Protecting people and wildlife along the Severn Estuary
Project overview
Working hand in hand with nature
Rising sea levels threaten local communities, commerce and wildlife. Our designs will provide flood defence, along with new wetland habitats, enhanced walking routes and views – and thousands of jobs.
The Severn Estuary between south-west England and south Wales has one of the highest tidal ranges in the world. Storm surges and large waves are common, putting coastal homes and businesses at risk of flooding. Forecast sea level rises, due to climate change, will exacerbate the flood threat.
The scheme includes creating 80ha – equal to 112 football pitches – of wetland habitat for overwintering birds
Mudflats, sandflats, rocky platforms and a mid-channel island also make the estuary home to the most extensive intertidal wildlife habitats in the UK. The Severn Estuary is designated as an international Ramsar site under the international convention on wetlands, and the estuary is recognised under UK legislation as both a special protection area and a special area of conservation of European Importance. Numerous coastal areas are sites of special scientific interest for natural heritage, wildlife habitats, geological features and landforms.
2500 homes and businesses protected
Improving flood protection for local communities and industries for the next 60 years, while enhancing natural habitats, is the aim of the Avonmouth Severnside Enterprise Area (ASEA) ecology mitigation and flood defence project. The scheme will deliver improved defences along 17km of eastern coastline, from Lamplighters Marsh in Bristol to Aust in South Gloucestershire, helping to reduce flood risk for least 2500 homes and businesses and unlock ASEA’s economic potential – potentially delivering 12,000 new jobs for local communities by 2026.
12,000 new jobs unlocked
The scheme will also provide environmental mitigation, with the creation of at least 80ha – the equivalent of around 112 football pitches – of wetland habitat for overwintering birds. The work will make it easier for people to enjoy nature too, as it includes improvement to walking routes and a new viewing area at Severn Beach offering spectacular vistas across the estuary. It’s an initiative that will encourage leisure activity while improving wellbeing for local residents and visitors.
£1.4bn - estimated economic value of scheme by 2038
We are working for Bristol City and South Gloucestershire councils and the Environment Agency in a joint venture with civil engineering contractor BAM Nuttall (BMMJV) on the detailed design and construction of the flood defences and wetland areas. This follows our successful collaboration with BAM Nuttall on other Environment Agency projects, including the Exeter flood defence scheme, phases one and two of the Leeds flood defence scheme, Torcross sea defence and the Boston Barrier. Since 2015, BMMJV has helped improve flood protection for more than 70,000 of the 300,000 homes targeted by the agency.
Looking after nature across 17km of construction works
A place for the birds… and much more
The salt marshes and mudflats on the estuary provide a winter home for migratory birds, as well as sheltering many other rare creatures and plants – not to mention humans.
74,000 birds live on the estuary
Almost 17,000ha of the Severn Estuary is designated as a special or protected ecological area of some kind. Migratory birds from the Arctic regions use the estuary to feed and roost. Each winter the salt marshes and mudflats of the estuary host an average of 74,000 birds, with dunlin, redshank, ringed plover, black-tailed godwit, shelduck, teal, pintail and shoveler just some of the species that use the area as a pit-stop.
17,000ha of the estuary is ecologically protected
Protected species such as badgers, bats, barn owls, great crested newts, otters and water voles are present, along with rare plants such as glasswort, common reed, sea barley and nationally important rare species like the bulbous foxtail. The estuary is also an important migratory route for salmon and rare fish, such as shad.
1800ha - size of enterprise area
But this area is more than an important wildlife sanctuary. The coastline is also home to communities as well as industries including docks, advanced engineering, aerospace, manufacturing, logistics and power generation. ASEA itself covers 1800ha and extends north from Avonmouth docks along the estuary.
Avonmouth docks and industrial estate
Future lines of defence
The works, which include raised earth embankments, sheet piling and reinforced concrete walls, cover six sites in Bristol and South Gloucestershire, as part of a fresh, joined up approach to flood risk mitigation.
40cm - potential water level rise by 2080
As the climate changes, communities, businesses and wildlife habitats along the estuary are at greater risk of flooding in the future due to sea level rise and greater frequency of winter storms. Water levels in the estuary are forecast to rise between 30cm and 40cm by 2080.
60 years - design life of new flood defences
The last significant improvements to the tidal flood defences were made in the 1990s. Since then, flood defence techniques have advanced and the project is providing a major upgrade, using a combination of flood defence structures appropriate for each location. The new defences have a design life of at least 60 years and our scheme provides a comprehensive, joined-up approach along the estuary, which will generate the confidence needed to increase business investment.
