assessment-Parrett TRAC Effect of construction works at the barrier on migratory fish species: During construction there could be effects on upstream and downstream movement of salmon, sea trout and European eel, due to noise and vibration generated during the construction works. As the piling hours will be restricted to between 5am to 8pm, there will be windows of no piling activity during the night when these species can move past the area when migrating. Lighting during the construction phase is unlikely to result in cessation of migration as it will be only be used when works are ongoing. Fish and eels will be able to use the bypass channel as for approximately half of the time, the tidal flow will be moving in the preferred direction of migration and individuals will be able to utilise these flows to migrate through the bypass channel. The construction works will be temporary and effects will cease when the works are completed (currently expect a 4 year period). Consequently, it is considered that there would be no permanent impact on fish populations causing deterioration of ecological status or prevention of meeting Good Ecological Status for the upstream water bodies during the Construction Phase. Effect of operational closures on migratory fish species, the assessment of effect for each species is provided below: Eels: Juvenile glass eel migration through the lower Parrett system peaks between March and May each year, though it is understood following discussion with EA area fisheries advisors that individuals migrate into the Parrett earlier than this, from December dependent upon flow and temperature conditions, and that the run may continue until June. Juvenile glass eels are understood to migrate through the estuary utilising Selective Tidal Stream Transport (STST) to move upstream on flood tides (Harrison et al., 2014). European eels within the Parrett Estuary, and upstream Parrett and Tone catchments are part of a pan-European population, with juveniles from the spawning grounds in the Sargasso Sea being carried by oceanic currents across the Atlantic Ocean to distribute across the Western Europe continental shelf each year (van Ginneken and Maes, 2005). Under the proposed operation, the barrier would close at low tide, remain closed for the duration of a surge tide and re-open upon equalisation of Pre 2055: Fish the fluvial and tidal water levels. Consequently, during operation, the barrier will prevent the upstream migration of glass eels with passage of significant numbers of eels then unlikely to occur until the following flood tide when the barrier is open (though some individuals may migrate Water body (No formal upstream immediately after barrier opening as water levels will be equalised). scale; baseline WFD From analysis of the West Quay tide data between 2003-2015, the barrier will be closed on an average (mean) of 0.39%, 1.43% and 3.73% of the Certainty: classification) flood tides during the upstream migration period of juvenile glass eels (December-June) for each representative year of predicated barrier High. No risk to NB: this operational frequency (i.e. 2024, 2055 and 2125), respectively. This equates to closures on up to 2, 7 and 16 tides per year (rounded up to the No risk to WFD compliance. None required. WFD section is nearest full tide) during the migratory period for each of the representative years, respectively. As discussed in Chapter 6, consecutive barrier Post 2055: compliance. relevant to all closures are included within the total number of closures predicted each year from the analysis of the West Quay tide data. For example, if there Water water bodies were a total of 2 closures for tidal flood risk management in a year, and there was a period of two closures on consecutive flood tides, then there body screened in for would be no further closures. scale; fish only (refer Under a scenario with a single barrier closure and the assumption that no upstream migration occurs during the ebb phase of the tide, eels Certainty: to tab 2a) attempting to migrate upstream beyond the barrier at the onset of a flood tide could be delayed for a maximum period of ~12.5 hours prior to the onset of the following flood tide. This period of delay could be further increased if closure of the barrier spans consecutive flood tides, and would Moderate. be a maximum of ~50 hours in 2024 (up to 4 consecutive closures), ~100 hours in 2055 (up to 8 consecutive closures) and ~115 hours in 2125 (up to 9 consecutive closures). For individuals that migrate upstream on the flood tide and encounter a closed barrier, they are likely to subsequently stay immediately downstream of the barrier following findings of studies on tidal barriers in the Netherlands (Dekker and van Willigen, 1997), with further glass eels accumulating as the barrier remains closed (Bult and Dekker, 2006). Given that the maximum delays will be ~50 hours in 2024, ~100 hours in 2055 and ~115 hours in 2124, upon re-opening of the barrier on the flood tide then individuals would be able to continue their upstream migration. In the current baseline situation, after their glass eel migration, many eels will reside in the estuarine environment or show nomadic behaviour between the coastal, estuarine and freshwater environments (Daverat and Tomas, 2006; Walker et al., 2013). Therefore, even if the migration of some individuals is halted by the closure of the barrier for the worst case duration, these individuals can seek residence within the estuary (Bardonnet and Riera, 2005) or move upstream into the River Parrett after the barrier is re-opened. Whilst a delay and accumulation of glass eels behind a permanent barrier may elevate the risk of predation or density dependent mortality of individuals, direct empirical evidence for this effect is limited for tidal barriers which are temporarily closed. Mouton et al. (2011) and Bult and Dekker (2007) found that opening of a permanent tidal barrier on the flood tide or saltwater ingress on the flood tide was an effective mitigation for delays in glass eel migration behind a closed barrier as the eels would use the opening period and tidal flows to migrate past the barrier. The proposed barrier will not be permanently closed, it will remain open on flood tides which do not reach the trigger level for a closure, allowing eels to continue to migrate. Given the evidence of the benefits of temporarily opening permanently closed barriers for the passage of glass eels, it is considered that the re-opening of the barrier following either