Abstract
Managed realignment (MR) involves the landward relocation of sea defences to foster the (re)creation of coastal wetlands and achieve nature-based coastal protection. The wider application of MR is impeded by knowledge gaps related to lacking data on its effectiveness under extreme surges and the role of changes in vegetation cover, for example due to sea-level rise. We employ a calibrated and validated hydrodynamic model to explore relationships between surge attenuation, MR width(/area) and vegetation cover for the MR site of Freiston Shore, UK. We model a range of extreme water levels for four scenarios of variable MR width. We further assess the effects of reduced vegetation cover for the actual MR site and for the scenario of the site with the largest width. We show that surges are amplified for all but the largest two site scenarios, suggesting that increasing MR width results in higher attenuation rates. Substantial surge attenuation (up to 18 cm km−1) is only achieved for the largest site. The greatest contribution to the attenuation in the largest site scenario may come from water being reflected from the breached dike. While vegetation cover has no statistically significant effect on surge attenuations in the original MR site, higher coverage leads to higher attenuation rates in the largest site scenario. We conclude that at the open coast, only large MR sites (> 1148 m width) can attenuate surges with return periods > 10 years, while increased vegetation cover and larger MR widths enable the attenuation of even higher surges.
Highlights
The combination of accelerated sea-level rise (Dangendorf et al 2019; Nerem et al 2018) and projected increases in episodic flooding, in regional hotspots such as north western Europe (Kirezci et al 2020), is expected to lead to increased flood risk for low-lying coasts and higher adaptation costs (Hinkel et al 2014)
Our results show that the capacity of the Freiston Shore Managed realignment (MR) site to reduce extreme high water levels (HWL) is generally low, ranging from −21 to 7 cm km−1 with an average of −3 cm km−1 for all extreme events and vegetation scenarios
For a comprehensive overview of HWL attenuation rates and the results of statistical tests for Freiston Shore and all MR width scenarios, we refer the reader to Table S1, provided in the online resource to this article
Summary
The combination of accelerated sea-level rise (Dangendorf et al 2019; Nerem et al 2018) and projected increases in episodic flooding, in regional hotspots such as north western Europe (Kirezci et al 2020), is expected to lead to increased flood risk for low-lying coasts and higher adaptation costs (Hinkel et al 2014). Traditional hold-the-line approaches for adaptation to rising sea levels and associated flooding limit accommodation space and can lead to large-scale losses of coastal wetlands. Such losses may be prevented by changing adaptation practices towards nature-based solutions (Spencer et al 2016; Schuerch et al 2018). Nature-based coastal adaptation can provide a cost-effective alternative (Reguero et al 2018) or complement conventional coastal defence schemes, as studies from the United Kingdom (UK) and the Scheldt estuary (Belgium) have shown (Turner et al 2007; Broekx et al 2011) Such practices involve the creation or restoration of coastal wetlands such as
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