Abstract
A summer polder had developed a deficit in surface elevation of about 20 cm in respect to rising sea level during its almost one-hundred-year period of embankment. We addressed the questions whether the distance of the restored site to the intertidal flats and continuation of livestock grazing in the restored site could hamper surface-elevation change during the first 10 years after de-embankment of the summer polder. The surface-elevation change showed similar positive linear relationships with annual tidal flooding in both the reference salt marsh and the restored site, indicating that the surface-elevation change in the restored site was not moderated by the distance from the sea. The surface-elevation change had a clear seasonal pattern with positive values in winter and negative values during summer. The surface-elevation change was 11 mm/year in the grazed reference salt marsh and 7 mm/year in the grazed restored site, but amounted to 17 mm/year in ungrazed exclosures in the restored site, showing that grazing retarded the catching up of the elevation deficit in the restored site. The surface-elevation change within the restored site was higher close to the constructed creeks indicating the inception of levee formation. The surface-elevation change was also positively affected by the proximity of breaches in the embankment, but this effect was less clear than the effect of creeks. We conclude that the surface-elevation deficit may be compensated in the Wadden Sea summer polders by their de-embankment when sediment supply is high, whereas livestock grazing retards this process. Dug creeks increase spatial variation in the restored site.
Highlights
Coastal salt marshes are broadly defined as areas vegetated by herbs, grasses or low shrubs, which are subject to periodic flooding as a result of fluctuations in the level of the adjacent water
This study focuses on surfaceelevation change (SEC) after de-embankment and aims to identify whether grazing and flooding regime are important when considering measures to restore a salt marsh by de-embankment through breaching
It is noteworthy that negative values of SEC occurred regularly in years with a low flooding frequency or Cumulative height of the water column (CHWC) values
Summary
Coastal salt marshes are broadly defined as areas vegetated by herbs, grasses or low shrubs, which are subject to periodic flooding as a result of fluctuations in the level of the adjacent waterCommunicated by Eduardo SiegleEuropean salt marshes, in particular, have experienced a long history of anthropogenic impacts, some of which can be traced back several millennia, including livestock grazing, land claim and lowering of the land surface due to both peat mining and drainage for agriculture (Davy et al 2009; Vos and Knol 2015). An economic trade-off and the growing awareness that salt marshes contribute to coastal protection by dissipation of tidal energy and wave attenuation (Shepard et al 2011) caused a partial replacement of costly traditional engineering practices, such as revetment works, by alternative softer approaches. These alternatives mostly involve a landward relocation of the coastal defence line, allowing intertidal habitats including salt marshes, to develop seaward of the new defence line. This landward relocation is termed managed retreat or coastal realignment (P.W. French 2006; Zedler 2017)
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