Introduction
Through recent engagement, we have heard a recurring question; how will the barrier actually work when a tidal surge occurs?
While many people understand that the scheme is being built, it can be harder to visualise how the different elements of the scheme will work together, particularly what happens to river flows upstream and downstream when the gates close. This can lead to uncertainty and questions about how flood water is managed and whether closing the barrier could displace water elsewhere.
It is not always easy to explain a system that changes and responds to tidal conditions using static images or written descriptions alone.
To help bring this to life, we have developed a new animated video explainer showing how the Bridgwater Tidal Barrier Scheme will operate as one connected flood risk reduction system.
Using animation, digital design imagery and footage from the local area, the video takes you through what happens when the barrier closes, how water continues to flow through the wider river system, the role of the downstream defences and how all the elements work together to reduce flood risk.
How will the Bridgwater Tidal Barrier Scheme work
Watch our animated video explainer to see how the tidal barrier and downstream defences at Chilton Trinity, Pawlett and Combwich will work together as one connected system to reduce tidal flood risk. Using animation, digital design imagery and local footage, the video shows what happens when the gates close, how river water is managed upstream, the role of downstream defences and what happens when the gates reopen as tide levels fall.
Your questions and answers
What happens downstream when the barrier closes?
The barrier is operated in conjunction with the wider flood defence system. Downstream flood defences, including primary and secondary embankments and other flood defence features, provide layers of protection to communities along the River Parrett. The operation of the barrier is carefully planned around predicted tide and river levels to manage flood risk across the wider system.
Exactly how long are the gate closure periods expected to be?
Gates will be closed near low water and start to be reopened as highest tide level is reached and begins to subside; exact timings will depend on the relative levels each side of the gates of the river and the tidal water.
How often will the barrier close?
The barrier will only close when forecast conditions indicate that closure is required to reduce tidal flood risk. The frequency will therefore depend on weather and tidal conditions.
Year 1
- Tidal flood risk management 6 hours: 1 to 5
- Testing and training 4 to 5 hours: Up to 5
- Maintenance 1 to 2 hours: 12 to 24
Year 30
- Tidal flood risk management 6 hours: 2 to 12
- Testing and training 4 to 5 hours: Up to 3
- Maintenance 1 to 2 hours: 10 to 20
Year 100
- Tidal flood risk management 6 hours: 20 to 55
- Testing and training 4 to 5 hours: 0
- Maintenance 1 to 2 hours: 6 to 12
How do secondary defences better protect the three communities of Chilton Trinity, Pawlett and Combwich in addition to Bridgwater?
The barrier is one part of a wider flood defence system. Downstream, embankments, tidal outfalls and other flood defence features work together to manage flood risk and protect homes, businesses and key infrastructure.
During high tides, the higher primary embankments along the river provide the first line of defence, helping to contain water within the main river channel. Behind these are lower secondary defences, set back from the river. If water were to overtop a primary embankment, the secondary defences provide an additional layer of protection.
This approach makes use of the natural floodplain, providing space for water and reducing the need for much higher primary defences along the river. Together, the primary and secondary defences create a deeper, more resilient system of flood protection, with each layer working alongside the others to manage flood risk.
Why is tidal flood risk determined by the height of the water rather than the volume of water?
The important factor in a tidal flood is how high the water level rises, rather than simply how much water is moving into the estuary.
Tides are the regular rise and fall of sea level, influenced mainly by the gravitational effects of the Moon and Sun. A storm surge is an additional rise or fall in sea level caused by weather conditions, particularly strong winds and changes in atmospheric pressure. When a storm surge coincides with a high tide, it can produce an unusually high-water level and increase the risk of flooding.
For the Bridgwater Tidal Barrier Scheme, it is therefore the predicted height of the water in the River Parrett and estuary that is particularly important. If the incoming tide and any surge are expected to raise water levels to a point where they could overtop flood defences, the barrier can be closed to hold the tidal water downstream and protect Bridgwater.
This doesn’t mean that the volume or movement of water is irrelevant. Large amounts of water are involved in the movement of the tide, and the shape, depth and characteristics of an estuary affect how the tide travels and how water levels respond. However, for understanding when tidal flooding may occur, the critical measure is the resulting water level in relation to the height of the flood defences.
This is why the barrier is designed around water levels and flood defence heights, rather than around trying to contain a particular volume of water.
How does the Environment Agency know there is enough room to store the river water?
Extensive hydraulic modelling has been used to calculate how much water the river channel can safely store during different river flow conditions. This information helps operators decide exactly when to close and reopen the barrier, ensuring there is sufficient capacity available throughout each closure.
Why wasn’t the barrier built somewhere else?
The location was carefully selected following years of engineering and environmental assessment. It provides sufficient river capacity upstream to temporarily store river water during gate closures while working effectively with the downstream flood defences.
How does the barrier know when to close?
The decision to close the barrier will be based on forecasts and monitoring of factors including:
- predicted tide levels
- river levels
- weather conditions
- expected river flows and tidal surges
The gates are closed in advance of the forecast tidal level reaching the barrier, allowing the system to manage the incoming tidal conditions.
What happens if high river flows and an exceptionally high tide happen at the same time?
The barrier has been specifically designed for these scenarios. Decisions on when to close and reopen the gates take account of both predicted river flows and tidal conditions, ensuring enough storage capacity is available throughout the event.
What happens if the barrier cannot operate?
The barrier has been designed with multiple safety features, backup systems and regular testing. In the unlikely event the gates could not be operated, temporary stop logs can be installed to provide the same level of flood protection.
What is the difference between a high tide and a storm surge?
A high tide is the natural rise in sea level caused by the gravitational effects of the Moon and Sun. A storm surge is an additional rise in sea level caused by weather conditions, particularly low atmospheric pressure and strong winds.
A storm surge can occur alongside a high tide, creating exceptionally high water levels and increasing tidal flood risk.
Is tidal flooding about the amount of water in the sea?
Tidal flood risk is primarily related to the level that water reaches, rather than simply the volume of water in the sea. Exceptionally high tides and storm surges can raise sea levels sufficiently for tidal water to travel further upstream and increase flood risk.
Will boats still be able to pass through the barrier?
The barrier and associated footbridge have been designed to maintain navigation on the River Parrett. The barrier does not prevent marine vessels from continuing to access the river upstream.
The scheme has been designed with navigation requirements in mind, including the available clearance for vessels. The existing Northern Distributor Road bridge upstream is already a consideration for navigation.
What happens to wildlife when the barrier closes?
The barrier has been designed with environmental considerations in mind. The scheme also includes improvements to fish and eel passage to support movement of aquatic species through the River Parrett and River Tone catchments.
Will fish and eels still be able to move through the river?
The scheme includes improvements to fish and eel passage at identified sites across the River Parrett and Tone catchments. These improvements are being delivered as part of a phased programme to support fish and eel migration and improve connectivity through the river system.