Coastal adaptation project at Northey Island

Our Coastal Adaptation Strategy

To mitigate the effects of climate change and rising sea levels, we've developed the Coastal Adaptation Strategy.

The work we do now will help to protect, strengthen and enhance this precious saltmarsh habitat in the Blackwater Estuary over the next 100 years. We’re using a range of approaches to achieve this, including:

• Realigning the south and south-east embankments to create new saltmarsh.
• Using dredged sediment at four locations around Northey to raise and regenerate the saltmarsh.
• Reprofiling other embankments to reduce the risk of failure and further saltmarsh loss.

Realigning the defences

Every year, large tides are overtopping (flowing over) the flood defences at the south and south-east of Northey Island. All the defences around Northey Island are in a poor condition. If they fail in an uncontrolled way, this will lead to erosion and further loss of habitat. It could also limit the way vegetation can become established, reducing the diversity of species compared to a managed situation.

Managed realignment

To tackle this, we’re using a technique called managed realignment. This involves removing parts of the flood bank to give space for the rising sea level. It also allows the saltmarsh plants to migrate inland in a natural way as the sea level rises, giving it a longer lifespan and better biodiversity.

We first implemented this plan at the south-east corner of Northey Island in 1991 and it has led to a healthy ecosystem that has existed for 30 years. Over 5,000 hectares of intertidal area has since been created across England using this technique. Since 2019 we’ve implemented two further managed realignments at Northey Island, creating 5ha of new saltmarsh in the immediate term.

 

Regenerating the saltmarsh

Work on the north-west side of the island has involved a different management technique. Here we have reused sediment that had been previously dredged to maintain the navigation for boats and barges to Maldon. Carefully placing the sediment avoids new channels being formed through the saltmarsh east to west and reduces the higher flows that have been causing further erosion and vegetation loss.

This technique modifies the flows within the saltmarsh itself. It takes some of the energy out of the tide so that sediment can naturally be deposited there. This in turn allows the health of the saltmarsh to improve, which then helps reduce the effects of wave action over the saltmarsh surface. To date we have regenerated a hectare of saltmarsh vegetation on what had been eroding, bare clay.

Following the success of this technique we now have works planned for three more sites around Northey. These include restoring and protecting saltmarsh on the Western Shore and causeway saltmarsh and creating a nesting bird island on the eastern side of Northey. This will creating a remote nesting habitat but also take energy out of the tides rushing into saltmarsh of Northey, protecting it from further erosion.

Improving diversity

The new areas of marsh created through the placement of sediment sit higher than the existing saltmarsh and regenerate naturally with saltmarsh plants. This improves the diversity of the saltmarsh and provides a niche area for some of the rarer saltmarsh species that are under immediate threat from coastal squeeze. We’ve seen plants establish within only a few months of placement.

Reprofiling the embankments

We don’t need to remove all the flood defences that exist. It's also not desirable for nature conservation. But where we do decide to keep embankments in place, we need to make sure they can cope with the higher tidal levels predicted over the next century to avoid any sudden and catastrophic failures and further loss of saltmarsh. To do this, we can use a technique known as reprofiling.

A shallower profile

On the north-east embankment at Northey Island, we have reduced the risk of the embankments breaching when flood waters come over the top. Breaching often happens from the landward side as the water floods over the bank and rushes down the relatively steep slope on the back face of the bank.

Once breached in this unmanaged way, deep scour holes form at the site of the breach and the surrounding saltmarsh is rapidly eroded. By making the back slope of the remaining north-east embankment shallower in profile, we have reduced the risk of this happening.

Releasing the flood water

The flood water can then be released back to the estuary in a controlled way through sluice pipes, avoiding loss of habitat there. In this way, the freshwater and slightly salty (brackish) habitats that exist in ditches, ponds and areas of terrestrial grazing marsh on the land can recover and survive into the future.

Funding our work

The work at Northey Island was part-funded under EU LIFE on the Edge. Other funding has come from National Trust Neptune Funds, Delia’s Fund, National Highways Environment and Well-Being Designated Funds, Defra’s Natural Flood Management Scheme and the Association of National Trust Members and Supporters in Belgium.