Tens of millions of people world-wide will be affected in the next few decades by coastal flooding due to sea-level rise and associated increases in wave action and surges [1,2]. In addition, coastal habitats are facing increasing risks world-wide as a result of human activity. These habitats provide a number of ecosystem services, or benefits, including coastal protection, fish production, recreation and other economic and cultural values [3]. In many instances the degradation of coastal habitats can result in a decrease in coastal protection and increase risk of coastal flooding [4,5]. Observations of extreme events like the Indian Ocean tsunami, Hurricanes Sandy and Katrina in the Atlantic and Typhoon Haiyan in the Pacific suggest that coastal habitats can help protect coastlines during such events [6,7,8,9]. There is hence great interest in identifying effective, and cost effective solutions that help conserve habitats and protect coastlines [10,11].

While there is important interest in the conservation of habitats for the natural defence they provide, there is a particularly strong interest in investments in restoring coastal habitats for nature-based defences. In this paper, natural defences refer to existing coastal habitats within which wave reduction has been measured. Nature-based defences refer to restoration projects that specifically include coastal protection as an objective (definitions adapted from the U.S. Army Corps of Engineers report on Natural and Nature-based Features [12]). A number of restoration projects world-wide are being implemented specifically for coastal protection [13]. These are increasingly driven by global conventions and their funding mechanisms, including the United Nations Framework Convention on Climate Change (UNFCCC) and the green Adaptation Fund (AF), as well as the United Nations International Strategy for Disaster Risk Reduction (UNISDR) and lending from the World Bank. They are also being driven by interest from national and multi-national agencies [10,12,14,15]. But critical questions remain about when these projects can be used effectively for coastal protection, for example about the costs of a habitat restoration project relative to other, more conventional alternatives.

The contribution of habitats to coastal protection is increasingly addressed in science, policy and practice [16,17,18]. There is also a growing interest in developing guidance about habitat restoration for nature-based defences but this has largely been identified based more on case studies than syntheses [12,18,19]. Insights on the success or failure of projects, and comparisons with hard structures that perform similarly, under the same conditions, are difficult to obtain [20]. While there are a number of studies analysing the physical aspects of coastal protection by coastal habitats, there is very little information to date that combines this knowledge with information on restoration projects, to assess the performance and viability of these projects. This can hinder decision-making with regard to future investments in restoration projects for coastal protection.

Widespread consideration and use of habitats for coastal protection still face significant challenges including: a) uncertainty around the effectiveness of habitats under different hydrodynamic and ecological conditions; b) a lack of synthetic information about the costs and effectiveness of projects that restore or manage habitats for coastal protection; and c) a paucity of studies that integrate and synthesise engineering and ecological knowledge to provide site-specific comparisons of the costs and effectiveness of nature-based defences versus hard structures. This paper integrates analyses of (a) natural defences with information from (b) nature-based defence projects, to address these gaps and improve understanding of how and where coastal habitats may be viable for coastal protection.

This is done by: a) analysing field measurements of wave reduction within coastal habitats and the parameters that may drive variability in this function; and b) mapping and combining information from these field measurements with information on nearby nature-based defence projects, to compare their costs with hard structures that will provide the same level of wave reduction. First we extend previous syntheses of wave reduction field measurements in coastal habitats [21,22,23] to include more measurements and improve understanding of the variability across habitats in reducing wave heights, focusing in particular on engineering parameters that will be critical in assessing and designing restoration projects. In their re-analyses of field data, Pinsky et al., [23] show high variability in wave reduction between habitats and investigate the influence of biophysical parameters on this variability–namely, the local flow conditions (Reynold’s number) and the resistance to flow provided by the habitat (the bulk drag coefficient). In our study, data from field measurements are used to directly investigate the influence of biophysical parameters on this variability (Fig 1). The field measurements are then mapped and, based on their location and habitat type, are combined with details of nearby nature-based defence projects. These nature-based defence projects are first analysed for their costs and benefits for coastal protection. Based on information from nearby field measurements, wave reduction extents are estimated for some of these projects and their costs compared to the costs of submerged breakwaters that will provide the same wave height reduction under the same conditions. These results provide insights on where and how coastal habitats and nature-based defence projects may be viable and cost effective, and also, on the key parameters that should be assessed when designing these projects.

This paper analyses sixty-nine field measurements to show that habitats have a significant influence on wave reduction, demonstrates the influence of specific engineering parameters on wave reduction effectiveness, reviews the costs and benefits of fifty-two nature-based defence projects (Fig 2), and demonstrates the cost-effectiveness of some of these projects relative to structures that provide the same wave reduction.

