Managing Change at Northey Island

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.

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.

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.

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.

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.

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.

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.

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.