Thorney Island Habitat Creation Scheme Information Page

Closes 3 Oct 2022

Opened 14 Oct 2020

Overview

More Information

Ground investigations and surveys are currently being completed to understand:
       • type of ground and geology
       • water levels
       • presence of contaminants
       • archaeology
       • unexploded ordnance

       The results of these will be used to inform the design and develop draft        options.

Summary

The Environment Agency is working in partnership with the Ministry of Defence (MOD), and Chichester Harbour Conservancy, to develop a habitat creation scheme through managed realignment at the Thorney Island barracks. With expert support from JBA Consulting and Volker Stevin a new set-back flood defence will be constructed, and the existing defence breached, which will allow new intertidal habitat to develop over time on the land seaward of the new defence.

 

Map showing the location of the proposed managed realignment site on Thorney Island

Figure 1: Location of the proposed managed realignment site on Thorney Island

Why do we need to create new intertidal habitat?

Intertidal habitat is rapidly declining in Chichester Harbour and the wider Solent due to past and on-going development and sea level rise. There are many urban areas in the Solent, including the large towns of Portsmouth and Southampton, as well as important industries. These are protected from coastal flooding and erosion by flood defences. However, as sea levels rise due to climate change, the natural intertidal habitats (mudflat and saltmarsh) are unable to naturally move landward as they become constrained against the defences. This process is resulting in a wide scale loss of intertidal habitat in the Solent. In recent years there has been a dramatic decline in saltmarsh habitat in Chichester Harbour. Natural England is currently assessing the extent of this. This process is called coastal squeeze (Figure 2).

The Environment Agency is legally obliged, under the Habitat Directives, to create new intertidal habitat to compensate for the losses resulting from coastal squeeze.

Diagram showing how the presence of coastal defences can lead to the loss of intertidal habitats resulting in coastal squeeze.

Figure 2: Diagram showing how the presence of coastal defences can lead to the loss of intertidal habitats resulting in coastal squeeze.

Thorney Island has been chosen as a preferred site to create new intertidal habitat as it has the required characteristics for intertidal habitat to thrive and is not currently protected by environmental designations. It has a varied natural topography, which allows the creation of a variety of habitats including mudflat, saltmarsh, transitional and terrestrial habitat with minimal engineering required.

Current Site

The MOD own the land on Thorney Island as part of their barracks. The original embankments and concrete defences around the whole of the island, are now in poor condition and there is a potential risk of a breach of the defences in a large storm event (Figure 3).

Photographs showing the current coastal embankment. The public footpath runs along this and it is in a poor condition.

Figure 3: The current coastal embankment.

The construction of a new set-back defence will not only allow the creation of new intertidal habitat, it will also improve flood defences for the island, and provide wider environmental benefits. Additionally, the public footpath which runs along the top of the current embankment and is in poor condition will be resituated to run along the top of the new flood defence. This will provide enhanced views of the developing intertidal habitat, provide better protection for birds and other species, and provide safer and easier access around this part of the island.

What are we doing?

To allow the new intertidal habitat to form, the Environment Agency plans to construct a new defence, landward of the current defence line, along the western boundary of the Thorney Island Barracks infrastructure. Two breaches will be made in the current defence (yellow lines on Figure 4) to allow the area to become inundated by the sea on high tides. This will provide the conditions needed for intertidal habitats to develop. These will look similar to the habitat present on the seaward side of the current embankment.

The project team are investigating the alignment of the set-back defence and what the defence will look like. Figure 4 shows indicative options for the location of the new set-back defence which are being investigated. There are a number of factors that will inform the design, including:

  •  ground conditions
  •  natural topography
  •  proximity to MOD infrastructure
  •  landowner requirements
  •  protected species and important terrestrial habitats
  •  known and unknown Heritage assets

Indicative locations of the set-back defence. A number of alignments are being investigated as shown by the red and dotted black lines. The yellow lines indicate the locations of the potential breaches

Figure 4: (Top) Indicative locations of the set-back defence. A number of alignments are being investigated as shown by the red and dotted black lines. The yellow lines indicate the locations of the potential breaches.

(Bottom) Preliminary modelling of the site to determine the type of habitat to be created based on Highest Astronomical Tide.

Preliminary modelling of the site to determine the type of habitat to be created based on Highest Astronomical Tide

The project team are talking with professional partners, stakeholders and communities that could be affected by, concerned about, or are interested in the future of this coastal frontage. The outcome of these conversations will help inform the design of the scheme as it develops.

What will the scheme look like?