Table 1 summarises the costs, coastal protection benefits, objectives and exposure of fifty-two nature-based defence projects in coral reef, oyster reef, mangrove, and salt-marsh habitats. A sizeable proportion of salt-marsh and mangrove projects state that such habitats provide improved protection from storm events (41% in salt-marshes and 50% in mangroves; see Table 1). Other coastal protection benefits include savings in damages during storm events, reductions in erosion and reductions in the costs of engineering for coastal protection, reflected, in a few cases, by positive benefit-cost ratios (e.g. also see [20]). Restoration objectives vary across habitat types, with most mangrove and marsh habitats reporting coastal protection as a primary objective. Among the coral reefs a majority of projects are targeted primarily at habitat restoration and only a small number for coastal protection, even though many of these reefs are situated in highly exposed regions. Unit restoration costs are lowest for marshes and mangroves, and coral and oyster reefs show higher, and more variable, costs (Table 1).

Analyses of the costs and wave reduction of thirteen nature-based defence projects (see Methods, S4 Table) in mangroves and salt-marshes show that these projects can be several times cheaper than alternative submerged breakwaters (Fig 3) for the same level of protection. Together with their ability to keep pace with sea-level rise [27] this suggests that nature-based defences can become increasingly viable on sheltered coastlines. Depending on the water depth, mangrove projects in Vietnam can be three to five times cheaper than a breakwater, and salt-marsh projects across Europe and the USA vary from being just as expensive, to around three times cheaper (Table 1, Fig 3). Fig 3 plots the total restoration costs of mangrove and marsh projects along with breakwater construction costs at these sites for a range of depths and wave height reduction values. The cross-shore width of habitat and the degree of wave height reduction are also indicated for each project. Water depth is a crucial factor, with both habitats showing an increase in cost effectiveness at higher depths, due to the relatively steep increase in breakwater construction costs. Habitat width is found not to be a sufficient indicator of cost effectiveness. Also, these nature-based defences are limited to wave heights less than half a metre and are not always cost effective.

Based on existing literature it was assumed that breakwater construction costs are uniform across the sites in Europe/USA and ten times lower for the sites in Vietnam [28]. Such regional differences are also reflected in the reported habitat restoration costs in these countries. While accurate estimates of construction costs require detailed information on structure profile, material and labour costs, etc., water depth is often a critical driver of construction costs [29] and therefore the main influence on cost effectiveness. Only total project costs and habitat extents were used, given the high variability in the relationship between restoration project costs and sizes (see Methods). The study does not explicitly account for increases in restoration costs due to adverse ecological or geomorphic site conditions which can significantly increase these values [30].

In the cost comparisons we look for structures that are equivalent to marshes and mangroves in function–i.e. wave reduction, as well as location–i.e. within the near-shore zone, and choose submerged breakwaters as the best alternative. Submerged breakwaters provide wave reduction to varying degrees, similar to coastal habitats and can be located within the near-shore zone. Though seawalls are a common substitute for mangrove and marsh habitats [20,31], these are often located at the shoreline and, in addition to blocking waves, also protect against flooding from high water levels. While there are some indications that mangroves and marshes can offer protection from high water levels (Table 1), we do not find enough evidence on this for a comparison of effectiveness, and as such, focus on their wave reduction function. While coral reefs are also very similar to breakwaters in structure and wave reduction function, we do not find enough information on reef restoration projects for a direct cost comparison. It is important to note that coastal habitats are usually one of several structural, nature-based and non-structural measures for coastal protection [32].

This study focuses on coastal protection by wave reduction, though habitats often provide other ecosystem services such as biodiversity, fish production, recreation and many other social, economic and cultural values [33]. The addition of these benefits, over and above their coastal protection value should make these natural approaches more appealing to coastal managers and decision-makers [34]. Also, the loss of existing coastal habitats and their replacement by man-made structures can result in loss of these ecosystem services [35]. In any case, policy decisions on where and how to conserve or restore habitats, rather than focusing on a single service, should consider multiple objectives for best allocation of available resources [36,37].

The data for the wave reduction analyses are all obtained from field observations of wave heights and hydrological variables. The datasets used in this study vary in terms of the type of data available for analysis, and these are described in S2 and S3 Tables. The wave reduction data are all field observations of wave heights through habitats (S2 Table). Almost all the studies provide information on habitat width, and most measurements in reefs also provide information on reef depth. Only a few studies–all in marshes, provide information on vegetation heights. The restoration project data are a mix of primary–i.e. observed and secondary–i.e. estimated costs and benefits (S3 Table). The coastal setting and exposure data for each project location are derived from other sources (see S1 Methods). Cost reporting by projects is highly variable (see Methods). All costs are reported on a per-m² basis, and use total project costs for the cost comparison analyses. Ideally, in future, cost reporting in projects should be consistent and report both unit and total restoration costs. More such comparisons with hard alternatives, along with detailed and consistent data on the extents, costs and coastal protection benefits of existing restoration projects, are needed to inform the design and implementation of future nature-based defences.

