Identifying global hotspots where coastal wetland conservation can contribute to nature-based mitigation of coastal flood risks

Journal Pre-proof Identifying global hotspots where coastal wetland conservation can contribute to nature-based mitigation of coastal flood risks

R. Van Coppenolle, S. Temmerman PII:

S0921-8181(20)30014-X

DOI:

https://doi.org/10.1016/j.gloplacha.2020.103125

Reference:

GLOBAL 103125

To appear in:

Global and Planetary Change

Received date:

6 March 2019

Revised date:

28 December 2019

Accepted date:

15 January 2020

Please cite this article as: R. Van Coppenolle and S. Temmerman, Identifying global hotspots where coastal wetland conservation can contribute to nature-based mitigation of coastal flood risks, Global and Planetary Change(2020), https://doi.org/10.1016/ j.gloplacha.2020.103125

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© 2020 Published by Elsevier.

Journal Pre-proof

Identifying global hotspots where coastal wetland conservation can contribute to naturebased mitigation of coastal flood risks R. Van Coppenolle1 and S. Temmerman1 Ecosystem Management Research Group, Department of Biology, University of Antwerp, Universiteitsplein 1C, 2610 Wilrijk, Belgium 1

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Corresponding author: Rebecca Van Coppenolle ([email protected]; Universiteitsplein 1C, 2610, Wilrijk, Belgium)

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Keywords: nature-based mitigation; salt marsh; mangrove forest; global hotspots; storm surge

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Abstract

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Low-lying coastal zones are increasingly exposed to flood risks due to global change including sea level rise, increasing storm intensity and growing coastal population densities. Local to regional-scale studies have demonstrated that conservation or restoration of coastal wetland ecosystems, such as salt marshes and mangroves, provides nature-based risk mitigation, as these wetlands have the natural capacity to mitigate the impacts of storm surges and related flood risks. Yet, it is unknown how important this nature-based mitigation of coastal flood risks is on a global scale. Here we present the results of a global-scale GIS model assessing the global distribution of inland surface areas and population numbers exposed to storm surges that would first propagate through tidal wetlands before they reach the inhabited land, and hence that would receive storm surge mitigation by the mangrove forests and salt marshes. Further our model quantifies the distance travelled by a storm surge through the tidal wetlands as a measure of the magnitude of storm surge mitigation. Results show that on a worldwide scale, about 30 % of the flood-exposed low-lying coastal plain benefits from nature-based storm surge mitigation by tidal wetlands, with the largest areas located in deltas (e.g. Pearl River, Yangtze, Mekong) and estuaries (e.g. Elbe). Areas protected by large wetlands, where a storm surge would first propagate through more than 5 km of tidal wetlands before it reaches vulnerable land and people, are located in river deltas such as of the Guayas (Ecuador), Mississippi (USA) and Ganges-Brahmaputra (India and Bangladesh). About 35 % of the global flood-exposed coastal population receives nature-based storm surge mitigation. The majority of that population (80 %) is located in five countries, i.e. China, Vietnam, the Netherlands, India and Germany. Areas more exposed to extreme storm surges (Eastern America, Caribbean Sea, Eastern Asia) include hotspot areas where storm surges are travelling through wider tidal wetlands generating higher risk mitigation, as for example in the Mississippi delta, Chesapeake bay, Ganges-Brahmaputra delta or Yangtze delta. Our global assessment aims to increase general awareness on the capacity of nature-based coastal flood risk mitigation, and to stimulate further local scale analyses in support of its wider application around the world.

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Journal Pre-proof 1 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 to 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).

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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, 2012b; Shepard et al., 2011). In the last decades, projects of naturebased 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 2

Journal Pre-proof (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).

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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.

2.1 Datasets

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2 Methods

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The following datasets were used (See Table 1).