a single or consecutive flood tide closures is likely to allow the passage of those glass eels which were halted by the closed barrier. Any increase in densities of migrating glass eels behind the closed barrier will be for a very short period of time as they are migrating upstream through the Parrett Estuary. Given the free passage of fish for the majority of the year, the project is not anticipated to influence the densities of Tab 5. WFD Assessment-Parrett TRAC through the Parrett Estuary. Given the free passage of fish for the majority of the year, the project is not anticipated to influence the densities of eels within the estuary or upstream catchments in the longer-term which could affect sexual differentiation of juveniles. Sexual differentiation in eels occurs during their phase of residency within the marine, estuarine or freshwater environment as yellow eels (Colombo and Grandi, 1996). Gonad differentiation in European eels commences at lengths over 200mm total length, and is often complete at 350mm total length (Davey and Jellyman, 2005). Given the lengths of glass eels migrating through the Parrett Estuary are unlikely to be longer than 90mm (Dekker et al., 1998), then they will not show gonad differentiation for a number of months or even years after migrating through the Parrett Estuary as glass eels. The delay of some glass eel individuals behind the barrier in any given year will have a very minor effect upon the population in the Parrett and Tone catchments as the individuals are likely to continue their migration shortly after re-opening of the barrier. If they do not continue their migration on the subsequent flood tides, then they can reside in the marine and estuarine environment without physiological impairment and can migrate upstream at a later point or show nomadic behaviours. Furthermore, given the nature of the European eel population, as a pan-European stock which arrives each year from the spawning grounds in the Sargasso Sea, the effect upon glass eels in any one year will not influence the glass eel run in the subsequent years (i.e. there would be no incremental effects given the scale of the wider population). Therefore, no reduction in abundance of glass eels within the Parrett and Tone catchments as a result of the project is expected. Adult silver eels typically migrate downstream during a migration window extending from August to December. Whilst silver eels are not as reliant on the tidal state for movement through estuarine waters as the juvenile glass eel life stage, the majority of downstream migration through these waters is thought to occur on the ebb tide to optimise energy expenditure (Barry et al., 2016). It is acknowledged that some movement of silver eels may occur during slack water but individuals migrating downstream in slack water prior to a flood tide would not encounter the closed barrier as it closes during the flood tide. Individuals migrating downstream in slack water prior to an ebb tide may encounter the closed barrier briefly but this barrier would open shortly afterwards as the tide turns to the ebb tide allowing free passage. As the closure of the barrier for tidal flood risk Pre 2055: Fish management purposes will primarily encompass the flood phase of the tidal cycle, it is not anticipated that there will be potential delays caused to Water downstream silver eel migration up to 2125 that may influence their escapement success. body (No formal Therefore, the project is not considered to result in a reduction in abundance of eels within the Parrett catchment, and thus pose no risk of scale; baseline WFD deterioration for WFD water bodies. This considers the very low proportion of flood tides affected during the glass eel migratory period; the Certainty: classification) likelihood of migration of any delayed glass eels continuing following re-opening of the barrier through their use of tidal flows to migrate and High. No risk to NB: this No risk to WFD compliance. inability to swim against tidal flows in the Parrett Estuary; the opportunity for delayed glass eels to reside within the estuarine and marine None required. WFD section is environment without physiological impairment if they do not continue their migration on the flood tides following re-opening of the barrier; the Post 2055: compliance. relevant to all pan-European nature of the European eel stock leading to annual replenishment of glass eels to the Parrett Estuary from the wider stock each year Water water bodies and no incremental losses to the population; and the lack of closures on the ebb tide when silver eels migrate resulting in no barrier to escapement body screened in for migration for adults. scale; fish only (refer Certainty: to tab 2a) Atlantic salmon: Numerous studies have demonstrated a distinct tidal component to the downstream movement of salmon smolts through Moderate. estuarine waters. Migration of wild smolts through the Conwy Estuary, North Wales, was shown to be predominantly by nocturnal selective tidal stream transport on the ebb tide, but with some variation in different parts of the estuary and in early and late running groups of fish. In the upper estuary they moved predominantly by night downstream with the ebb tide, staying in the upper layers of the water column, and holding station or swimming against the flood tide sometimes being displaced slowly upstream (Moore et al., 1995). The principal variation to this pattern was the loss of selective nocturnal movement in later running smolts, which consequently move faster through estuaries than earlier migrants, possibly as an adaptation to reach marine habitat at an optimal time (Moore et al., 1995). In another study of wild smolts, Moore et al. (1992) showed that salmon smolts passed through the smaller Hampshire Avon estuary in less than one tidal cycle, moving on the ebb tide at night. Similarly, Moore et al., (1998) observed the majority of downstream smolt migration to occur during the ebb cycle on the River Test. Considering that closure of the barrier for flood prevention purposes will primarily encompass the flood phase of the tidal cycle, there are not anticipated to be significant impacts to downstream salmonid smolt migration under the initial or future operating proposals. The closure of the barrier may cause the potential for downstream migrating Atlantic salmon smolts to not hold position on the flood tide, and instead move downstream on the flood tide to reach the barrier. This would result in individuals being held up behind the barrier