We are interested in general conclusions about the parameters that influence wave reduction across multiple habitats and physical conditions. Therefore, the study uses average values of vegetation height and water depth for the parameter analyses. It is worth emphasising that the measurements of waves in the analysed studies are all under ‘normal’ conditions of low waves. Mean wave height values are used for the meta-analyses. Variations in wave height measurements at each site are accounted for within the analyses (see S1 Methods). However, when analysing extreme value measurements, it will be necessary to include analyses of variances to assess the effect on wave reduction. Also, site-specific variations in all these parameters will need to be considered when designing a nature-based defence project. For instance, the slope of a coral reef can influence variations in wave reduction over that reef [38] and hence, its effectiveness as a nature-based defence. Wave height is the response variable for the meta-analyses, following a number of the reviewed studies that report reductions in terms of wave heights. Field measurements and analyses of wave energy, rather than wave height, may provide a better picture of the processes that drive wave reduction at each site [21].

Field evidence of the protection offered by habitats is generally difficult to obtain. However, clear differentiation of measured parameters–i.e. physical reduction of wave heights or storm surges, versus economic savings in damage costs during extreme events–is essential to understand the extents to which, and conditions under which, different habitats offer protection. For instance, the review of nature-based defence projects suggests that mangroves are effective protection measures against flooding from storms (Table 1, S3 Table). The meta-analyses of wave heights however show that wave height measurements in mangroves have so far been limited to lower waves than in salt-marshes (Table 1, S1 Table).

Future studies of effectiveness and cost-effectiveness would also be strengthened by paired measurements of wave height reduction with and without habitat [39] accompanied by information on habitat parameters such as height, density and roughness [40,41,42]. A small but growing number of field observations, laboratory experiments and numerical models suggest that reefs and wetlands can act as buffers against extreme waves and water levels [8,43,44,45,46], though the observed data for extreme events is scant. It will also be critical to get similar field measurements of wave and water level reductions by habitats during extreme events [47]. When evaluating restoration projects for coastal protection, it would be useful to follow monitoring and evaluation procedures set out within established coastal engineering frameworks. These could usefully include demonstrations of projects implemented in different physical settings [20], theoretical design frameworks [48,49], or even, evaluations of nature-based defences within national accounts [37]. Such evaluation typically involves a before-after comparison of the coastal hazard at the site. However, a restoration project can typically have multiple objectives, the evaluation of which will require monitoring of outcomes at multiple impact and reference sites.

This paper is, as far as the authors are aware, the first attempt at synthesising evidence from field measurements and restoration projects to provide an overview of the wave reduction by natural defences, in combination with site-specific comparisons of the costs of nature-based defences versus alternative structures. The paper also synthesises information on the benefits of restoration projects for coastal protection. These analyses and syntheses demonstrate the following: a) coastal habitats–particularly coral reefs and salt-marshes–have significant potential for reducing wave heights and providing protection at the shoreline; b) restoration projects for which data are available–i.e., mangrove and marsh projects–can be cost-effective relative to submerged breakwaters in attenuating low waves and become more cost-effective at higher water depths; c) a number of nature-based defence projects, especially in mangroves and marshes, have been observed to offer protection during storms. Variations in wave reduction and cost effectiveness are dependent on multiple parameters including water depth and vegetation / reef height.

Examples of nature-based defence projects are growing rapidly in number, but much better reporting of effectiveness and cost effectiveness is necessary, for better understanding of their viability. Data from post-project monitoring of the success or failure of restoration projects are not easily available. As with any hard engineering structure, information on nature-based defence project failures–i.e. the reasons why a particular project did not work can also be very valuable when developing guidelines and methodologies for project design. This would include, for instance, before and after observations of whether a restoration project designed for coastal protection has achieved its stated objectives. Ideally, project costs, site conditions and wave reduction extents should be measured at the same location. This will allow better understanding of variations in project costs with variations in water levels, wave conditions and habitat characteristics. This is particularly important for a future where variations in rates of sea-level rise and other environmental factors can result in a spatial variability in wave heights [50,51]. Also, better estimates of maintenance costs and the additional services and benefits (including coastal access, fish production, carbon sequestration) or lack thereof, for both artificial and nature-based defences are required when evaluating the overall costs and benefits of a restoration project that includes coastal protection as an objective. Finally, inclusion of dune and also beach habitats [52] would vastly improve the richness of existing nature-based defence databases.