Thorney Island Habitat Creation Scheme Information Page

Overview

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 of the coast at the south-western edge of Thorney Island barracks. With expert support from JBA Consulting and Volker Stevin a new set-back flood defence will be constructed to protect the barracks, and the existing defence breached, allowing seawater to cover the low-lying land as the tide goes in and out. This will enable new intertidal habitat to develop on the land seaward of the new defence over time.

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.

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).

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. We plan to breach the current defence to allow the area to become inundated by the sea on high tides. This will provide the conditions needed for intertidal habitats to develop.

The project team are investigating the alignment of the set-back defence and what the defence will look like. 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

(Bottom) 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.

Figure 5: (Top) Intertidal habitat creation scheme at Cobnor. (Bottom) 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?

Ground investigations and surveys were completed in November 2020 to help inform the design of the new defences and to understand:

• type of ground and geology
• water levels
• presence of contaminants
• likelihood of finding archaeology
• likelihood of finding unexploded ordnance.

The results of the ground investigations were returned in winter 2020 and DDT was found in low concentrations in a number of sites across the project area, which was expected.

We have reviewed past investigations into the presence of DDT and it was found to be widely present in the existing saltmarsh and intertidal habitats around Thorney Island. This gives us some confidence that the project will not impact the wider harbour negatively. Although the levels of DDT are too low to pose a risk to human health and the land-based (terrestrial) environments, there is limited information and guidance on the impacts to the marine environment. Therefore, the project team are currently investigating the implications of our findings in more detail.

We need to assess what the impacts of DDT could be on the marine environment when the soils at Thorney Island are covered by tidal water, and whether there would be any effect on wildlife. This impact assessment will enable us to determine what remediation works may be required.

To determine what the DDT impact assessment needs to include, the project team have consulted with experts and specialists across the Environment Agency (Water Quality, Estuarine and Coastal Assessment, Contaminated Land and Eco-Toxicology); MOD; Centre for Environment, Fisheries and Aquaculture Science (CEFAS); Chichester Harbour Conservancy; Chichester District Council and specialist consultants.

We have slowed other elements of the project as the team investigate the impacts of the DDT found in the ground investigations. This is an important step we need to complete to ensure that our project does not cause any harm to the environment.

We will work with the experts and specialists to define the scope of the DDT impact assessment and following this, it could take up to a year to complete depending on the level of analysis required.  Further clarity on timescales will be available once we have discussed the method with the Marine Management Organisation as the regulatory body.

The result of this additional work may lead to changes to the design of the scheme. Although DDT has been found on the site, concentrations are low. We remain confident in designing a scheme that will benefit habitat and wildlife in Chichester Harbour without risking harm.