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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 SRTM, NASA JPL, 2013) digital elevation model of the continents. Both datasets have a resolution of 30 arc-second. The SRTM dataset is found to be the best known global digital elevation model (Rodriguez et al., 2006; Sun et al., 2003). The worldwide distribution of the tidal wetlands was determined by the Global distribution of Mangroves (Giri et al., 2011) and the Global distribution of Saltmarshes (McOwen et al., 2017) (USGS, www.unep-wcmc.org). Only the polygon features from the salt marsh dataset (McOwen et al., 2017) were included in the analysis, as point features do not have a spatial extent and can not be accounted for to calculate storm surge height reduction over a certain distance. The coastlines delimiting the land and sea environment were defined by the combination of the country boundaries as existing in January 2015 (ESRI, DeLorme Publishing Company, Inc., 2015), and the mangrove forests and salt marshes extent, as the different datasets do not perfectly overlap (Lichter et al., 2011). The storm surge heights for a 1 in 100 year return period that are used in this analysis, are derived from the Global Tide and Surge Reanalysis (GTSR) dataset (Muis et al., 2016). It is 3

Journal Pre-proof based on the coastline segments of the Digital Chart of the World (DCW, Environmental Systems Research Institute, ESRI, 2002) and corresponds to the near-coast global reanalysis of storm surges over the period 1979-2014, with extensive validation of the results against observations. The population distribution originates from the LandScan 2013 Global Population Database (Bright et al. 2013). It represents the population over a 30 arc second grid resolution and integrates the diurnal movements and collective travelling behaviour of the world population, i.e. the so-called “ambient population”, averaged over 24 hours (Bright et al., 2013; Dobson et al., 2000).

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The distribution of the historical tracks of the cyclones is based on the Global Cyclone Hazard and Frequency Distribution that is a compilation of more than 1 600 storm tracks over the period of January 1980 to December 2000; the wind speeds around the tracks have been modelled using the Holland’s model (1997). The value of each cell corresponds to a decile ranking. A higher ranking implies a greater frequency of the hazard relative to the other cells (Center for Hazards and Risk Research - CHRR - Columbia University, Center for International Earth Science Information Network - CIESIN - Columbia University, International Bank for Reconstruction and Development - The World Bank, 2005; Dilley et al., 2005)

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There are local differences in the exact position of the coastlines between the different datasets, e.g. tidal wetlands and land areas can locally appear on the seaward side of the storm surge coastline segments. However, the goal of the analysis is to account for all the tidal wetlands and land areas that can influence the landward propagation of the storm surge. Therefore, the coastline segments of the storm surge datasets were not directly used as the source of the flooding. Alternatively a ‘flood source line’ was defined via the creation of a buffer of 15 km around the original storm surge coastline segments. Only the offshore limit of this buffer was kept and defined as the ‘flood source line’. As such, the flood source line corresponds to a simplified coastline 15 km offshore of the original storm surge datasets coastline segments, to assure that all tidal wetlands and land areas that influence the propagation of the storm surge are located on the landward side of this flood source line. The storm surge heights of the different segments stored in the GTSR dataset are transferred to the flood source line segments of various lengths (average length of 59.04 ± 87.03 km) with a shortest Euclidean distance algorithm (for further information see Van Coppenolle et al.(2018)). The model has a resolution of 30 arc second in accordance with the original resolution of the bathymetry (GEBCO), topography (SRTM) and population (LandScan) datasets. As such, the other datasets, i.e. the countries boundaries, the salt marshes and the mangrove forest areas, were transformed to raster datasets of 30 arc second resolution. This manipulation generated the loss of the smallest salt marshes and mangrove forest areas (< 1 km²), however, due to the global character of the model, those losses are unavoidable. Hence our model accounts only for storm surge mitigation by wetland patches larger than 1 km². We argue that this is acceptable, as storm surge attenuation rates are up to 25 cm/km in salt marshes and 50 cm/km in mangroves (e.g. Zhang et al. 2012; Mcivor et al. 2012; Krauss et al. 2009; Wamsley et al. 2010; Lovelace 1994), hence less than 1 km wide wetlands provide a relatively low degree of storm surge height reduction and are not considered here. The global datasets used present some limitations in regards to local data accuracy and local data artefacts. As such, our analysis is subject to limitations that are similar to other published 4

