Frequently Asked Questions

The Avonmouth Severnside Enterprise Area (ASEA) Ecology Mitigation and Flood Defence Project will deliver a new flood defence scheme for Avonmouth Severnside, along a 17 kilometre stretch of coastline to protect local communities and reduce flood risk to at least 2,500 homes and businesses, now and in the future. The project also includes creating new habitats for important wildlife specific in the area and to help enhance and grow the Avonmouth Severnside Enterprise Area to reach its economic potential.

This is a joint partnership project with South Gloucestershire Council as the lead partner, Bristol City Council as a partner and the Environment Agency as a partner and delivery partner.

The Avonmouth Severnside Enterprise Area extends for five miles along the Severn Estuary and covering some 1,800 hectares and located between Bristol and the River Severn. It is currently a mix of industrial and former industrial areas and greenfield sites immediately adjacent to the M5 and M49 motorways. It consists of two main areas of economic activity – Avonmouth in the south and Severnside in the north. The scheme extends outside of the ASEA and this is to ensure that flood risk is reduced to the ASEA and surrounding properties which will result in 17km total length along the coast. Click here to view our interactive map.

Within Avonmouth Severnside there are areas which are at risk of tidal flooding and some which are at risk of fluvial (river) flooding. An estimated 1,051 existing properties are currently at risk of flooding. There are some existing defences, but these don’t provide consistent levels of protection, and to unlock the potential of the Enterprise Area and to reduce flood risk to communities, new effective flood defences will be essential.

The Environment Agency exercise their permissive powers to maintain 8km of the existing flood defences, with riparian owners being responsible for the other sections, which provide varying levels of protection. Through this scheme we will achieve a consistent level of protection across the project area. The Environment Agency have agreed to take on the responsibility to operate and maintain most of the new defences subject to an appropriate maintenance regime being agreed and appropriate legal agreements for access.

New, more effective flood defences for the area is considered to be essential both in order to attract new development and to reduce future flood risk to existing properties and infrastructure.

The Avonmouth Severnside location is considered internationally significant and is expected to continue attracting major manufacturing, logistics and distribution companies over the next 30-40 years. Improving the flood defences in this area will help give businesses more confidence to encourage investment in the Avonmouth Severnside Enterprise Area and help to support the creation of 12,000 new jobs from new commercial development by 2026.

The recommended standard of flood protection for new developments at risk from tidal flooding is 1 in 200 years (0.5% Annual Exceedance Probability), plus an appropriate allowance for climate change impacts such as rising sea levels. The work required to upgrade the flood defence will include raising existing earth embankments and walls, as well as using innovative techniques such as glass panel flood barriers to ensure views out to the Severn Estuary are retained. For each section of coast, the project has considered the following to arrive at the actual proposed flood defence levels (levels determined in Metres Above Ordnance Datum (mAOD)):
  • Joint-Probability Analysis: providing worst-case scenarios of estimated still water flood levels and wave actions (utilising Environment Agency Coastal Boundary Data – established from long-term level gauge records). The levels and action vary depending on the section location.
  • Residual Uncertainty Allowance: an assessment of the statistical error that may exist in the coastal boundary data and wave run-up calculations.
  • Overtopping: (due to sea level and wave overtopping) must not exceed 10l/s/m during a 0.5% AEP (1 in 200 years) event. This allows for sea level rise and increases in extreme wave height due to climate change, as well as wave period, defence geometry and the nearshore slope geometry.
  • Additional factors depending on whether the defence structure proposed in each section consists of an earth embankment or is a hard defence.

For sections where earth embankments are proposed allowances for the following factors have also been incorporated into the design:
  • Initial settlement during construction period (0.1m).
  • Settlement over 60-year design life (for up to a 3.0m high embankment) (0.1-0.3m).
  • Desiccation risk (drying/shrinkage). This is managed by allowing for soil blending and admixtures within the upper layers to reduce risk of desiccation cracking.
  • Topsoil (considered as not waterproof) above flood defence level (0.1m).
  • Safety factor, to guard against lower grade material or changes to sea level rise allowances (0.2m).
UK Climate Projection data has been utilised (based upon the UKCP09 dataset). While the UKCP18 dataset was published in November 2018, guidance to authorities designing flood defence schemes is yet to be updated. That said, the methodology outlined above is precautionary so as to produce a design outcome broadly in line with the 2018 data.

