Research articleIdentifying global hotspots where coastal wetland conservation can contribute to nature-based mitigation of coastal flood risks
Introduction
Coastal areas are increasingly exposed to flood and erosion risks due to sea level rise, increasing intensity of storms (Hinkel et al., 2014; Vitousek et al., 2017), and land subsidence by human actions such as reduction of sediment supply by river dams or soil compaction after conversion of coastal wetland ecosystems into human land use (Auerbach et al., 2015; Kirwan and Megonigal, 2013; Tessler et al., 2015). In parallel, the coastal population will continue to grow, reaching globally averaged densities of 405–534 people/km2 by 2060 (or ten times the current world's average) (Kron, 2013; Neumann et al., 2015), with more and more people concentrated in large coastal cities (Sengupta et al., 2018; United Nations, 2012), increasing the number of people and assets exposed to coastal flood risks (Hanson et al., 2011; Small and Nicholls, 2003).
The standard strategy for coastal protection is the construction of hard engineering structures such as dams or dikes that protect low-lying coastal areas from coastal flood and erosion risks (Adriana Gracia et al., 2018; Pranzini, 2018; Rangel-Buitrago et al., 2018). However, those structures are more and more challenged because they may have negative consequences for the natural environment (disturbance of natural habitats, disturbance of sediment supply, accelerated erosion) and because of practical and financial difficulties to maintain them in the face of projected climate and socio-economic changes. Nature-based solutions, or combined hybrid solutions, are more and more regarded as a sustainable, self-sufficient and cost-effective strategy to mitigate coastal flood and erosion hazards (Adriana Gracia et al., 2018; Temmerman et al., 2013). Nature-based solutions are based on the conservation, restoration or creation of coastal ecosystems, such as mangrove forests and salt marshes (further referred to as tidal wetlands), for their capacity to reduce the inland propagation of storm surges, to reduce wind waves and shoreline erosion, and to adapt to sea level rise by sedimentation (Krauss et al., 2014; McIvor et al., 2012a, McIvor et al., 2012b; Shepard et al., 2011). In the last decades, projects of nature-based coastal protection were developed in several coastal areas around the world, as along the Mississippi delta plain (Boesch et al., 2006; Coastal Wetlands Planning Protection and Restoration Act (CWPPRA), 1990; Day et al., 2007) or along coastal plains and estuaries in the UK, Belgium and the Netherlands (Gardiner et al., 2007; Meire et al., 2014; SigmaPlan, 2017).
Tidal wetlands are increasingly recognized as having the capacity to attenuate storm surges. The mechanisms of storm surge reduction rely on the friction exerted by the tidal wetlands' geomorphology and vegetation on the water column during the landward propagation of the surge (Lyddon et al., 2018; Smolders et al., 2015; Stark et al., 2016). The storm surge attenuation rate is often expressed as a rate of storm surge height reduction per unit of distance travelled through the tidal wetlands. Attenuation rates derived from observations range from a couple of centimetres to 25 cm/km for salt marshes (Krauss et al., 2009; Stark et al., 2015; Wamsley et al., 2010; Zhang et al., 2012), and up to 50 cm/km for mangrove forests as reported by the hydrodynamic modelling study of Zhang et al. (2012). The large ranges of attenuation found in the literature are representative of the various conditions that could affect the storm surge attenuation capacity of salt marshes and mangrove forests, such as specific properties of the wetlands (e.g. the vegetation canopy height, density or stiffness, the density and width of wetland channels) (e.g., Hu et al., 2015; Loder et al., 2009; Temmerman et al., 2012), specific storm characteristics (e.g. magnitude, duration, track of the storm) (e.g., Liu et al., 2013; Resio and Westerink, 2008; Wamsley et al., 2010), and properties of the larger-scale coastal landscape (e.g. coastline geometry, off-shore bathymetry, etc.) (e.g. Wamsley et al., 2009). All those parameters influence the storm surge attenuation capacity of tidal wetlands and therefore generate large ranges of storm surge attenuation rates (see Leonardi et al., 2018, for a review).
Existing studies on storm surge risk mitigation by tidal wetlands, discussed above, mostly focus on local to regional scales and are mostly concentrated on specific locations in the USA (e.g. Arkema et al., 2013) and to a lesser extent in Europe (e.g. Stark et al., 2015), while studies elsewhere in the world are much scarcer. As a consequence, until now there is poor insight in the global scale possibilities for nature-based storm surge mitigation by mangroves and salt marshes. For this study, we aimed to identify the global distribution of coastal land surface areas and population numbers that can receive nature-based flood risk mitigation by existing mangrove and salt marsh ecosystems. To do so, we further developed the GIS based model of Van Coppenolle et al. (2018), which provides a simplified procedure to estimate, for a given coastal area, the flood routing of a 1-in-100 year storm surge and which estimates the land surface area and population number that would be reached by a storm surge that has first travelled through salt marshes or mangroves. In Van Coppenolle et al. (2018), the model is developed and applied to the case of 11 deltas around the world, illustrating the capacity of exiting salt marshes and mangroves to attenuate storm surge impacts in the specific deltas. Here in this study, the model is further developed and applied on a global scale, with the aim to increase insights in the global distribution of land surface areas and population numbers that can receive nature-based flood risk mitigation by existing salt marshes and mangroves. As such, we aim to stimulate further local-scale studies and developments in nature-based mitigation policies as a strategy against increasing coastal flood risks.
Section snippets
Datasets
The following datasets were used (See Table 1).
The values for the topography and the bathymetry are provided by the General Bathymetric Chart of the Oceans (GEBCO) (British Oceanographic Data Centre, 2017) that represents a gridded bathymetry of the oceans coupled with the NASA Shuttle Radar Topography Mission (NASA Shuttle Radar Topography Mission SRTM, 2013) digital elevation model of the continents. Both datasets have a resolution of 30 arc sec. The SRTM dataset is found to be the best known
Coastal plain areas benefiting from storm surge mitigation
The results show that for a 1-in-100 year storm surge event, without accounting for any flood protecting structures and without the existing tidal wetlands (scenario 2), 281,750 km2 of the world's coastal plain is exposed to storm surge flood risks. However, when accounting for the currently existing tidal wetlands (scenario 1), 80,307 km2 (i.e. 29% of the previous number) of the world's coastal plain benefits from a reduction in storm surge height as the storm surge pathway passes through
Discussion and conclusion
In the face of global climate change and the associated increasing risks of coastal flooding from more severe storm surges and expected sea level rise (Hallegatte et al., 2013; Hinkel et al., 2014; IPCC, 2013; Woodruff et al., 2013), the conservation of tidal wetlands can contribute to the nature-based mitigation of coastal flood risks by their ability to attenuate storm surges, reduce the impact of waves and shoreline erosion, and accumulate sediments in balance with sea level rise (Lovelock
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.
Acknowledgements
The author would like to thank the University of Antwerp who funded this study.
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