Journal Pre-proof studies on global assessments of coastal flood risks (e.g., Lichter et al., 2011; Dasgupta et al., 2011; Neumann et al., 2015). Such limitations include the vertical accuracy of the land elevation in the global SRTM dataset, which is limited to 1 m and which implies that the calculation of the spatial extent of the floodplain for a 1-in-100 year storm surge (see calculation procedure in next paragraph) is approximate. Further, the SRTM elevation dataset may include artefacts such the canopy of vegetation, which may result in a local overestimation of the land elevation by one to several meters (Rodriguez et al., 2006; Sun et al., 2003). Other limitations include the moderate resolution of the tidal wetlands datasets that can locally result in over- or underestimations of the surface area of the tidal wetlands (Giri et al., 2011; McOwen et al., 2017). The variable lengths of the DIVA coastline segments involve that some very local characteristics of the coastal plain or some possible local increase or decrease in storm surge height due to the geomorphology of the coast may not be accounted for in the model.

Source

General Bathymetric Chart of the Ocean (GEBCO) & Shuttle Radar Topography Mission (SRTM) Global Distribution of Mangroves Global Distribution of Salt Marshes ESRI, January 2015

Storm Surge Heights Population Distribution

Global Tide and Surge Reanalysis LandScan 2013 Global Population Database Global Cyclone Distribution Hazards and Frequency Distribution

Reference

(British Oceanographic Data Centre, 2017); NASA SRTM,

NASA JPL, 2013

Giri et al., 2011 McOwen et al. 2017 ESRI, De Lorme Company, Inc., 2015 Muis et al., 2016b Bright et al., 2013

Publishing

(CHRR, CIESIN, Columbia University, International Bank for Reconstruction and Development - The World Bank, 2005)

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2.2 Model

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Mangroves Forests Salt Marshes Country Boundaries

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Topography & Bathymetry

Cyclone Tracks

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Dataset

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Table 1 Summary of the datasets used in the study

The model corresponds to the GIS procedure described in Van Coppenolle et al. (2018), but applied on a worldwide scale, while in Van Coppenolle et al. (2018) it was tested for 11 large deltas around the world. The model was developed in ArcGIS (10.3.1) and Python (2.7). The model is similar to previously published procedures that assess the coastal areas and number of people vulnerable to storm surge flooding on regional to global scales (Arkema et al., 2013; Dasgupta et al., 2011). The model simulates how a storm surge flood wave would be routed from the above-described flood source to the coastal plain, i.e. the land area below 10 m of elevation (corresponding to the Low Elevation Coastal Zone and to the maximal storm surge height in the GTSR dataset). The model resolution did not allow accounting for flood protecting structures like dikes or dams. Hence, the presented results correspond to an over-estimation of the coastal areas and populations that will be flooded during a storm surge event in the current situation, and consequently also an over-estimation of the areas and populations that could benefit from storm surge mitigation by tidal wetlands. However, the results can relate to a situation where the existing flood protecting structures would fail. 5

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Four land covers are considered, i.e. (1) open water and channel areas, (2) salt marshes, (3) mangrove forests and (4) remaining land area. For each of them, a storm surge attenuation rate, i.e. reduction of the surge height with distance the surge has travelled over these land cover types (in cm/km), was defined based on literature (See Supplementary Information Table SI 1). For tidal wetlands, a distinction is made between salt marshes and mangroves as mangrove forests are expected to exert more friction on the water column than salt marshes due to their higher vegetation canopy. Open water and channel areas are attributed a very low attenuation rate of 0.1 cm/km, while the remaining land area is assumed to have an attenuation rate of 6 cm/km, which is lower than for tidal wetlands (Table 2) (Van Coppenolle et al. 2018). The storm surge propagation pathways were defined by the cost distance algorithms that account for both the distance travelled and the friction generated by the land covers. As such, every pixel of the coastal plain is associated with a given travelling cost that is subsequently used in the cost distance algorithm, and corresponds to the attenuation rate (Table 2) the storm surge will undergo by its propagation from the flood source towards the pixel.