The creation of two new wetlands is thus crucial for the region’s economic future too. They will compensate for the loss of greenfield land allocated for development as part of the enterprise area and provide alternative feeding and roosting areas for over-wintering birds.
The flood defence work will include 5850m of raised earth embankments, 2500m of sheet piled walls, 600m of insitu reinforced concrete walls and 2300m of precast reinforced concrete walls.
Where we’re working
The scheme is made up of six areas and lies across the boundary of two local planning authorities – Bristol City and South Gloucestershire councils (see detail below). Existing defences are generally in good condition, though some need refurbishing or raising. Some locations require new defence walls, earth embankments and flood gates. Several large outfalls also need improving.
South Gloucestershire
1 Aust to Severn Beach (Severnside)
- design and construct new flood defence walls, embankments and flood gates
- raise existing flood defence walls and embankments
- raise sections of coastal road
- improve Cake Pill, Chestle Pill and Cotteralls Pill outfalls
3A Severn Beach railway (north)
- design and construct new flood defence walls and embankments
- raise existing flood defence walls
- improve New Pill outfall
5 Northwick
- construct wet grassland habitat and ponds
Bristol
2 Avonmouth docks
- design and construct new flood defence walls and embankments
- improve Holes Mouth and Kings Weston outfalls
- construct a maintenance access track within Bristol port
- construct new flood defence walls and embankments in Lamplighters Marsh open space and a new flood gate
3B Severn Beach railway (south)
- design and construct new flood defence walls and embankments
- raise existing flood defence walls and embankment
- improve Stup Pill and Mitchells Salt outfalls
4 Hallen Marsh
- construct wet grassland habitat
A rich history
Remains of the Aust Ferry jetty today
Building the best protection
Earth embankments respect the existing environment. Raising existing embankments with material excavated from the sites of two new wetlands reduces the project’s carbon footprint by minimising the amount of imported material required. This method needs a much larger working area so is not suitable in all locations.
5850m of raised earth embankments
2500m of sheet piled walls
600m of in situ reinforced concrete walls
Steel sheets driven deep into the ground are being used in areas such as Avonmouth, where there is limited space. Unlike concrete walls, sheet pile walls do not suffer from erosion of the horizontal base, known as the toe. Sheet piled walls are also quicker to install than insitu concrete walls poured on site and can potentially be installed without removing existing flood defences, avoiding the need for temporary bunds. Delivering the steel sheets for the flood defence walls at Bristol port by boat also cuts carbon emissions.
Glass panels to view nature
Sections of insitu reinforced concrete walling incorporate 188m of glass panels at New Passage, a popular viewpoint and birdwatching spot on the Severn Way footpath, while at Aust Wharf Road, in the shadow of the Severn Bridge at the scheme’s northern tip, the wall features a stone render-effect finish and built-in habitats. There are 63 glass panels at New Passage. This hybrid wall provides the same level of protection as the other flood defences, while allowing residents and visitors to enjoy views across the estuary. The natural-looking stone render wall finish at Aust Wharf Road is overlooked by local homes. On the other side of the wall, facing onto the estuary, built-in habitat shelves made from oak contain vegetation suitable for invertebrates.
2300m of precast reinforced concrete walls
New concrete defences
At six sections of the project, including Severn Beach north and south, precast concrete units manufactured across the Severn in Newport will form the new flood barriers. Compared with pouring the concrete directly on site, precast units are quicker to install, minimising disruption to residents and the environment.
8 improved outfalls
Raising outfalls to new heights
The height of existing outfalls must increase in line with the new flood defences. Most also need to be wider to support the extra load from the increased height. To reduce the weight of the existing earth over the outfall at Chestle Pill, at the scheme’s northern end, we are replacing some earth with lightweight attenuation crates, made of high-strength plastic modular cells. Similar to milk crates, these can be recycled at the end of service life.
Geotechnical innovation
Planning a construction project in an environment teeming with protected wildlife and fragile ecosystems is not easy. Complex ground conditions such as saturated ground, along with significant contamination in the port area, diverse existing structures, multiple landowners and an inherited reference design, all added further levels of difficulty.
4 hours - noise restrictions around each high tide in winter
Work is also subject to noise limits to minimise the disturbance to wildfowl and waders during the overwintering period. Construction operations above a certain noise threshold are restricted for two hours either side of high tide to avoid disturbing high-tide roost sites during the winter. And of course, comprehensive ecological and ground investigations and assessments were needed before any work could commence.