until the subsequent ebb tide for a maximum of 6 hours, but likely to be substantially less by the time individuals have actively migrated downstream to the barrier in the absence of favourable flow direction. However, this behaviour would also result in positive net migration for the Atlantic salmon smolts and not delay their migration, as they would be further downstream than under baseline conditions. Also, as they would be able to pass the barrier on the subsequent ebb tide and downstream migration of Atlantic salmon smolts occurs swiftly (Moore, 1997; Moore et al., 1998; Drenner et al., 2012; LaCroix and McCurdy, 1996; Thorstad et al., 2012; Lothian, 2016), then they would only be present upstream of the barrier for a short period of time before moving out to sea. This effect is therefore considered to represent a very low risk to subsequent mortality and fitness of the migrating smolts to those individuals that it occurs. Furthermore, it is acknowledged that if the presence of the barrier whilst in the ‘open’ position reduces the cross-sectional area of the channel Tab 5. WFD Assessment-Parrett TRAC Furthermore, it is acknowledged that if the presence of the barrier whilst in the ‘open’ position reduces the cross-sectional area of the channel slightly then flow speeds will be elevated, and Atlantic salmon smolts have been shown to be hesitant to move into areas of accelerating flow (Enders et al., 2008; Enders et al., 2012; Haro et al., 1998; Kemp et al., 2005; Kemp et al., 2008; Russon and Kemp, 2011). Therefore, an increase in flow speeds may reduce the migration rate of individuals through the Parrett Estuary as they encounter the accelerating flow. However, whilst the channel width is narrowed into two rectangular channels either side of the central column of the barrier, part of the channel will be deepened, the banks of the channel will be raised, and the slope of the channel will not be altered. Overall, therefore, the flow velocities through the barrier whilst it is open are not likely to be significantly different from the baseline flows present. If any increases are present, they are likely to be substantially lower than the flow speeds evaluated in the cited papers and unlikely to pose a delay to migrating individuals. Furthermore, detailed modelling will be considered to inform later stages of the scheme as required. It is envisaged that this modelling will aim to support the detailed design and construction of the barrier to minimise hydraulic impacts in the vicinity of the structure. Previous studies on rivers draining into the Severn Estuary have demonstrated correlations between adult salmon migration and tidal state. Aprahamian et al., 1998, found that salmon migrated upstream predominantly on the flood tide during a radio tracking study of 56 adult fish in the Usk Estuary. Little movement was observed during the ebb tide, with movement recommencing at the bottom of the ebb cycle and continuing into the following flood tide. In a review of tracking studies, Milner, 1990, noted that upstream movement incorporating selective tidal stream transport was frequently seen in the main estuary, with fish taking advantage of the flood tide; but also swimming strongly upstream on the ebb, seemingly when committed to river entry. Such contrasting behaviours may reflect salmon in differing phases of readiness to enter the river, or differences between natal fish and straying non-natal fish (Milner, 1990). The upstream migration of Atlantic salmon in many rivers peaks during the summer and autumn, as shown for other rivers in the region such as the Pre 2055: Fish River Tawe in South Wales (Environment Agency, 2002) and River Frome on the south coast of England (Welton et al., 1999). Uncertainty exists, Water however, in the precise timing of migration of returning adult individuals through the Parrett Estuary and to the River Tone population. Following body (No formal discussion with EA fisheries advisors it is considered that Atlantic salmon could theoretically migrate through the Parrett Estuary at any point scale; baseline WFD between February and December, and it is uncertain when the actual migratory period and migration peaks are during these months. Certainty: classification) From analysis of the West Quay tide data between 2003-2015, the barrier will be closed on an average (mean) of 0.24%, 1.07% and 3.42% of the High. No risk to NB: this flood tides during the potential upstream migration period of adult Atlantic salmon (February to December) for each representative year of No risk to WFD compliance. None required. WFD section is predicated barrier operational frequency (i.e. 2024, 2055 and 2125), respectively. This equates to closures on up to 2, 8 and 23 tides per year during Post 2055: compliance. relevant to all the migratory period for each of the representative years, respectively. Water water bodies Based on the predicted proportion of flood tides that the barrier will be closed in the potential adult Atlantic salmon migration period, during initial body screened in for operation of the barrier from 2024 (0.24%), it is considered extremely unlikely that the barrier would impede the upstream migration of adult scale; fish only (refer Atlantic salmon. Any individuals which are migrating during the small number of flood tides that the barrier would be closed, would be able to Certainty: to tab 2a) continue their migration shortly afterwards when the barrier reopens after up to 6 hours as the barrier would only be operated for a mean of ~2 Moderate. tides during the migratory period. This short delay would pose a limited risk of increased predation or reduced fitness for the very small proportion of the population that it may affect, as the species has been shown to adopt downstream and subsequent upstream movements upon reaching a migratory barrier to attempt passage numerous times. If individuals were migrating during the small number of flood tides that the barrier would be closed, then they would be able to pass the barrier on their subsequent attempts while it is open. For example, Lundqvist and Rivinoja, 2008, recorded individuals attempting passage up to 11 times at a migratory barrier, over a median delay time of 9 days (min-max 1-82 days). Newton et al., 2018, also recorded individuals attempting passage up to 11 times at a migratory barrier, over a mean delay time of 47.8 hours (min-max 15 minutes-31 days). Newton et al., 2018, also found that of the 41 fish encountering the migratory barrier and attempting passage