Managing Change at Northey Island

A view across the salt-marsh at Northey Island, Essex. A small creek at Northey Island, Essex. A remote island in the Blackwater Estuary and cut off at high tide, visiting Northey's a unique pleasure.

A significant conservation project is underway at Northey Island to help retain vital saltmarsh habitat in the Blackwater Estuary over the next century. We are working in partnership with the RSPB funded through the EU Life programme to protect, strengthen and enhance this precious saltmarsh habitat.

The saltmarsh is far smaller than it used to be, eroding on the outer edges due to wave action and stronger tidal flows caused by sea level rise and climate change. The inner parts of the saltmarsh are also being eroded along creek margins as more salty water flows over and through them. Sea level rise is also now at a rate that the vertical growth (from sediment deposition) is struggling to keep pace with.

As a result of sea level rise and climate change the saltmarsh habitat is changing. Some of the plant species usually found at lower saltmarsh elevations are dying out as the areas where they live are now more frequently under water. These plants are creeping further up the saltmarsh, encroaching on the “higher” saltmarsh plants as sea levels rise, but the higher species have no space to migrate to as we have historically built seawalls to claim land from the sea and to protect from flooding; they are being squeezed out of existence by a process known as ‘coastal squeeze’ between the rising tides and the fixed man-made defences. This has led to the loss of many hectares of saltmarsh in the Blackwater Estuary, a pattern that is repeated across much of south eastern England.

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Responding to the threat of sea level rise

There are a range of management approaches the National Trust aims to apply over the next few years, with a common goal of maintaining saltmarsh at Northey Island over the next 100 years. Taking no action will see the loss of almost all the saltmarsh, so to be effective the measures taken need to work together and with the natural estuary tidal ebb and flow and rising sea levels.

Realigning the defences

The defences to the south and south-east of Northey Island are being overtopped annually on large tides. All the defences around the island are in a poor physical condition so could fail in an uncontrolled way; such failure leads to erosion and further loss of habitat and can create conditions behind the defences that limit the way vegetation can become established, this can reduce the diversity (10% less saltmarsh species colonise) compared to a managed situation.  Through actively managing the process the saltmarsh can be created or managed to adapt and migrate more naturally giving it a longer lifespan and better biodiversity.

Managed Realignment involves removing parts of the flood bank to give space for the rising sea level and space for the saltmarsh plants to migrate inland in a natural way as the sea level rises. This management action was first implemented in England at the south-east corner of Northey Island in 1991. It has already created a healthy, functioning, ecosystem that has existed for 30 years and demonstrated a technique that has now allowed over 5,000 hectares of intertidal area to be created across England to date.

Following on from the success of the project in 1991, during the summer of 2019 we lowered part of the flood defence along the south embankment of Northey Island. The dug clay was placed back into the ditch behind it, from where it was originally excavated, returning the area to its previous natural landscape.

Silhouettes of the geese coming in to land with an orange

Habitat creation

This element of the project has allowed new saltmarsh to form and over a year on saltmarsh plants like sea beet and samphire are flourishing, whilst birds including curlew and lapwing are making use of the area for feeding and resting over high tide.

Saltmarsh created at Northey Island in 2019

Sustainable saltmarsh

This area at Northey can now sustain itself into the future as the man-made walls have been removed and the system is functioning naturally, re-creating saltmarsh on land that was claimed from the sea in the past.

The next step in our work at Northey Island is to improve the Closing Bank in the centre of the island to allow for managed realignment in the South-East of the island. The central Closing Bank will prevent flooding between the eastern and northern fields when the embankment is realigned and is an important step in implementing the Coastal Adaptation Strategy for the island. This Closing Bank will also offer protection to a badger sett and allow for adaptation of the north east corner of the island to cope with rising tide levels for the next 50 to 100 years.

Regenerating the saltmarsh

On the north-west side of the island we have applied a different management technique by beneficially using locally dredged sediments. Here we have placed sediment that has been dredged to maintain the navigation for boats and barges to Maldon. The placement of the sediment has been carefully designed to avoid new channels being formed through the saltmarsh east to west and reduce the higher flows over the saltmarsh surface that have been causing further erosion and vegetation loss.

