Attenuation of high water levels over restored saltmarshes can be limited. Insights from Freiston Shore, Lincolnshire, UK

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The managed realignment (MR) of flood protection on low-lying coasts, and the creation, or re-creation, of intertidal saltmarsh habitat between old and new, more landward sea defence lines is an intervention designed to help protect coastal infrastructure and communities against the impact of storm waves and surges. However, the effectiveness of such schemes has rarely been proven in the field. Environmental monitoring has generally been limited to the first few years after implementation and has focussed on sediment accretion and surface elevation change, vegetation establishment and habitat utilization, to the neglect of the study of biophysical processes, such as wave energy dissipation and High Water Level (HWL) attenuation. We address this knowledge gap by analysing HWL attenuation rates in saltmarshes from within, and in front of, the open coast MR site of Freiston Shore (Lincolnshire, UK).For this purpose, a suite of 16 pressure transducers was deployed along four sections (two within and two outside the MR) of identical setup to measure water level variations during the highest spring tides of the year 2017.Our results show that for the conditions encountered during the field monitoring period, the capacity of the Freiston Shore MR site to provide HWL attenuation was limited. HWL attenuation rates were significantly higher over the natural saltmarsh (in front of the MR), where HWL attenuation ranged between 0 and 101 cm km−1 (mean 46 cm km−1). Within the MR site, rates varied between −102 and 160 cm km−1 (mean −3 cm km−1), with even negative attenuation (i.e. amplification) for about half of the measured tides.We argue that the weak performance of the MR site in terms of HWL attenuation was a result of internal hydrodynamics caused by scheme design and meteorological conditions. The latter may have counteracted the HWL attenuating effect caused by the additional shallow water area provided by the restored saltmarsh.