at least once, 40 made it successfully over the migratory barrier with only one (2.44%) not attempting passage again. Therefore, the probability of the barrier resulting in a failed migration for an individual (i.e. combining the probability of an individual encountering the closed barrier, and the probability of an individual subsequently being deterred from its migration) is very small. Increased closure frequency and the potential for closures to span a larger number of consecutive tidal events are anticipated in the future, however, given the very small proportion of flood tides that the barrier will be closed for up to 2055 (refer to Chapter 6), the majority of the returning spawning population is likely to be able to migrate uninhibited. However, considering the uncertainty within the timing of migration of returning adult Atlantic salmon individuals through the Parrett Estuary and to the River Tone population, and the increased number of closures predicted up to 2125, there is a greater risk of overlap between the migration of some returning adults and the months with the greatest numbers of closures (April/May and September/October) up to 2125. This overlap, and the greater number of flood tide consecutive closures, may mean that on a precautionary basis some returning adult Atlantic salmon individuals could be delayed in their migration for a longer period, with subsequent additional risks of increased mortality and reduced spawning success. As the population of Atlantic salmon in the River Tone is very small (West, 2019), then the delay of a small number of individuals could affect an Tab 5. WFD Assessment-Parrett TRAC the population of Atlantic salmon in the River Tone is very small (West, 2019), then the delay of a small number of individuals could affect an appreciable proportion of the total run of spawning adults. Therefore, the project is considered to potentially result in a reduction in abundance of Atlantic salmon within the Parrett catchment up to 2125. However, as Atlantic salmon are not represented within the FCS2 model used to classify the River Parrett and Tone catchment water bodies, then a deterioration in status is not expected in any of the water bodies in the Parrett catchment. This assessment considers the lack of closures on the ebb tide when Atlantic salmon smolts migrate, resulting in no barrier to migration for juveniles; the very small proportion of flood tide closures up to 2055 (~1% or less) being unlikely to impede and delay migrating individuals for a period of time that might reduce spawning success, and the Sea trout: Similar migratory behaviour to Atlantic salmon has also been established for sea trout; Milner (1992) observed sea trout displaying tidally directed movement through the estuary of the Afon Glaslyn, North Wales. Prior to river entry they held up in saline waters, moving upstream when detecting the influence of river flow which quickly reduced salinity. Tidally directed movements tended to be strongest in the lower to middle reaches of estuaries, with fish moving upstream on the flood tide and back on the ebb, or pausing while moving up or downstream, if suitable holding sites were present. Such movement patterns are commonly observed in a number of rivers, e.g. the Fowey (Potter, 1988), the Ribble (Preide et al., 1988) and the Aberdeenshire Dee (Smith & Smith, 1997). Past studies have also demonstrated that salmonid migration through estuarine waters is predominantly nocturnal in nature (e.g. Smith & Smith, 1997; Gauld et al., 2016). Whilst upstream migration of adult sea trout can occur over the same general period as for Atlantic salmon (February to December), upstream Pre 2055: Fish migration of sea trout is more closely confined to the summer months, predominantly in June and July, and with a further peak in October and Water November identified following discussions with EA fisheries advisors. Sea trout populations are supported by freshwater residents of the species, body (No formal who do not migrate to the estuary and therefore will not be affected by closure of the barrier. Therefore the population of the River Tone is not scale; baseline WFD considered to be vulnerable to the delay of a small number of individuals migrating through the Parrett Estuary. Certainty: classification) From analysis of the West Quay tide data between 2003-2015, the barrier will be closed on an average (mean) of 0.24%, 1.07% and 3.42% of the High. No risk to NB: this No risk to WFD compliance. flood tides during the potential upstream migration period of adult sea trout (February to December) each year up to 2024, 2055 and 2125 None required. WFD section is respectively. This equates to closures on up to 2, 8 and 23 tides per year (rounded up to the nearest full tide) during the migratory period up to Post 2055: compliance. relevant to all 2024, 2055 and 2125 respectively. Water water bodies Under the proposed 2024 and 2055 operation regimes there is only a single instance of barrier closure within the sea trout peak upstream body screened in for migration window in June and July (based on the West Quay data from 2003-2015). While the mean number of closures per year will increase over scale; fish only (refer the lifetime of the structure up to 2125, closures are still not predicted to occur during June or July. It is therefore considered unlikely that the Certainty: to tab 2a) barrier would significantly impact upon upstream sea trout migration during operation. Moderate. Therefore, the project is not considered to result in a reduction in abundance of eels within the Parrett catchment, and thus pose no risk of deterioration for WFD water bodies. This assessment considers the lack of closures on the ebb tide when sea trout smolts migrate resulting in no barrier to migration for juveniles; the very small proportion of flood tide closures up to 2024 and 2055 (~1% or less) are unlikely to impede and delay migrating individuals for a period of time that might reduce spawning success; the more temporally confined peaks in sea trout migration during the summer and autumn avoiding the higher proportion of flood tide closures in the spring in 2024, 2055 and 2125; that no closures are predicted during sea trout migration peak periods in June and July; and that the biology of sea trout means that populations in the Tone and Parrett are supported by the freshwater resident brown trout which do not migrate and therefore will not be affected by the closure of the barrier, reducing the vulnerability of the population.