For the new flood defence earthworks the width is dictated by the future operational requirements of the Environment Agency. In most cases this means a crest width of 5m and side slopes of 1:4 and therefore, the thickness of the earthworks is related to these requirements and the necessary flood defence level.

The Severn Estuary is an important ecological area and is home to protected species including water voles, bats, breeding wading birds and a range of rare plants. Parts of the area are designated as either Special Protection Area (SPA) a Special Area of Conservation (SAC), a Site of Special Scientific Interest (SSSI) or a Ramsar site – an internationally important wetlands site.

Coastal wetland habitat is one of the most valuable natural habitats for the UK, as it is under threat from rising sea levels due to climate change. The Severn Estuary’s protected status means that new habitat will need to be created to replace habitat affected by new development within the Enterprise Area. We will create a minimum of 80 hectares of new wetland habitat in the Hallen Marsh and Northwick areas to provide “high-tide roosts” for the birds. This will include wet grassland to encourage wading birds and open water to encourage wildfowl. We will use a number of different methods to create this habitat, including new ponds and areas of shallow water over the winter months called scrapes.

Local considerations have been given greater importance to minimise the ecological/environmental effect of the scheme design. The scheme design is predicated on the basis that all works will need to take place outside of the SSSI/RAMSAR/SPA/SPC/Nautica 2000 boundary. While this will cause a short-term impact on the local ecosystem, the benefits over the medium to longer term will be considerably advantageous.

The planning consents incorporate several pre-commencement planning conditions and these include approval of a Pre-Construction Environmental Management Plan (CEMP); a Landscape and Ecological Management Plan (LEMP); pre-construction ecological surveys and arrangements for longer-term biodiversity monitoring.

Local considerations have been given greater importance to minimise the environmental impacts of the scheme. The scheme design ensures that all works will take place outside of the SSSI/RAMSAR/SPA/SPC/Nautica 2000 boundary. While there is a short-term impact on the local ecosystem, the benefits over the medium to longer term will be considerably advantageous. Pre-commencement planning conditions, included a Pre-Construction Environmental Management Plan (CEMP); a Landscape and Ecological Management Plan (LEMP); pre-construction ecological surveys and arrangements for longer-term biodiversity monitoring.

We are ensuring that a thriving environment will be established in the area for people and wildlife. A line of hybrid poplar trees along the Severn Way footpath near Northwick, and some hedgerows have been removed to allow the new defence. As part of the planning consent in Spring 2019, we obtained permission to remove the hybrid poplar trees along the Severn Way footpath that are about ten years away from the end of their expected lifespan. This has allowed us to complete the improvements to the earth embankment flood defence from Passage Road to New Passage.

The poplar trees that have been removed were planted approximately forty years ago to obscure the now disused Northwick landfill site. Poplars are generally regarded as low-quality tree species and due to their limited lifespan, are rarely used to enhance wildlife habitats.

We could not build the new, larger flood defences in front of the existing poplar trees south of Cake Pill outfall because this would encroach onto the Severn Estuary intertidal marsh, which is an internationally protected area.

In agreement with landowners, we removed these trees and cleared the nearby vegetation in January 2021.  Removal of these trees was included in the Planning Application and approved.

We continue to replace the poplars with high quality, longer living, native species, to be planted on a raised bank behind the location of the existing poplar trees and protected by the new flood defence embankment. Extensive belts of native woodland planting have been planted. The new plantings will extend along the landward side of the new flood defence integrating existing woodland and creating a new species rich wildlife corridor, aiding species diversity resilience to climate change and aesthetic value for users of the Severn Way footpath. We are planting areas of tree and shrub copse, using species including but not limited to oak, hornbeam, field maple, cherry, birch, hazel and hawthorn. This mix will help to provide a valuable source of native species to aid biodiversity. The copse style planting merges into traditional native broadleaved plantings as it follows the flood defence embankment southwest towards the Northwick historic landfill site. The species selected here include but are not limited to oak, hornbeam, Cherry, field maple birch hawthorn and hazel.

In the Severnside area, this work is scheduled to be complete during the planting season in Winter 2022/23.