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The values of the land use dependent storm surge attenuation rates (Table 2) are kept constant in the present global application of the model. However, storm surge attenuation rates may vary depending on site-specific properties (e.g. of the tidal wetlands, coastal land use types, coastal geometry, off-shore bathymetry, etc.) and on properties of the specific storm surge (e.g. magnitude, duration, storm track, etc.) (e.g. Leonardi et al., 2018; Marsooli et al., 2016; Smolders et al., 2015). In Van Coppenolle et al. (2018) we assessed the sensitivity of the model output to different input values of the attenuation rates for the case of 11 deltas, and found that the model output is relatively insensitive to the different values.

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The following procedure is used to identify the coastal areas and populations that would be flooded by a storm surge that would first travel through tidal wetlands before it would reach the areas and populations at risk. First, the model is run with high attenuation rates of 10 cm/km within existing mangroves and 8 cm/km within existing salt marshes (i.e., called here scenario 1). Secondly, the model is rerun with attenuation rates within both wetland types that are replaced by a lower attenuation rate of 6 cm/km (the value we considered for “remaining land area”) (i.e., called here scenario 2). The only reason why we simulate these two scenarios, is to identify which pixels are flooded by a storm surge that passes through the existing tidal wetlands, i.e. this is identified as the pixels with a reduced flooding depth in scenario 1 as compared to scenario 2. Hence the two scenarios are not meant to quantify the impact of tidal wetland reclamation on storm surges. Similarly, it is also not our objective to quantify the impact of drowning of wetlands. The probabilities for tidal wetland reclamation or drowning depend on multiple, local factors, which are difficult to assess in a global scale analysis, and hence this is beyond the objective of the present study.

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Journal Pre-proof Table 2 Attenuation rates attributed to the land covers considered in this study, in accordance with the model approach presented in Van Coppenolle et al. (2018).

Attenuation rate (cm/km)

Substrate Tidal wetlands Mangrove

10.0

Salt Marsh

8.0 6.0

Open water and Channels

0.1

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Remaining land area

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For every pixel located on the coastal floodplains, the length of the flood pathway crossing through tidal wetlands was defined as an indication of the magnitude of the storm surge mitigation by the tidal wetlands, as propagation through longer distances of tidal wetlands are expected to generate a larger reduction of the storm surge height. The distance travelled through tidal wetlands was calculated by dividing the difference in storm surge height reduction between the two scenarios by the difference of attenuation rates between the tidal wetland type, either mangrove or salt marsh, and the remaining land (i.e. 10-6 cm/km, or 4 cm/km for mangrove forests and 2 cm/km for salt marshes). In the situation were the two types of tidal wetlands were present, the value used for the division corresponds to the averaged value, i.e. 3 cm/km.

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The model does not simulate the full complexity of atmospheric and hydrodynamic processes involved in flood propagation and therefore is not able to calculate accurate flood depths and absolute values of reduction in flood depth behind tidal wetlands during specific storm surge events. Instead it calculates the surface area and population numbers flooded via flood pathways crossing through tidal wetlands, and it calculates the distance or length of the flood pathway crossing through tidal wetlands, which are variables that are not dependent on complex atmospheric and hydrodynamic processes during specific storm surge events. As such, it has the major advantage to be globally applicable to compare coastlines and coastal plains around the world. For representation purposes, the model output will be presented as aggregated values for the coastline segments defined by the Digital Chart of the World. A section of the coastal plain is associated to each coastline segment via Euclidean distance. For each segment the model output exists of (1) a value for the total surface area within the associated coastal plain that is flooded via pathways crossing through tidal wetlands – further called “area benefiting from storm surge mitigation”; (2) a value for the total population number within the associated coastal plain that is flooded via pathways crossing through tidal wetlands – further called “population benefiting from storm surge mitigation”; (3) mean distance travelled by a storm surge through tidal wetlands.