Summary: Therefore, the operation of the barrier is not considered to pose a risk of changing the presence or abundance of Atlantic salmon, sea trout or European eel in upstream water bodies, or in the Parrett TRaC water body. Therefore, the project is not considered to pose a risk of causing a deterioration in status in the upstream river water bodies (assessed via the FCS2 model) or in the Parrett TRaC water body (assessed via the TFCI model).

Tab 5. WFD Assessment-Parrett TRAC Effect of water chemistry changes on migratory fish and estuarine fish species: During a barrier closure the upstream movement of salt water from the incoming tide is prevented. This will temporarily reduce the salinity upstream of the barrier. During those extreme tides when the barrier is closed, the frequency and extent of this weak salt water intrusion will be reduced. For diadromous species, given their natural tolerance to salinity fluctuations encountered during migrations, no effects upon these fish species are anticipated. For other designated fish species of the Severn Estuary, the brief reduction in salinity upstream will have limited influence given that they will be excluded from the area by the barrier itself for a short period of time. Therefore, effects are not considered likely and no further consideration is therefore required within this assessment. Organic matter discharged from the nearby Chilton Trinity Sewage Treatment Works (and arriving from upstream) may exert a higher oxygen demand during barrier closures, especially during periods of low flows and velocities, when dilution is low, reaeration is limited and sedimentation can occur. An initial high-level assessment of potential water quality impacts of the project has indicated that the barrier will never be closed for sufficiently long periods for this to be significant (i.e. will not exceed 5-percentile UKTAG WFD standards for waterbodies to achieve ‘good’ status). Even during consecutive closures the barrier will re-open on each ebb tide. Any potential dissolved oxygen sags would be most pertinent during the summer months during which time there is likely to be comparatively fewer barrier closures under both the initial and future scenarios. Therefore, effects are not considered likely and no further consideration is therefore required within this assessment. Pre 2055: Fish Water body (No formal Effect of changes in turbidity on migratory fish and estuarine fish species: The construction of the tidal barrier may result in a temporary increase scale; baseline WFD in turbidity, in particular following piling activities, or other in-channel works such as the creation of cofferdam(s) or the temporary bypass channel. Certainty: classification) However, the cofferdam(s) will be constructed early in the construction programme and this will ensure the vast majority of the construction work High. No risk to NB: this No risk to WFD compliance. is isolated from the main river channel. Whilst there will inevitably be some mobilisation of silt during the construction of the cofferdam(s) and None required. WFD section is associated with the limited works outside of the cofferdam, this should be seen in the context of the existing highly turbid environment. Any silt Post 2055: compliance. relevant to all mobilised will be insignificant when compared with the existing turbid environment. It is important to note that the designated fish species in the Water water bodies Severn Estuary are naturally exposed to very high levels of turbidity and are well adapted to swimming through and surviving within highly turbid body screened in for waters. Consequently, given the naturally turbid conditions within the Severn and Parrett Estuaries and the ability of fish to swim past or avoid any scale; fish only (refer areas of increased turbidity, any effects on designated fish species are expected to be negligible. Once operational, during barrier closures there is Certainty: to tab 2a) likely to be a slight increase in turbidity downstream of the barrier due to reduced mixing of tidal waters with the less turbid freshwater flow. Moderate. Upstream of the barrier there is expected to be a slight decrease in turbidity due to the reduction in sediment-laden tidal waters moving upstream, with turbidity more closely linked to the sediment load of the river following closure. Barrier closures are likely to be short-term and infrequent and therefore changes in turbidity will be temporary. Based on this assessment and the fact that the designated fish species present are well adapted to changes in turbidity, this effect will not be considered further within this assessment.

