Bridgwater Tidal Barrier Scheme WFD Compliance Assessment
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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.