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Journal Pre-proof 3 Results 3.1 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 tidal wetlands (see Supplementary Information Figures SI 1 and 2).

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The locations having the largest coastal plain area benefiting from storm surge mitigation by tidal wetlands (> 1 000 km² of the coastal plain associated to one segment, see Method section, segments are highlighted in red in Figure 1), are mainly found in or close to deltas, estuaries and lagoons. Those hotspots include the Northern part of the Yangtze delta, in front of the city of Yancheng (China),with 4 693 km² benefiting from storm surge mitigation associated to one segment of the delta; the Yukon-Kuskowim delta (Alaska, USA) with two segments having respectively 2 969 and 2 159 km² benefiting from storm surge mitigation; the Wash Bay (UK) with one segment having 2 287 km² benefiting from storm surge mitigation; and the Northern part of the Elbe estuary (Germany), with one segment having 2 018 km² of coastal plain benefiting from storm surge mitigation by tidal wetlands.

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Not all coastline segments have the same length, and therefore, in order to standardize the results, we also plotted what we call the standardized surface area benefiting from storm surge mitigation, i.e. the absolute surface area divided by the length of the associated coastline segment (km²/km) (Figure 2). The results show that hotspots for storm surge mitigation by tidal wetlands per unit of shoreline length also correspond to bays, lagoons, deltas and estuaries, yet, the locations show some divergences with the absolute surface area benefiting from mitigation. The hotspots are mainly located along the Northern European coasts ( Belgium, Netherlands, Germany and United Kingdom) and along the East Asian coasts.

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Figure 1 Absolute surface area benefiting from a storm surge pathway crossing through tidal wetlands represented on the associated coastal segment (km²), with circles highlighting the segments for which the coastal plain area benefiting from storm surge mitigation is greater than 1 000 km².

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Figure 2 Standardized area, i.e. surface area per unit of shoreline length (km²/km), benefiting from a storm surge pathway crossing through tidal wetlands represented on the associated coastal segment, with circles highlighting the segments for which the surface area benefiting from storm surge mitigation is greater than 15 km² per 1 km of shoreline.

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Journal Pre-proof 3.2 Magnitude of Storm Surge Mitigation

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Not only the surface area benefiting from a storm surge pathway crossing through tidal wetlands, but particularly the distance travelled by the storm surge through the tidal wetlands is a very relevant parameter determining the magnitude to which wetlands can contribute to nature-based mitigation of storm surge flood risks. Therefore the distance travelled by the storm surge through tidal wetlands was defined for every pixel of the coastal plain and averaged over the areas associated to each segment (Figure 3). The locations having the longest distance (> 5 km, highlighted in red in Figure 3) travelled by the storm surge through tidal wetlands are considered as having the highest degree of storm surge mitigation. They are mainly found in areas where large tidal wetlands exist, again in large deltas, such as in the Guayas delta in Ecuador, where parts of the coastal plain benefit of storm surge mitigation by more than 12 km travelled through the mangrove forests. The coastal plains in the Western Mississippi delta (USA), the Saint Simonds Sounds (USA) and The Everglades (USA), in the Ganges-Brahmaputra delta (India and Bangladesh) or the Tidung estuary in the North Kalimantan region of Borneo (Indonesia) also benefit from more than 5 km travelled by a storm surge through tidal wetlands.

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The comparison of the areas with a long distance of tidal wetlands along the storm surge flood pathway (> 5 km) with the areas that are exposed to cyclone conditions is presented in Figure 3. The value of the global cyclone hazard distribution represents the frequency of the hazard relative to the other areas. Seven hotspots having a coastal plain area benefiting from storm surge mitigation by a long distance (> 5 km) of tidal wetlands are also areas where the likelihood of being exposed to cyclone hazards is greater than elsewhere (indicated in red colours in Figure 3). Those areas, whilst exposed to higher frequency of cyclones, could benefit from higher storm surge mitigation by the tidal wetlands.

