Hydrodynamic and sedimentary processes in sedimentation fields: the requirement for continued maintenance to preserve restored intertidal habitat

Globally, approximately 46.4% of saltmarsh has been lost or degraded. Sedimentation fields are one method of restoring and creating compensatory intertidal habitat, by enclosing mudflat to reduce hydrological forcing, trap sediment, and encourage saltmarsh establishment. To date, sedimentation fields have predominantly been studied using numerical models or with a focus on the initial deposition of sediment and plant colonisation. Consequently, it remains unknown whether the created habitat in sedimentation fields can become self-sustaining or whether there is a need for continued maintenance to prevent subsequent erosion. Here, we present a novel empirical investigation of Rumney Great Wharf, Wales, where sedimentation fields were constructed between 1989 and 2005 but no maintenance has been carried out since 2010. Hydrodynamic measurements indicate greater spatial differences in current velocity during the summer, and that suspended particulate matter was higher outside the sedimentation fields in comparison to inside the enclosed area during the summer but not during the winter. Between May 2023 and 2024, 87% of the surface area of the sedimentation fields experienced erosion resulting in the net loss of 9531 m3 of sediment. This loss occurred despite indications from on-site sediment trap data that there is the potential for sediment to accrete at more than 10 cm/year at mudflat sites and up to 9 cm/year at saltmarsh sites. These results suggest that the created marsh is not self-sustaining, and continued maintenance of the sedimentation fields might be required. Further research is required into sedimentary processes in sedimentation fields, both to inform the management of such schemes and to identify suitable locations for future sedimentation field construction. Assessments of marsh functioning and ecosystem service delivery in sedimentation fields are also needed to provide justification for future implementations of this restoration method.