The art of investigation
Desktop studies and field surveys had investigated the presence of protected species – badgers, bats, birds, dormouse, great crested newts, hedgerows, otters and water voles – to inform the environmental impact assessment for the planning application. Before work began on site in April 2019, our JV team carried out further ecological surveys, mapping the area for wildlife and developing mitigation plans.
Ecological surveys to inform mitigation plans
They found all manner of wildlife in addition to the legally protected species, including bats, otter and newts, as well as invasive species such as three-cornered garlic and Japanese knotweed, which then had to be carefully removed.
To investigate and monitor ground conditions, our geotechnical experts used cable percussion boreholes to extract samples and cone penetration testing to establish strength profiles, while trial pits and dynamic cone penetrometers were used to investigate areas between boreholes. Leapfrog Works geological modelling software analysed onsite and laboratory test data to establish characteristic geotechnical parameters and build a conceptual 3D model.
Across most of the project, ground conditions were characterised by soft compressive tidal flat deposits. Considerable contamination was discovered in the Port area and in Lamplighters Marsh, so onsite treatment and removal had to be undertaken before piling or excavation works could commence.
Optimising the design of embankments
Settling the main geotechnical issue
Across the project, the crest level of the almost 6km of embankments must not rise or sink more than 100mm above or below the design flood level one year after takeover by the client. Initial settlement sensitivity calculations indicated a significant risk of under or over building the embankment height due to uncertainty in the geotechnical parameters.
100mm - maximum permissible change in embankment crest level after one year
12 months of settlement monitoring on the trial embankment
100t of carbon saved by optimised design due to settlement monitoring
To investigate this risk further, we constructed a 100m long, 4.2m high trial embankment, which was 5m wide. The crest and height dimensions were selected to match the highest embankment along the flood defence. Monitoring instruments – vibrating wire piezometers, settlement plates, magnet extensometers and surface settlement monitoring points – were installed to measure the response of the underlying ground to embankment loading during construction and for 12 months afterwards.
About 180mm of settlement was observed during construction, with a further 240mm occurring in the first four months of post-construction monitoring. This was greater and more rapid than anticipated based on laboratory data. During those four months, pore pressures declined from a maximum of 80kPa at the end of construction to 62kPa, suggesting that at least half of the primary consolidation settlement had occurred. Back analysis of the trial data using 3D settlement and foundation software predicted a total settlement of 580mm, with most happening within two years of construction.
The results of the trial allowed the settlement parameters to be refined and increased confidence in the settlement estimations, resulting in carbon and cost savings.Senior principal engineering geologist
The findings enabled us to optimise the design of embankments across the project, using our representative settlement parameters to adjust the banks in specific areas, based on location ground conditions. This will reduce the volume of embankment fill to be transported to site and compacted by 5000m3, saving 100t of carbon.
“The results of the trial allowed the settlement parameters to be refined and increased confidence in the settlement estimations, resulting in carbon and cost savings,” explains Gareth Mason, BMMJV geotechnical project lead. “The risk of additional remedial works following demobilisation was also significantly reduced.”
Learning legacy
The trial embankment results comprise important research into the behaviour of tidal flat deposits under embankment loading, which can be used on future flood defence projects. The results were published in a technical paper and presented at the 11th International Symposium of Field Monitoring in Geomechanics in 2022.
Cross-section showing ground model and instrumentation
Same outcome, lower cost
Value engineering (VE) is formalised on the ASEA project, enabling us to identify, discuss and agree more sustainable solutions, with a smaller carbon footprint and lower cost. In some locations, we have demonstrated that a “do nothing” option can be implemented without compromising on project outcomes.
Identifying sustainable solutions
60 value engineering interventions (by December 2022)
15x savings - every £1 spent on value engineered items saves £15 in construction
£100,000 - potential cost saving on Kings Weston outfall
£200,000 - potential cost savings on Holes Mouth outfall
By the end of December 2022, we had advanced more than 60 VE items for approval, from significant interventions to minor adjustments. Examples include:
- The Kings Weston outfall is located in Bristol port, and the inherited reference design included sheet piling to create a levelled maintenance area, construction of two foreshore access ramps and the replacement of the existing trash screen and access footbridge. BMMJV proposed an alternative that maintained the access provision demanded by the client while greatly reducing the scale of works. Our solution included a single reinforced concrete foreshore ramp, reprofiling of the downstream channel bank, and removal (without replacement) of the trash screen and access footbridge. This minimised both construction work in the tidal zone and the interface with the existing structure, for which no records were available. The capital cost saving is between £80,000 and £100,000.
