Harbour-dredged mud used for ‘remarkable’ restoration of protective saltmarsh

Dredged mud from a harbour on the south coast is being used to restore surrounding saltmarsh, in an innovative scheme to protect the local economy and wildlife.

The team behind the scheme to restore the eroding saltmarsh that protects Lymington Harbour, Hampshire, hope the “remarkable” recovery of natural habitat could be replicated around the country to conserve coastal areas at risk from rising sea levels.

Lymington has a ferry link to the Isle of Wight, 1,600 leisure moorings and sees 20,000 visiting yachts a year, with the harbour worth tens of millions of pounds a year to the local economy.

But its precious saltmarsh is eroding at a rate of two or three metres a year in the face of rising seas and increased storminess driven by climate change, which risks leaving the harbour exposed and unviable.

Building rocky breakwaters and dumping dredged mud in front of the saltmarsh have been used in recent years to slow the erosion.

But now a partnership led by Lymington Harbour Commissioners and Land & Water Group is trialling new methods to try to place dredged sediment at a height that will allow plants to recolonise and restore the habitat.

The scheme is backed by the Crown Estate, which owns the majority of the foreshore, seabed and tidal riverbed at Lymington Harbour, including the saltmarsh being restored.

Ryan Willegers, chief executive of Lymington Harbour Commissioners, said that the harbour had started seeing a decline in the saltmarsh since around 1920, like many other areas around the UK, and it had been eroding ever since.

He added: “At the beginning of this century we started to get very concerned that if it carried on, we would be in a situation where the saltmarsh that protects the harbour, most of it wouldn’t exist.

“Without the saltmarsh a lot of the harbour would be exposed and no longer viable to support the economy.”

James Maclean, chief executive of Land & Water, said saltmarshes were not only important for coastal defences, they also support 80% of marine species that grow in inshore waters, leading to the potential “collapse” of marine ecosystems if nothing is done to protect them from rising sea levels.

“They are a really important part of the UK coastal story,” he said.

At the same time, he said, around 20 million tonnes of nutrient-rich sediment dredged from the UK’s harbours and ports each year, is almost entirely dumped offshore.

So the team are testing different technologies and working with a number of bodies to unlock regulatory issues to make it easier to use sediment in nearshore areas.

They are using technology that is inspired by Victorian steam drag boxes, described by Mr Willegers as a “giant scoopy sled” with a hydraulic winch attached to a 28 tonne excavator, to rebuild the saltmarsh.

In 2024, the first year of the trial, which was funded by the Environment Agency, they put around 800 cubic metres of mud to the height needed to regenerate habitat deep into the saltmarsh in the least exposed area possible.

When they returned 12 months later, “we’ve got really good quality saltmarsh growing in that location”, Mr Willegers said.

Last year, funded by the Crown Estate, the scheme involved transporting the sediment a far shorter distance, allowing the team to move two and a half times the amount of sediment in a third of the time.

“Saltmarsh won’t grow until the spring, we had to get through this winter and hope it’s still there at the right level, then if it makes it through to the spring we are fully expecting it to colonise and have some very good saltmarsh,” Mr Willegers said, adding that it was all still there so far.

Mr Maclean said that sediment had to have the right chemical and physical composition, and be raised to the target height – but then it would naturally recolonise with wind-blown and bird-carried seeds, with pioneer species appearing within a year.

“Everybody involved has been dumbfounded by how quickly in 2024 the material colonised.

“It’s remarkable how over one winter, it’s turned into a really quite exciting habitat.

“So we know if we get that target height, nature will take over,” he said.

The Crown Estate says it is backing the scheme to protect its assets and align with its “guiding nature principles” approach to treat restoration, protection and sustainable investment in nature as a priority in its marine estate.

And it is hoped it could be replicated in other threatened saltmarshes.

Caroline Price, head of nature and environment at the Crown Estate, said more traditional approaches to restore saltmarshes could be slow and expensive, and the organisation wanted to think about how to use the sediment that needs to be removed to keep harbours operational.

“We’ve been keen, alongside Land & Water, the Environment Agency and others to look at whether there were more innovative, lower cost, more scalable ways to bring these two things together.

“It’s important that we can have the sort of innovative projects, like what’s happening at Lymington, to help us understand the art of the possible and how we can move forward and change and then replicate that elsewhere,” she said.