Managed and unmanaged realignment as a nature-based solution to saltmarsh habitat loss: A sedimentary perspective

Managed realignment is the deliberate inundation of the coastal hinterland as a nature-based solution to saltmarsh habitat loss. Managed realignment sites are often designed and landscaped to encourage the development of a mosaic of targeted habitat types. However, despite these pre-breach engineering works, it has been demonstrated that managed realignment sites have a lower abundance and diversity of key plant species, and an increased percentage of bare ground. In contrast, unmanaged realignment is an emerging area of interest and describes sites where no landscaping or engineering works take place prior to site breaching. However, little is known of the evolution and development of unmanaged realignment sites. We argue that from a sedimentary perspective, with emphasis on the centrality of sediment in the evolution of realignment sites, the two approaches should not be classified into distinct categories. Through this sedimentary perspective, we synthesise the scientific evidence on the design and implementation of realignment in terms of the sediment structure, content, movement, and landform evolution. In doing so, we highlight that by adopting a sedimentary perspective a more integrated approach to realignment can be taken. The future research agenda is discussed, concluding that a more managed approach is needed in locations where unmanaged realignment could occur.