Flood Defence

When complete in 2026/ 2027, the Avonmouth Severnside Enterprise Area (ASEA) Ecology Mitigation and Flood Defence Project will help protect communities from the increased risk of flooding from climate change and rising sea levels. The project will ensure flood risk is reduced for at least 60 years.

Reducing flood risk to homes and businesses

We are building 17km of improved flood defences from Lamplighter’s Marsh in the south to Aust in the north.

Within Avonmouth Severnside there are areas at risk of tidal flooding and some areas at risk of fluvial (river) flooding. There are some existing defences, but these do not provide consistent levels of protection. 

The new flood defences will reduce flood risk to 2,500 homes and businesses, in particular around Avonmouth Village and Chittening in Bristol and the Severn Beach area in South Gloucestershire, including the railway line, New Passage, Pilning and Marsh Common.

The new flood defences will also create improved walking routes, making it easier for everyone to enjoy nature and the spectacular views of the Severn Estuary

Watch our flythrough video showing what flood defences between Aust Wharf Road and to south of Severn Beach will look like when complete.

How we build flood defences

Given the scale of the ASEA project, we need to use a combination of different types of flood defence structure to help protect communities and businesses around the Severn Estuary.

Earth embankments

Earth embankments make up around half of the total flood defences. We are using earth embankments to reduce the risk to communities from flooding. In some areas we are raising existing earth embankments. These will help protect residents and businesses while respecting the existing environment and reducing our carbon footprint. This method needs a much larger working area so is not suitable in all locations.

Earth embankments offer a number of benefits that include:

Precast units

In parts of the project area, including Severn Beach North and South, we are using precast concrete units to form the new flood barriers. Thames Valley Construction produces and builds these units off-site in Newport, Wales. They are then transported directly to site and lowered into position.
This method minimises disruption for local residents and the environment, compared with pouring the concrete directly on site. Its benefits include:

We expect to install a total of around two kilometres of precast concrete units by the time we complete the works.

Sheet pile walls

We will use sheet piles to form the flood defence wall in some areas. Sheet piles are steel sheets that we directly drive deep into the ground, until we reach rock or firm ground.  We use these in areas with very limited space; where there is not enough room to build earth embankments. We can only use sheet piles where we know there is little risk of finding obstructions in the ground. 

The benefits of sheet pile walls include: 

Glass panels

We will use glass panels in the flood defence walls at New Passage. As with the precast concrete units, these glass panels are manufactured off-site and delivered directly to us when and where we need them.

These flood defence walls incorporate a series of individual glass panels installed using a glass vacuum lifter (which looks like a very large magnet) and lifting device, such as a telehandler, digger or crane. The panels can be replaced individually which is of real benefit if one were to be damaged. In this situation, a temporary structure could be put in place while the glass panel was repaired or a new one made.

When we pour the concrete foundations for this section of the defences, we will place stainless steel anchor plates into the concrete, ready to bolt on the glass panels. After the foundations have cured the manufacturers will install the glass panels without needing to drill into the newly cast concrete.
These barriers will offer the same protection as the other flood defences while also retaining visibility; allowing residents and visitors to continue to enjoy the breathtaking views of the Severn Estuary and beyond.

Concrete wall with anchor plate
A similar glass flood defence wall in Hull

Outfalls

There are a number of existing outfalls throughout the project area. We need to raise the height of the outfalls, in-line with the new flood defences. In many cases, in order to support the increased height, we need to widen the outfalls, to support the extra load.

The height of the existing structure has been raised – September 2021

Cutting-edge technology

At Chestle Pill Outfall, the existing outfall structure would not have supported the additional load from the increased height. To overcome this, we looked at ways of reducing the weight of the existing earth over the outfall. We identified the best way to do this was to replace some of the existing earth over the structure with lightweight high strength modular cells (that look like milk crates, see below). This network of cells has a 95% void ratio – being much lighter in weight than the ground they will replace but still robust enough to support the same load. It is also recyclable at the end of its service life.

The lightweight, high strength modular cells used at Chestle Pill

Road ramp and flood gates

The improved flood defences cross roads at a number of points, for example at either end of Passage Road. Depending on the location, we are either building a ramp over the road to the same height as the flood defences, use a flood gate or a combination of both. The new flood gates, which will be operated by Environment Agency staff, can be closed to provide a continuation of the flood defences across the road when needed. Flood gates are also being installed to allow access on the seaward side of the new raised flood defence at Aust Wharf Road, New Passage and Lamplighter’s Marsh.
Example of a flood gate – the ones used on the ASEA project will be galvanised or painted.

Watch this short interview with Emma Baker - Environment Agency, Area Director Wessex

Play Video

This project will protect not only present but future generations from the increasing risk of flooding as a result of climate change. It’s really exciting to see such progress. The Severn Estuary is internationally important for its habitats for birds and over 80 hectares of wetlands will provide safe roosts for the wintering birds during high tides on the estuary.

Emma Baker
Environment Agency, Area Director Wessex