Brushwood-fenced sedimentation fields for saltmarsh restoration at Rumney Great Wharf, Severn Estuary, UK

Fairley, I., Jenkins, D., Dale, J. ORCID: https://orcid.org/0000-0002-5242-8071, Rimington, N., Pauls, L. and Bennett, C. (2026) Brushwood-fenced sedimentation fields for saltmarsh restoration at Rumney Great Wharf, Severn Estuary, UK. In: ICE Coastal Management Conference, 16-18 September 2025, Bristol.

Abstract/Summary

A series of sedimentation fields, or ‘polders,’ were constructed by Natural Resources Wales (NRW) at Rumney Great Wharf (Severn Estuary, UK) in summer 2024, replacing a previous scheme built in 1989 and 2005 that had shown some promise but was not adequately maintained or monitored. The aim of the new scheme is to rotect and restore saltmarsh and mudflat by facilitating mud deposition, through dampening waves and current velocities, and subsequent vegetation colonisation; thereby reversing the erosive trend in the region. This should have benefits for both biodiversity and flood risk. The project is being considered a pilot study for the suitability of polders as a nature-based solution in higher energy environments similar to Severn Estuary; therefore, a secondary aim is to develop understanding of the site evolution to inform future projects. This paper details the design process, construction method and monitoring plans; highlighting the lessons learned to date. Historically, polders have been constructed in areas of lower tidal range and the mega-tidal range at Rumney Great Wharf complicated the design process, particularly the spring-neap variability. Hydrodynamic modelling has shown that polder installation would slow currents sufficiently to promote accretion within the fields and previous research has shown there is enough sediment in the system to make polders an appropriate tool. Wave exposure was identified as the biggest risk, both in terms of structural damage and mobilisation of newly deposited sediment. The large tidal range meant that if the seaward fences were installed at a high enough level to fully mitigate the wave-associated risk, the polders would only be submerged on a limited number of tides (reducing the amount of time when sediment would be brought into the fields) and the area enclosed by the polders would be too small to be cost effective. Therefore the design of the new scheme has accepted this risk and installed polders lower in the tidal frame. The project is committed to ongoing maintenance to give the scheme the greatest chance of success. Construction largely went smoothly, but the brushwood compacted more than expected which means the fence height did not meet the design height for the whole scheme. Structural and morphological monitoring has commenced: in both cases this has been based on uncrewed aerial system (drone) data due to the health and safety risks of traversing unconsolidated mud, especially as the scheme starts to accrete. The structural monitoring has shown that the fences have successfully withstood the winter period, with only minor repairs needed. The most recent morphological survey was conducted one month after the polder construction was completed and therefore the only noticeable changes related to the polders were compaction of the mud during construction, which anecdotally has already filled in. Biannual surveys are planned to capture post-summer and post-winter changes as the scheme evolves.

Item Type	Conference or Workshop Item (Paper)
URI	https://centaur.reading.ac.uk/id/eprint/128900
Refereed	Yes
Divisions	Science > School of Archaeology, Geography and Environmental Science > Department of Geography and Environmental Science
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