Tidal-flat reclamation aggravates potential risk from storm impacts
Introduction
Most developing countries with a high population density in Asia, including China, Vietnam, Bangladesh, and the Philippines, primarily rely on seawalls for coastal storm protection (Temmerman et al., 2013; Barbier, 2015). Unfortunately, the natural storm flood mitigation functions of tidal flats (i.e., here refer to both unvegetated mudflat and saltmarshes) have been underappreciated (Möller et al., 2014). Since the middle of the last century, many of these countries have reclaimed large littoral areas for land demands (Bi et al., 2012; Barbier, 2015). The newly reclaimed polders that have been moved further seaward are generally low-lying, and thus highly sensitive to storm impacts. In order to address this issue, China, for instance, had constructed long, hard-engineered defenses of approximately 14,000 km length in total along the 34,000 km-long coastlines to protect its coastal population of roughly 600 million (Liu et al., 2019). The benefit of hard-engineered structures in mitigating economic loss and casualty is universally recognized. However, environmental changes of sea-level rise, land subsidence, and record-breaking extreme storm events are eroding the seawall's protective ability (Temmerman et al., 2013). Maintenance costs are hence expected to rise with time (Liu et al., 2019). Tidal flats are increasingly recognized as “recumbent seawalls”, providing long-term protection to the conventional hard-engineered defenses (Willemsen et al., 2020). These natural interfaces with the sea have an inherent resilience against sea-level rise (Kirwan et al., 2016) and hence, in contrast to human-constructed defenses, do not need structural maintenance, and further provide valuable ecosystem services that vertical seawalls do not offer (Reed et al., 2018).
From all intertidal ecosystems, vegetated foreshores are most efficient in attenuating waves due to their highly elevated position in the intertidal zone (Bouma et al., 2014). Therefore, vegetated foreshores, like saltmarshes (Möller et al., 2014) and mangrove forests (Menéndez et al., 2018), are well recognized as ecosystem-based flood protection to reduce storm impacts. Wave mitigation by vegetated foreshores depends both on plant traits like shoot stiffness and shoot biomass (Bouma et al., 2005, 2010), stem height, stem diameter, and stem density (Reed et al., 2018), as well as ecosystem traits like marsh width (Willemsen et al., 2020). Even if plant stems break during an extreme storm or are absent, tidal foreshores may still attenuate waves by morphological effects like topographic slopes (Loder et al., 2009; Vuik et al., 2018), bottom friction (Möller et al., 2014), and depth-induced wave breaking (Altomare et al., 2016). Translating this kind of knowledge on wave attenuation to designing hybrid flood defense systems, which consists of a seawall behind a tidal flat, requires numerical models (Vuik et al., 2018). In this study, we aim to integrate the effect of extreme water levels with storm wave run-up into a single long-term modeling effort to quantify how the flood risks of a hybrid flood defense system changes after land reclamation. That is, we model under extreme storm conditions, how wave loading and wave overtopping changes in response to reclamation of tidal areas.
Our study site is the Fengxian Coast, located on the northern bank of Hangzhou Bay, China. Since the 1950s, a large-scale coastal embankment program has been implemented, aiming to improve flood defense and navigation (Xie et al., 2017; Zhang et al., 2018a). At the beginning, embankments only occurred above the high-water level (i.e., high marsh area); however, they were gradually extended onto the intertidal zone (i.e., including low marshes and bare tidal flats), and now reclamation is being performed beyond the low-water level at the −5 m sub-tidal zone to fulfill the increasing land demand (Zhang et al., 2018a) (Fig. 1b). Embankments above the high-water level are believed to be beneficial for flood mitigation (Kundzewicz et al., 2019; Wang et al., 2012). However, little is known about the actual consequences of intertidal reclamation for coastal flood safety and if such measures could reduce or magnify coastal flood risk due to the change of wave run-ups. Moreover, in recent years the Fengxian Coast has changed from being an area of accreting to eroding (Xie et al., 2017), which will lead to an even lower and narrower foreshore at the toe of the seawall in the future. It is reported that the coastal embankments in Bangladesh in the intertidal zone have not always made a positive contribution to flood mitigation (Adnan et al., 2019). Similarly, it is questionable whether the substantial inter- and sub-tidal embankments have reduced flooding risk along the Hangzhou Bay. Therefore, there is a need for a better understanding of the role of tidal flat in helping to mitigate the flood hazard.