- Proposed works south of the Holes Mouth outfall had included sheet piles spanning 440m along the estuary. However, our design development determined that levels along the existing earth embankment crest were either already at the required defence level or within about 2m, while checks indicated that topping up the bank was a viable solution. As well as achieving a capital cost saving of between £100,000 and £200,000, our solution minimises maintenance requirements and delivers programme and carbon savings.
- A permanent bridge over pipelines that carry fuel to airports in the south of England was originally to be built landward of the existing Holes Mouth outfall. After discussions with stakeholders, our design team determined that access was not specifically required at this point and an alternative route would remove the need for a bridge. Using vehicle tracking software, we set out an alternative route for future maintenance access. The cost saving is estimated at £20,000. However, the main benefit was to health and safety, by removing the need for construction work above and next to the fuel pipelines.
- Changing the material for habitat shelves in the flood defence walls from concrete to timber will deliver an estimated cost saving of £5000. Timber also provides a better habitat and is naturally durable.
Habitat shelves along the defence walls
Space for species to thrive
To minimise risk to valuable ecosystems during construction, we developed a construction environmental management plan. In terms of biodiversity, this requires contractors to:
- assess the risks of potentially damaging construction activities
- identify biodiversity protection zones
- take practical measures (both physical measures and sensitive working practices) to avoid or reduce impacts
- plan the location and timing of sensitive works to avoid harm to biodiversity.
We installed the barn owl boxes away from our new wetlands working area in disused buildings and a nearby tree so they could nest without being disturbed. We are all so pleased and excited that the owls have successfully raised these three chicks.Senior ecologist
One example of the measures implemented to protect wildlife is installing nest boxes to encourage barn owls to stay away from a suitable nesting tree likely to be disturbed by the works. If an owl had nested in this tree, the team would have had to set up a significant buffer with a major impact on the works programme. Three owl chicks were discovered in one of the boxes, and our specialist ornithologist ringed and weighed the birds then recorded the information with the British Trust for Ornithology. “We installed the barn owl boxes away from our new wetlands working area in disused buildings and a nearby tree so they could nest without being disturbed. We are all so pleased and excited that the owls have successfully raised these three chicks,” says BMMJV lead ecologist Kath Thorne.
In addition to the owl nest boxes, ecological mitigation measures include:
- more than 100 bat boxes
- 12 ponds for great crested newts
- one otter holt, ensuring otters can continue to use an established commuting route
- one kestrel box
100 bat boxes
3 football pitches’ worth of new native trees and shrubs
40% of plant and animal species globally depend on wetlands
135ha of wet grassland, ponds and other ecological mitigation areas created
For the mitigation areas, the historic landscape has been preserved through the retention of field boundaries, hedgerows, ditches, and ridge and furrow. A total of 2ha – about three football pitches – of native species of trees, including silver birch and hornbeam, as well as shrubs, will be planted across the project. Some existing trees are being replaced by native species such as oak, and overall more trees will be planted than will be removed during construction.
Valuable new habitats
The natural infrastructure of a wetland habitat is particularly important for hosting species of over-wintering waterbirds. The Severn Estuary area was designated a Ramsar site (under the Convention on Wetlands of International Importance) and special protection area in 1995 and as a special area of conservation in 2009.
Creating new waterbodies and wet grassland habitat alongside the estuary as part of the ASEA project will bring other. Wetlands are effective carbon sinks, and about 40% of the world’s plants and animals depend on them.
The two new wetlands areas at Northwick and Hallen March have been designed to attract a variety of rare species:
- Northwick Marsh in South Gloucestershire will become a 60ha ecological mitigation area and includes 40ha of freshwater seasonal (winter months) wet grassland habitat and 5ha of permanent open water in the form of ponds.
- At Hallen Marsh, north-east of Avonmouth docks, a 75ha ecological mitigation area will mostly (70ha) consist of freshwater seasonal (winter months) wet grassland habitat.
Wetlands at Northwick and Hallen Marsh
Planting wetland grasses will provide the habitat the birds need when the mudflats where they feed are underwater, while a network of ponds and areas of shallow water, which dry up in summer and are called scrapes, will enable wetland birds to feed and rest at high tide. At Northwick, ponds have also been created for great crested newts.
Soil dug out for the wetlands is being used elsewhere in the project, including for the construction of embankments.
Creating the Northwick Marsh wetland