New fences can save restored saltmarsh

Saltmarsh is disappearing from coastlines around the world, and while brushwood fencing can help bring it back, a new study has found it only works if the fences are kept in good repair.

Around 46% of the world's saltmarsh has been lost or damaged, and restoring it is not always straightforward. Sedimentation fields, which use rows of brushwood fencing to trap mud and encourage marsh plants to take root, offer an option where conventional methods cannot be used.

New research from the University of Reading, published in the Journal of Environmental Management, is the first to look at what happens at a sedimentation field site once the marsh has formed, rather than just in the early stages after construction.

The research team studied a site at Rumney Great Wharf in Wales, where five sedimentation fields were built between 1989 and 2005. The fencing was not repaired after 2010, and the willow bundles that formed the barriers washed away, leaving only the wooden stakes behind. The site was selected due to Natural Resources Wales (NRW) interest in the location: NRW re-instated and extended the fencing at the site in August 2024, funded by the Welsh Government’s Nature Networks programme. This programme looks to increase biodiversity and resilience in our protected areas. By implementing the polders, the project aims to increase saltmarsh habitat for both biodiversity benefits and flood risk mitigation in the local area.

Drone surveys carried out in May 2023 and May 2024 showed that 87% of the enclosed area had lost sediment during that 12-month period, a total of 9,531 cubic metres. That is roughly the same as filling four Olympic swimming pools with mud.

The mud was washing away even though measurements showed it could build up at more than 10 centimetres per year in some parts of the site. Without fencing to slow the water, waves and tidal currents were strong enough to carry the sediment away. It is hoped that the new fencing will reverse this trend before continuing to create even more saltmarsh at the site.

Dr Jonathan Dale, lead author at the University of Reading, said: “Saltmarsh protects our coastlines from flooding, stores carbon and provides habitat for wildlife. We are losing it fast, and sedimentation fields are one of the few ways to bring it back in places where other methods cannot be used.

“This research shows that the job is not done once the fencing goes up. The fencing needs to be kept in good repair until the marsh can survive without help. Exactly when that point is reached remains unknown, and finding the answer is the next stage of the research.”

Following these findings, NRW have instigated an ongoing maintenance programme for the fencing at Rumney Great Wharf, giving the saltmarsh the best chance to recover.