In this study, we determine the long-term changes in flood risk using a numerical model that combines extreme tidal levels with wave overtopping analysis, using the foreshore profiles measured before and after reclamation (i.e., wide vs. narrow tidal flat) at the Fengxian Coast. That is, we primarily assess the impacts of reclamation on flood risks by comparing the wave impact on the seawall behind the wide high marsh tidal flat as present in 1984 versus the seawall behind the narrower bare tidal flat as formed after the reclamation that had taken place between 1984 and 1997 (immediately prior to the No. 9711 typhoon event, http://typhoon.zjwater.gov.cn/, accessed on 15 August 2019). These analyses are done on a selected length of the Fengxian Coast, where crucial industries and a college town suffered severe coastal flooding during the No. 9711 typhoon event, resulting in approximately $5.5 billion direct damages (Wang et al., 2012). Particularly, we explored whether reclamations had intensified wave overtopping and increased damage of the No. 9711 event, and to what extent the damage is due to human interventions. A quantitative relationship between the width of intertidal flat (in the horizontal scale) and the height of seawall (in the vertical scale) on equivalent flood protection has been derived to inform the discussion on the non-stationary seawall-foreshore redesign, and how this contributes to or detracts from coastal flood protection.
Section snippets
Study area and foreshore bathymetric measurements
The Fengxian Coast is situated on the northern bank of Hangzhou Bay, the largest embayment of China, and on the southern bank of Shanghai Municipality, which hosts the largest economy in China (Fig. 1). It is a region of high importance, having crucial nuclear energy infrastructure, petrochemical industry, and a college town (Fig. 1b). The closest tide gauge station shows that storm surges influencing the Fengxian Coast propagate primarily from Hangzhou Bay, which is a typical funnel-shaped
Methodology
The investigation into the impacts of intertidal reclamations on aggravated risks from storm surges have been addressed in the following three steps: (1) a framework of numerical models hindcasting tides and waves with improved accuracy is used to downscale the offshore sea states to those at the toe of the seawall; (2) the outputs of tidal levels (TLs) and significant wave heights (SWHs) of independent storms are used for the extreme value analysis, and the resulted marginal distributions are
Return levels of stationary and non-stationary estimations
In order to examine the changes of extreme tidal levels (TLs) and significant wave heights (SWHs) due to reclamations, a detailed analysis of all independent storms is provided using the plots of probability distributions (Fig. 6a–c). A more explicit comparison of the change of return levels versus the corresponding return periods under both stationary (ignoring the observed trend, Fig. 6d–f) and non-stationary assumptions (Fig. 6g-o) are also presented. The initial goodness-of-fit of the GEV
Discussion
Although vegetated foreshore is demonstrated to provide storm protection (Möller et al., 2014; Reed et al., 2018), it is proved challenging to quantify the evidence of the impacts of the tidal flat reclamation program on coastal flood risks due to the facts that (i) intertidal reclamations are always accompanied with procedures of seawall strengthening and heightening, (ii) tidal flat morphology consisting of a sediment body in front of the dike is continuously evolving especially shortly after
Conclusions
With coastal reclamations, tidal flats are lost, while wave run-ups at the toe of the seawall are increased, threatening the sustainability of coastal development around Shanghai. As a safety measure to protect against flooding, seawall management requires a derivation of the quantitative relationship between tidal flat loss and the increase in extreme events intensity. The results show that the intertidal reclamations are the primary source of increased flood risks in locations dominated by
Author statement
All authors contributed to the design and development of the work. The numerical experiments were originally carried out by Huayang Cai. Zhijun Dai and Jiahong Wen carried out the data analysis. Min Zhang built the model and wrote the manuscript. Tjeerd J. Bouma, Jeremy Bricker, Ian Townend and Tongtiegang Zhao reviewed and revised the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research is supported by the National Natural Science Foundation of China (Project no.: 41701001, 2018YFE0109900, 51761135024), China Postdoctoral Science Foundation (Project no.: 2018M630414) and Guangdong Provincial Department of Science and Technology (2019ZT08G090). The authors would like to thank Dr. Elisa Ragno and Prof. Oswaldo Morales Napoles from Delft University of Technology for their helps on non-stationary Copulas. Great thanks are due to the reviewers for a number of very
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