Effect of changes in bed level and water depth on migratory fish and estuarine fish species: In the long-term, deposition as a result of the operation of the barrier is estimated to cause an increase in bed level of 100mm in the Parrett Estuary. This small increase is unlikely to affect the movements of individuals migrating through the Parrett Estuary. Also, as few species are known to spawn in the tidal reaches of the Parrett, then the change in bed level will not affect the spawning success or post-hatch mortality of the majority of species. The change in bed level is also not expected to influence the foraging behaviour of species resident within the Parrett Estuary. Therefore, it is considered that the small change in water depths will not affect the population abundance, structure or extent of the fish species present within the Parrett Estuary transitional water body.

Tab 5. WFD Assessment-Parrett TRAC Effect of changes in bed substrate on migratory fish and estuarine fish species: An area of the bed adjacent to the barrier structure will require rock armour bed protection to be installed to reduce scour risk. It is anticipated that the total length of hard bed protection from the stilling basin and rock armour is approximately 60m along the channel and this extends across the channel width. This represents a change in habitat type over less than 1% of the length of the water body. This change in substrate may be sub-optimal for some species residing within the estuary (such as flatfish species), but these individuals will have wide areas of estuary to utilise that remains optimal. For some species, this substrate will be optimal for residency (such as wrasse species) and there may be limited habitat elsewhere within the water body where this habitat exists. Overall, the change in substrate may increase the diversity of fish species within the estuary and this would improve the TFCI score for the water body. The Pre 2055: Fish change in substrate would have no effect on migrant species which are moving through the area. There is the potential for reduced tidal Water penetration upstream of the barrier at high tides, which may result in a fining of sediment (as the marine derived sediment is coarser than fluvial body (No formal sources). This is unlikely to influence fish species as the majority present within the Parrett Estuary will find fine sediment suitable for foraging and scale; baseline WFD residency. Increased siltation due to ponding of freshwater upstream of the barrier may also lead to some fining in bed sediment close to the Certainty: classification) barrier, but given the requirement for rock armour bed protection, this fining is unlikely to influence the behaviours of fish in this area. This will not High. No risk to NB: this affect the population abundance, structure or extent of the fish species present within the Parrett Estuary transitional water body. No risk to WFD compliance. None required. WFD section is Post 2055: compliance. relevant to all Effect of changes in intertidal habitat structure and extent on estuarine fish species: As the barrier is not predicted to change the overall width, Water water bodies depth or tidal prism of the estuary, it is not expected to have a significant effect on the overall structure of the intertidal zone including the location body screened in for and extent of mudflats and saltmarsh within the estuary. Locally, there is potential for the barrier operation to lead to increased accretion of scale; fish only (refer cohesive material on the intertidal banks, leading to vegetation growth and bank steepening. Current understanding of the Parrett suggests this Certainty: to tab 2a) typically fails due to oversteepening, maintaining the overall estuary regime cross-section over time. Therefore, intertidal habitats across the Moderate. estuary will remain unchanged as a result of the project and this will not affect the population abundance, structure or extent of the fish species present within the Parrett Estuary transitional water body. The construction of physical structures of the barrier, bank and bed protection will have a localised impact on the intertidal zone over approximately 60m length of the channel and banks, which will disturb an existing strip of narrow, steep intertidal habitat. This will be a permanent impact but localised within the water body. As the current intertidal habitat at the location of the barrier is both narrow and steep, then it is unlikely to be utilised as suitable habitat by fish species for residency or foraging behaviour and changes to this habitat will not affect the population abundance, structure or extent of the fish species present within the Parrett Estuary transitional water body.