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When up-scaled to the country level, the results show that on the 114 countries having tidal wetlands in their coastal plain, 93 benefit from flood risks mitigation by the tidal wetlands (See Supplementary Information, Figure SI 1). The 21 countries that have tidal wetlands but no coastal area buffered by tidal wetlands are mainly countries where the coastal plain is rapidly gaining in altitude, as in Equatorial Guinea, where the 142 km² of mangrove forests are bordered by land areas with an altitude rapidly reaching 5 m, while the 1 in 100 years storm surge is less than 2 meters high.

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Figure 3 Mean distance (m) travelled through tidal wetlands by a 1-in-100 years storm surge during its landward propagation. The circles highlight the segments for which the mean distance travelled through tidal wetlands by a storm surge is longer than 5 km, while red colours indicate hotspots were the long distance of wetlands coincides with high exposure to cyclones.

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Journal Pre-proof 3.3 Coastal Population Benefiting from Storm Surge Mitigation

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Globally around 38.3 million people are exposed to coastal flood risks from a 1-in-100 year storm surge in the case of scenario 2 (no protecting structures and no tidal wetlands). When considering the current tidal wetlands (scenario 1), around 13.5 million people (i.e. 34.5 % of the previous number) benefit from nature-based storm surge mitigation (See Supplementary Information Figures SI 3, 4 and 6). Hotspot areas (i.e. coastline segments with > 10 000 people benefiting from storm surge risk mitigation) are predominantly located in large deltaic and coastal lowland areas in Asia and Europe, such as in the Mekong delta (Vietnam), the Northern Yangtze (China) and the Rhine-Meuse-Scheldt delta (Belgium and Netherlands)(Figure 4). At a country level, the highest number of people benefiting from storm surge mitigation by tidal wetlands is found in China with 7.1 million people, followed by Vietnam (1.6 million people), The Netherlands (1.3 million people), India (0.8 million people)and Germany (0.6 million people) (See Supplementary Information Figure SI 3). Those five countries together make up for 80 % of the global population benefiting from nature-based flood risks mitigation.

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As for the coastal plain area, the population benefiting from storm surge mitigation was also standardized by calculation per unit of shoreline segment length (Figure 5). The hotspots where the population benefiting from storm surge mitigation per 1 km of shoreline is the largest partly correspond to the hotspots of the absolute number of people benefiting from storm surge mitigation associated to one coastal segment, i.e. the Rhine-Meuse-Scheldt delta (The Netherlands and Belgium) in Europe, and most of the deltas and bays highlighted in Asia, except from the Northern Yangtze delta.

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Figure 4 Absolute number of people benefiting from a storm surge pathway crossing tidal wetlands represented on the associated coastal segment, with circles highlighting the segments for which the population benefiting from storm surge mitigation is higher than 100 000 people.

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Figure 5 Standardized population, i.e. people per unit of shoreline length (number of people/km), benefiting from a storm surge pathway cro ssing tidal wetlands, with circles highlighting the segments for which the population benefiting from storm surge mitigation is greater than 10 000 people per 1 km of shoreline.

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Journal Pre-proof 4 Discussion and Conclusion

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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 et al., 2015; Sandi et al., 2018). As such, nature-based risk mitigation can reduce the threats to flood-exposed coastal areas and populations (Duarte et al., 2013; Sutton-Grier et al., 2015; Temmerman et al., 2013). Current assessments on the role of tidal wetlands for coastal flood risk mitigation are based on in situ observations (Krauss et al., 2009; McGee et al., 2006; Stark et al., 2015) and/or on modelling studies (Arkema et al., 2013; Stark et al., 2016; Zhang et al., 2012) at local to regional scales. Such site-specific studies have substantially advanced our understanding of the mechanisms determining the rate of storm surge mitigation by salt marshes and mangroves (i.e. how much the peak water level is reduced per distance that the storm surge has travelled through salt marshes or mangroves). However, we currently lack a global scale assessment of the possibilities for nature-based mitigation of coastal flood risks. Therefore, an assessment is needed of the location of global hotspots of large flood-exposed coastal areas and populations that can receive nature-based risk mitigation from existing salt marsh and mangrove ecosystems. Our present analysis is based on the model developed in Van Coppenolle et al. (2018), which showed for 11 deltas around the world that a simple GIS model at a regional scale (delta scale) can provide relevant estimations on storm surge mitigation capacity of tidal wetlands. Here, this model approach is up scaled and applied on a global scale. Our study is to our knowledge the first worldwide assessment identifying the coastal area and number of people that can benefit from nature-based coastal flood risk mitigation.

