Is unmanaged realignment an appropriate coastal management strategy?

Managed realignment, the process of breaching flood defence structures for habitat restoration and flood defence purposes, is becoming an increasingly popular form of coastal management across Europe and North America. Typically managed realignment has been implemented at former intertidal locations that have been embanked and reclaimed, on the assumption these areas should be able to support intertidal habitats again. Despite this assumption, during the construction of managed realignment sites extensive engineering and landscaping works are often carried out, including the construction of drainage channels rather than utilising remnant pre-reclamation intertidal drainage channels. These engineering works are intended to encourage a range of habitat types and support the intended post site breaching land use, such as grazing. However, it has been demonstrated that managed realignment sites have more simplified creek and drainage networks, and lower topographic variability, than natural saltmarshes, which might restrict drainage, impact the plant communities that can colonise, and prevent widespread sedimentation and seed dispersal.

 

In contrast to managed realignment, unmanaged realignment is the natural breaching of flood defences without any costly engineering or landscaping works. Unmanaged realignment sites provide an opportunity to assess the “natural” morphological evolution of restored saltmarsh sites without the influence of extensive site design, engineering, or landscaping features. However, there remains no analysis of the evolution of ‘recent’ unmanaged realignment sites, with most studies focusing on historic breaches. This study provides an assessment of the occurrences of unmanaged realignment on the coast of the United Kingdom, which has the most realignment sites in Europe, since from 1996. The subsequent morphological evolution of these sites is then evaluated through an assessment of the change and development of the creek and drainage networks. Results indicate differences in post-breach morphology in relation to the history and former land use management of the sites. Findings are discussed in terms of the benefits of unmanaged realignment, and considered in context of long-term shoreline management planning, including habitat creation, flood defence and carbon storage. It is recommended that further data are collected on unmanaged realignment sites to understand their development and to enable comparison with managed realignment sites, with examples of such comparisons included in the discussion.