Key: - - High risk to WFD objectives - Some risk to WFD objectives (requiring mitigation) / Neutral / negligible effect on WFD objectives + Slight positive contribution to WFD objectives + + Good positive contribution to WFD objectives

Tab 5. WFD Assessment-Parrett TRAC STEP 3 - Assess potential changes in quality elements (A - risk of deterioration) and contribution to mitigation measures (B - achieving GEP). 3B - MITIGATION MEASURES Cycle 2 SW RBMP HMWB Mitigation Measures classification data (2015 Comments in relation to WFD assessment for Bridgwater Barrier Catchment Data Explorer) EA NEAS and Environmental Planning specialist discussed at the preliminary screening stage (Aug 2016) that the Parrett Transitional is designated as a HMWB, and data shows it is failing Overall Mitigation Measures the mitigation measures assessment and needs these measures to be put in place. However no other monitored biological or chemical elements were failing so they questioned whether Moderate or less Assessment there is a need to put any MMs in place if it is effectively at GEP. In light of the current uncertainty, a basic review of the listed generic waterbody mitigation measures has been completed. Realign flood defence Measure is not 'in place' Scheme will not deliver this measure and will increase height of some existing embankments. This does not prevent this measure being achieved in the future.

Good positive and significant contribution to WFD objectives due to improved fish and eel passage at 12 sites upstream of the barrier to be delivered as part of the BTB Scheme. Refer to Fish passes Measure is not 'in place' separate WFD Assessments for details of the measures proposed. Enhance ecology Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Remove obsolete structure Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Changes to locks etc. Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Avoid the need to dredge Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Dredging disposal strategy Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Reduce impact of dredging Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Reduce sediment resuspension Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Retime dredging or disposal Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Sediment management Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Dredge disposal site selection Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Manage disturbance Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Retain habitats Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Remove or soften hard bank Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Indirect mitigation Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Scheme will not directly deliver or prevent this measure. Project has considered measures to create habitats at the barrier site with reference to sources such as the Estuary Edges: Preserve or restore habitats Measure is not 'in place' Ecological Design Guidance {Environment Agency, 2008} but the construction and maintenance of any such measures within the highly silty environment of the River Parrett is not considered to be viable. In channel morph diversity Measure is not 'in place' Scheme will not directly deliver or prevent this measure. Bank rehabilitation Measure is not 'in place' Scheme will not directly deliver or prevent this measure.

Key: - - High risk to WFD objectives - Some risk to WFD objectives (requiring mitigation) / Neutral / negligible effect on WFD objectives + Slight positive contribution to WFD objectives + + Good positive contribution to WFD objectives

Reference: Environment Agency (2008) Estuary Edges: Ecological Design Guidance

Tab 5a. Mitigation Measure Assessment STEP 4 - In-combination assessment Projects listed have been screened in as having some effects. The effect is given as either DIRECT (if within the Parrett Transitional water body) or INDIRECT if not within the water body. Plans (such as local development plans) have not been included.