A number of managed realignment schemes have already been achieved within the Solent. Figure 5 shows the small scheme at Cobnor, and the much larger Medmerry scheme.

 

Photo showing Intertidal habitat creation scheme at Cobnor

Figure 5: (Top) Intertidal habitat creation scheme at Cobnor. (Bottom) Managed Realignment scheme at Medmerry.

Photo showing Managed Realignment scheme at Medmerry.

Chichester Harbour is internationally designated for its mosaic of marine, intertidal and coastal habitats and the birdlife that use those habitats. The site will be designed to replicate some of that variety. The area will be a mixture of intertidal mudflats, saltmarsh, small islands and grassland incorporating the existing topography. The intertidal habitats will become new feeding areas for wintering and passage birds such as Brent Geese, Curlew, Dunlin, Redshank, Grey Plover and Bar-Tailed Godwit. The islands and higher areas will become high tide roosting sites for birds in winter and double as breeding sites for Terns, Gulls, Oystercatchers and Ringed Plovers in summer. Many of these species are declining in the harbour or nationally, and their roosting and breeding sites are threatened by sea level rise and are often impacted by human disturbance.

The new saltmarsh, islands and grassland will also support a range of specialised plants and invertebrates that will naturally colonise from elsewhere in the harbour, and the shallow intertidal creeks will be important nurseries for fish such as Bass, Mullet and Flounder.

Additionally, as the habitat develops it will attenuate wave energy in the wider Chichester Harbour, improve water quality by filtering pollutants, and provide opportunities for carbon sequestration.

When will the scheme be constructed?

The project team are currently developing the design of the scheme and will be completing environmental surveys and ground investigation works over the autumn to inform the design.

Once this information has been collated, it will feed into the development of a draft option, currently programmed to be completed in winter 2020. At this point, we will discuss the scheme further with key stakeholders, sharing more detail with them about:

  •  the alignment of the set-back defence and what this  might look like
  •  the types of habitat that will form
  •  timelines
  •  any further questions stakeholders may have

These discussions will be used to finalise the draft option and develop a business case for funding approval. Approval of the business case will unlock Flood and Coastal Risk Management Grant in Aid (FCRM GiA) funding to complete the detailed engineering design of the scheme in preparation for planning permission to be submitted in 2023. It is intended that construction will begin in 2024 and be complete by the end of 2025.

Key messages

  • The Thorney Island Habitat Creation scheme aims to allow new intertidal habitat to develop over time to compensate for the loss of this important habitat in Chichester Harbour and the wider Solent. The Environment Agency has a legal requirement to deliver this habitat.
  • The creation of this intertidal habitat will provide a variety of benefits for Chichester Harbour including: supporting international designated habitat (SAC and SPA); providing fish nurseries; attenuation of wave energy in the wider Chichester Harbour; filtering pollutants to improve water quality; and carbon sequestration.
  • Thorney Island has been chosen as the preferred site as there are no environmental designations and the natural topography will allow a variety of habitats to be formed. The size of the site will also ensure that there is space for the habitat to naturally move landwards with sea level rise over time.
  • The option will consist of a set-back defence, and 2 breaches in the current embankment to allow the site to be inundated and provide conditions for intertidal habitat to develop.
  • The project team are completing investigations and surveys during autumn 2020 to inform a draft option. The project team will then engage with key stakeholders in winter 2020 to discuss this further and finalise the business case to unlock further funding for detailed design.
  • Construction of the scheme is planned to be complete by the end of 2025.

 

 

 

Get in touch

Thank you for visiting our project page.

Please contact:

Thorney.Island@environment-agency.gov.uk

if:

  • you are not able to access any of the information on this page
  • your have any comments or feedback on our information page
  • your have any questions about the project

If you would like to be added to our distribution list to receive updates on the project, please complete and return the form below.

For more information on how we will use and share your data, please see our Privacy Notice below and our Personal Information Charter

 

Audiences

  • Businesses
  • Charities
  • Statutory organisations
  • NGOs
  • Members of the public
  • Elected representatives, including MPs
  • Community groups
  • Flood action groups
  • Members of the public
  • Community groups
  • Non-governmental organisations with an interest in environmental issues
  • Environment Agency colleagues
  • Lead Local Flood Authorities
  • Local Risk Management Authorities
  • Flood Resilience Forums
  • Members of the public
  • Town and parish councils
  • Regional Flood and Coastal Committees
  • Internal Drainage Boards

Interests

  • Business and industry
  • Flood management
  • Coastal management
  • Fishing and boating
  • Habitats and wildlife