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The presented model allows a global-scale assessment of the locations where tidal wetlands are expected to play a role in the mitigation of storm surge flood risks. Nevertheless, as global-scale assessments of coastal flood risks intrinsically imply limitations in local accuracy (e.g., Lichter et al., 2011; Small and Nicholls, 2003), such limitations also apply to our global modelling approach. Due to its global scale, the model resolution cannot account for tidal wetlands smaller than 1 km². Although this may generate in some areas an under-estimation of the potential storm surge mitigation, we expect that this effect is rather limited, because the model accounts for storm surge attenuation rates of 10 cm/km and 8 cm/km for mangroves and salt marshes, respectively, and hence tidal wetlands smaller than 1 km2 are expected to have only a limited impact on the reduction of storm surge height (less than 10 cm). Further, the values used for the storm surge attenuation rate for the two wetland types, for remaining land surfaces, and for open water surfaces (see Table 2), are average approximations, and in reality may vary depending on multiple factors including wetland and other land use properties, coastal geometry, off-shore bathymetry, and last but not least, specific storm properties. In order to obtain a more precise evaluation of the attenuation of a specific storm surge due to the presence of a specific tidal wetland area, a hydrodynamic modelling approach would be needed based on high resolution input data regarding the storm characteristics (duration, intensity, track, wind velocity field...), the wetlands’ vegetation and geomorphology (vegetation type, density and continuity, soil surface topography...) as well as the geomorphology of the surrounding coastal area (off-shore bathymetry, shoreline shape, flood protection structures...) (Leonardi et al., 2018). However, the high computational demand of such a hydrodynamic modelling approach 16

Journal Pre-proof does not enable a worldwide assessment, while the more simple approach of our model and the use of globally available datasets enable its worldwide applicability with a much lower computational demand than hydrodynamic models (Van Coppenolle et al., 2018). As a consequence of our model limitations, we do not report on local-scale spatial variations in flooding depths and reduction of flooding depths generated by the wetlands, because our model can not deliver this level of detail. Instead our model output is aggregated at the scale of large coastal segments (average length of 59.04 km with a standard deviation of 87.03 km), and for each segment we report on the land surface area and population number that can be flooded via a flood pathway travelling through wetlands, and we report on the average distance travelled through the wetlands, as a proxy for the magnitude of flood risk mitigation delivered by the wetlands.

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The results show that about one third of the global flood-exposed coastal plains and of the global flood-exposed population experience nature-based storm surge mitigation by existing mangrove forests or salt marshes. The coastal plains with the largest absolute surfaces benefiting from storm surge mitigation are mainly found in deltas (e.g. Northern Yangtze Delta, Pearl River and Mekong delta) and estuaries (e.g. Elbe estuary) (Figure 1), where large lowlying lands are favourable to both the establishment of tidal wetlands and to storm surge mitigation over large areas (Leonardi et al., 2018). The areas with the highest absolute number of people benefiting from nature-based coastal flood risks mitigation are located in densely populated deltas and estuaries, mainly in North-western-Europe and East-Asia (Figure 4). Of the 35 % of the flood-exposed population benefiting from nature-based storm surge mitigation, the majority (i.e. 80 %) is living within five countries, being China, Vietnam, The Netherlands, India and Germany. The results are similar for the standardized surface area and population (i.e. calculated per unit length of the coastline) benefiting from storm surge mitigation by the tidal wetlands. Whilst for both, the number of hotspots is smaller in the case of the standardized value, suggesting that some areas with large absolute areas or population numbers benefiting from storm surge mitigation are associated with long shoreline segments.