Actions for WFD Compliance (including Project In-combination effect (DIRECT / INDIRECT)* Assumptions proposed mitigation during design and implementation of works):

Requirement for future monitoring of effects of DIRECT long-term dredging upstream of the tidal barrier Continuing discussions between the Environment Agency and the IDB/SRA. Potential impact to WFD compliance for Bridgwater Bay coastal and Parrett Transitional water is included within future monitoring bodies. requirements for the BTB Scheme. A protocol has been set up between the Environment Agency and the IDB (Environment Agency, 2018). The IDB intend to undertake dredging acting Initially undertaken as trials, in 2018 the Environment Agency agreed a 5-year protocol for the To reduce as much as possible the potential for under the Environment Agency’s powers related to flood risk works on main IDB to follow to ensure the WID works are legally compliant with environmental legislation. in-combination effects on sediment rivers. The SRA are therefore acting as a Competent Flood Risk Management The IDB are developing extensive plans for further dredging upstream of the barrier and WID transport/deposition, channel morphology and Parrett Internal Drainage Authority and is responsible for legal compliance with environmental is expected to be the main method of annual maintenance. The EA therefore considers the navigation, the WID protocol provided by the Board (IDB) Water Injection legislation. The Environment Agency have provided advice and a set of dredging to be a permanent annual event along the Parrett water body. Environment Agency to the IDB will be revised Dredging (WID) upstream of conditions for the IDB on how they can undertake a rolling 5-year programme before the BTB Scheme becomes operational. Bridgwater of annual maintenance in areas previously dredged without causing There are anecdotal reports of increased sedimentation occurring in the vicinity of Dunball The revised protocol will be informed by ongoing environmental “harm” (as defined by Section 161A of the Water Resources Act (downstream of the Barrier) from the trials undertaken to date. Given the potential long-term monitoring (refer to the Bridgwater Tidal Barrier 1991) and other breaches in environmental legislation. Full details of the frequency of the WID along the Parrett and Tone there is the potential for in-combination Estuary Monitoring Plan), and discussion with the conditions are provided in Environment Agency (2018). Within the protocol the effects. WID is most effective on and targeted for high spring tides when the strong ebb tide is IDB, but is expected to state that WID cannot be advice states that WID should be restricted to December to January to avoid most effective at moving the disturbed sediment downstream. These are the tides that carried out on the tidal cycle preceding or at the impacts on migratory fish. It is assumed that the IDB will heed this advice and barrier closure is most likely to be required. There is therefore potential for significant same time as any barrier closures (operational or therefore there are no expected pathways to effect on qualifying fish and eels enhanced silt deposition at the barrier site. maintenance). The Environment Agency will keep from increased turbidity expected. the IDB informed of planned or predicted barrier closures. The areas due to be subject to the 5-year rolling programme of WID have been DIRECT dredged as part of earlier projects to improve the capacity of the Rivers Parrett Potential impact to WFD compliance for Bridgwater Bay coastal and Parrett Transitional water Long term changes to morphological conditions and Tone (Environment Agency, 2018). The 5-year programme will maintain Somerset Rivers Authority bodies. shown through monitoring due to both projects the dredged areas therefore there will be no alteration from the current Dredging Programme acting in combination to be reviewed in the baseline. There will be therefore, no changes to the surface water flooding of Capital and maintenance dredging on the Parrett and Tone beyond that covered by the 5 year future as part of the Adaptive Environmental adjacent habitats. WID programme. Management Plan so that action to prevent effects on the water body are taken.

EDF Energy Hinkley Point C DIRECT Nuclear Reactor & associated Neutral impact - Construction periods are likely to overlap, however considered unlikely to Assumed that construction programmes will overlap. None required projects have in-combination effects with the BTB Scheme.

INDIRECT Avonmouth Severnside The project aims to manage the risk of flooding to keep pace with climate change and rising Project will proceed and will be WFD compliant. Habitat creation will seek Enterprise Area (ESEA) sea levels. It includes the creation of a new area of habitat to compensate for that lost within additional enhancement opportunities relevant to WFD if possible. Project will None required Ecology and mitigation flood the Severn Estuary Natura 2000 as a result of the development. Potentially positive impact be completed prior to construction of the BTB Scheme. defence project through the possible provision of enhancement opportunities along the water body and species diversity.

Reference: Environment Agency (2018) R. Parrett Water Injection Maintenance Dredging – Environmentally Acceptable Protocol. Report Version 2.0 (08/10/18)

Tab 6. In-combination Assessment