The project has been divided into five geographical areas listed below and an overview of the work required:

  • Area one – Severnside defence.
  • Including on-line raising of existing earth embankments and new flood defence walls and flood gates, local realignment, secondary defences for overtopping. Improvements to the Cake Pill Outfall, Chestle Pill Outfall, and Cotteralls Pill Outfall.
  • Area two – Avonmouth docks defence.
  • Including on-line strengthening and new defences set back from the waterfront at Lamplighter’s Marsh.
  • Area three –Central outfalls and Existing Embankments.
    Including on-line raising of existing earth embankments. Set back earth embankment on the landward side of a section of the railway embankment. Improvements to the existing outfalls to take water out into the estuary from the land drainage rhine network.
  • Area four – Hallen Marsh.
    Including creating additional ponds and scrapes and wetland landscape.
  • Area five – Northwick.
    Including creating additional ponds and scrapes and wetland landscape.

In 2012, the three organisations commissioned a study to establish the possible options for the future flood defence of Avonmouth Severnside Enterprise Area. This identified a preferred flood risk management option, taking into account the socio-economic, environmental and financial components to ensure sustainable development, together with engineering considerations.

In April 2019, South Gloucestershire Council awarded planning permission for the project Bristol City Council also passed a ‘resolution to approve’ planning permission in March 2019, which is to be confirmed shortly. The decision to proceed to the next phase of the project will see the detailed design and build of the proposals.

Watch our video (waiting on link to come through) about progress so far to December 2020 and what happens next.

The construction phase of the project started in summer 2020 and is taking place in a number of separate stages between 2020 through to 2026/27. The sites which are identified as being currently at highest risk of flooding will be tackled first, as priorities.

The Environment Agency will lead the project management of the design and construction stage as delivery partner and will manage the awarded contractor, who are a multi-disciplinary contractor to carry out the works. The project will last until 2026/27.

The Spring and Autumn periods are as important as the overwintering period to the species of wader bird that is designated/protected. Construction time limits have been set to minimise the disturbance to the protected species during the over wintering period only.

The Traffic Plan submitted as part of the planning application documentation identified local fields of traditional ridge and furrow will be used for construction compounds.

A Construction Traffic Management Plan will need to be agreed with the Local Planning Authority prior to commencement of works. This will also need to address the issue of the approach towards the use/non-use of fields containing traditional ridge and furrow.

A Construction Traffic Management Plan will need to be agreed with the Local Planning Authority prior to commencement of works. This will need to address the issue of traffic movement and materials movement into, within and out of the project area. There will need to be a number of HGV movements as part of the construction but the CTMP will be agreed with the Local Planning Authority.

Construction and future maintenance costs are estimated to be around £80 million.

The project is being funded through a combination of the below channels:

  1. West of England Local Enterprise Partnership Economic Development Fund: A fund created from retained business rate growth across the West of England Enterprise Zone and Areas. The fund runs for 25 years from 2014 until 2039 and is focussed on supporting capital infrastructure to unlock job creation in the Enterprise Zone and Areas.
  2. Flood Defence Grant in Aid (FDGiA): Funding from central government for managing flood risk in England. The total national amount of FDGiA available is distributed across several bodies responsible for managing flood risk. These include the Environment Agency, Local Authorities and Internal Drainage Boards.
  3. Local Levy: Local income raised by a Regional Flood and Coastal Committee (RFCC) to fund activities within its region that are a local priority. Lead Local Flood Authorities (LLFAs) contribute to the local levy fund.

South Gloucestershire Council and Bristol City Council have committed to borrowing £1.9m (phase 1: development stage) and up to a further £63.9m (phase 2: design & build stage). The borrowing is secured against the West of England Economic Development Fund. Although the project expense will be incorporated into the joint councils capital budgets, the net level of the council expense is £0.

We shared information on the design proposals throughout 2017, where we sought feedback from local stakeholders. Consultation was also undertaken on the planning application for the scheme through the planning process managed by the Local Planning Authorities in 2018. We are now in the construction phase and are using a range of communication channels such as social media and this website to encourage people to sign up to receive updates.

You can sign up to receive newsletter updates from us and our contractors by emailing  Watch our video about progress so far to December 2020 and what happens next.

Please click here to view the FAQ PDF


The Environment Agency will shut the flood gates in advance of certain high surge tides (which meet a criteria) and open once the tide has passed. The Environment Agency will also complete checks on the gates on a regular basis between surge tide closures to ensure that they remain functional.

Parking at this location will be at the owners’ risk. Signposts will be erected warning that the road may be closed.

The glass panels are designed to be self-cleansing, but cleaning will be undertaken by the Environment Agency or a window cleaning contractor if occasionally required.

We will look to remove the footpath diversions as quickly as possible. If we are able to open short lengths ahead of removing overall full closure, we will. Updates on footpath closures including diversion route maps can be found here.