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Furthermore, the results illustrate that the areas more likely exposed to cyclones also include hotspots for which the nature-based storm surge mitigation can be the highest in terms of mean distance of tidal wetlands crossed by the storm surge (Figure 3) (Leonardi et al., 2018; Loder et al., 2009; McIvor et al., 2012b; Stark et al., 2016). This is the case in the Mississippi delta, along the Saint Simonds Sound and the Everglades, in the Ganges-Brahmaputra delta or along the Dee River. Whilst other areas exposed to severe storms benefit from a limited protection by tidal wetlands as for example in the Zambesi delta and the coastal areas in Mozambique. Our relatively simple but global scale model highlights that numerous hotspot areas around the world, e.g. the Mississippi delta, Western European estuaries, Eastern Asian deltas and estuaries, would benefit from the conservation of salt marshes or mangrove forests as part of strategies to mitigate and adapt to increasing coastal flood risks associated with climate warming, sea level rise and increasing storminess. For some of those hotspots nature-based coastal protection strategies are actively implemented, mostly in addition to classical engineered flood defence structures like dikes and levees, such as in the Mississippi delta (Boesch et al., 2006), the Chesapeake Bay (Chesapeake Bay Program, 2000), the Rhine-MeuseScheldt delta (Meire et al., 2014)or the Humber estuary (Elliot et al., 2016) (Boesch et al., 2006; Chesapeake Bay Program, 2000; Elliott et al., 2016; Garbutt et al., 2017; SigmaPlan, 2017). In other hotspots, policy and decision-makers are only beginning to account for the coastal 17

Journal Pre-proof protection value of tidal wetlands and nature-based strategies are starting to be implemented along with classical hard engineering, as in the Ganges-Brahmaputra delta, the Yangtze delta or the Mekong delta (Käkönen, 2008; Seavitt, 2013; Ysebaert et al., 2017). Nevertheless, the application of nature-based strategies, through conservation of coastal ecosystems, can be compromised by high demands for coastal wetland reclamation and conversion into human land use, such as in rapidly developing coastal zones as in China for instance (Ma et al., 2014; Meng et al., 2017; Wang et al., 2014).

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Creating sustainable and cost-effective coastal protection strategies to adapt to the increasing coastal flood risks would require policy makers to account for the presence of tidal wetlands, and coastal ecosystems in general, in the design and development of coastal protection structures and renounce to the practice of large-scale tidal wetland reclamation (McLeod et al., 2011; Pendleton et al., 2012). Indeed, salt marshes and mangroves have been converted on large scales to human land use such as for agriculture, aquaculture, industry and urbanization, and this is still actively going on in several places around the world, especially in fast developing regions such as in Southeast Asia (Jia et al., 2015; Jiang et al., 2015; Tian et al., 2016). Apart from the fact that such large-scale tidal wetland reclamation implies the loss of biodiversity and valuable ecosystem services, we argue that wetland reclamation should be planned carefully and avoided as much as possible in order to maximize the flood risk mitigation function of remaining tidal wetlands. Our global-scale assessment aims to increase the awareness on the value of nature-based coastal protection strategies and stimulate further site-specific studies as a first step towards a more worldwide implementation of nature-based mitigation policies as a strategy against increasing coastal flood risks.

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The author would like to thank the University of Antwerp who funded this study.

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Journal Pre-proof Declaration of interests 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.

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30 % of the world’s coastal plains benefit from nature-based storm surge mitigation 40 % of the world’s flood-exposed population receives nature-based storm surge mitigation 5 countries combine 80 % of the flood-exposed population receiving storm surge mitigation Hotspots for nature-based mitigation often coincide with high storm surges exposure

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