The sediment placement has also modified the flows within the saltmarsh itself.  This management action has taken 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 and that helps reduce wave action over the saltmarsh surface (which also leads to loss of saltmarsh condition). To date, the adaptation of the local physical processes has trapped 1.5 times the volume of sediment placed, thus showing that the marsh is adapting by turning a section of saltmarsh that was eroding into one that is now building (known as accreting) sediment, with the vegetation becoming healthier. In this process we have regenerated a hectare of saltmarsh vegetation on what was eroding, bare clay.

On the Northey Island north-west corner, locally dredged sediment is used to regenerate saltmarsh habitat

Placing Sediment

This photo was taken when we first placed the local material along the bank in 2017. Once placed, the material is re-shaped and allowed to naturally colonise with plants.

View across Northey Island showing how plants have recolonised the saltmarsh

Taking Root

As the tide comes in and out it drops seeds which start to grow on the bare mud relatively quickly, as shown in this photo taken in 2018. The first plants to take root on the bare mud are called saltmarsh ‘pioneers’.

Regenerated Saltmarsh at Northey Island

Succession in action

The ‘pioneer’ species are quickly succeeded by saltmarsh plants that grow at their relevant niche compared to tidal inundation, creating a healthy and diverse saltmarsh habitat, as can be seen in this 2019 photo.

The area created is higher in elevation than the existing saltmarsh and has regenerated naturally with saltmarsh plants. This is improving the diversity of the saltmarsh and providing a niche area for some of the rarer saltmarsh species that are under immediate threat of loss from coastal squeeze. This is also providing protection from the increased forces caused by climate change and sea level rise to the remaining saltmarsh with 4.5ha visibly improved already.

The works between 2018 and 2020 were been completed with funding from the National Trust Neptune fund and from a grant from Defra’s Natural Flood Management (NFM) fund. The Trust has monitored this beneficial use and through NFM is helping advise the use of the technique elsewhere. On the back of the success of the NFM project, we are now extending this work using an EU LIFE grant to improve the resilience of more of the saltmarsh in the north-west corner of the island. Hopefully, this kind of intervention can be used in other suitable areas around our precious estuaries to help sustain saltmarsh habitats elsewhere.

Maldon District Conservation & Design Award 2019 Plaque

Recognition of achievement

In November 2019 the Northey Island Coastal Adaptation Project was named the winner of the Nature Conservation category at the Maldon District Conservation & Design Awards.

Re-profiling the embankments

It is not necessary, nor indeed desirable for nature conservation, to remove all flood defences that exist, but keeping them all in place as they are will result in the further loss of saltmarsh. Where we decide to keep embankments in place, we need to consider how they can survive in the face of the rising tides and waves as much as the saltmarsh itself and avoid creating a situation where a sudden and catastrophic failure arises. This is about adapting the embankments themselves to cope with higher tidal levels and more frequent overtopping by the tide over the next century.

Northey Island North East Embankment

Re-profiling

On part of Northey Island the options to realign or let the banks fail would lead to loss of saltmarsh habitat through connectivity across it, and of small creeks turning into large channels eroding away the remaining part of healthy saltmarsh. In this area we are applying a technique, which is to change the profile of the embankment itself so that overtopping causes little direct damage to the embankment.

Northey Island North East Embankment

Overtopping

As sea levels rise, more and more defences will come under increasing risk of being overtopped more than they were designed to be. The land behind becomes flooded and can remain so for a long time as only small pipes exist to release water back to the estuary. If we can evacuate salty flood water in these overtopping events within 7-10 days then 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.

The ability to evacuate salty flood water rather depends on the embankment staying in place and not breaching when the flood waters come over the top. Breaching of embankments 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. In the past this was not an issue as the sea was lower and overtopping less frequent. Once overtopped, the water causes scour (erosion) of the bank face and slumping making the bank thinner until eventually it breaks through in a narrow breach. Once breached in this un-managed way, deep scour holes form at the site of the breach and the surrounding saltmarsh is rapidly eroded away.

To reduce the risk of this failure we aim to make the back-slope of the remaining North East embankment shallower in profile, so that overtopping may occur without it scouring and thus reducing the risk of the bank breaching. The flood water can then be released back to the estuary in a controlled manner through more sluice pipes avoiding loss of habitat there.

Life on the Edge

From 2021, much of the work at Northey Island is possible thanks to EU funding under the LIFE on the Edge (LOTE) Project. We are working in partnership with the RSPB across several sites to create more and better coastal habitat to benefit breeding, wintering and migratory water birds, as well as other wildlife.  Northey Island is a showcase for different management techniques for the underpinning habitat that support these birds.

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