Terminology in coastal climate change adaptation

Besides engineered coastal protection measures, NbS have emerged as viable alternatives in areas where time and space allow for their implementation. These solutions effectively address sea-level rise challenges, mitigate wave impacts and influence morphodynamic processes such as erosion and sedimentation. The evolution of coastal NbS from a concept (World Bank, Reference MacKinnon, Sobrevila and Hickey2008; IUCN, Reference Galland and Herr2009, Reference Cohen-Shacham, Janzen, Maginnis and Walters2016) to a recognised approach for addressing sea-level rise (see box 4.3. in IPCC, Reference Portner, Roberts, Masson-Delmotte, Zhai, Tignor, Poloczanska, Mintenbeck, Alegria, Nicolai, Okem, Petzold, Rama and Weyer2019) occurred alongside a scientific consolidation phase through ongoing scholarly and expert discourse.

The evolution of implementing natural elements and aspects into coastal protection measures has been shaped by two key frameworks: “Building with Nature” and “Working with Nature.” The “Building with Nature” initiative started as a research programme in 2007, presented at the 18th World Dredging Congress (WODCON) by the World Organization of Dredging Associations (WODA). This programme ultimately led to the formation of the Dutch EcoShape foundation (Raalte et al., Reference Raalte, Dirks, Minns, van Dalfsen, Erftemeijer, Aarninkhof and Otter2007), promoting the “Building with Nature” approach. EcoShape is a consortium comprised of private partners, government agencies and knowledge institutes, all working at the interface of nature, engineering and society (Slobbe et al., Reference Slobbe, Vriend, Aarninkhof, Lulofs, de Vries and Dircke2012; Vriend et al., Reference Vriend, van Koningsveld, Stefan, de Vries and Baptist2015). Meanwhile, “Working with Nature” was introduced in 2008 by the World Association for Waterborne Transport Infrastructure (PIANC) in a position paper (PIANC, 2008).

Considering the origin and semantics of “Building with Nature” and “Working with Nature”, both have strong ties to engineering and the dredging industry and, as such, consider the use of sediment-based protection as an appropriate natural mean within the scope of their frameworks. From this, a set of concepts in the coastal engineering community evolved, dealing with coastal ecosystems as NbS for coastal protection (a systematic etymological overview is given by Jordan and Fröhle, Reference Jordan and Fröhle2022).

In contrast, other frameworks emerged from the Disaster Risk Reduction (DRR) community (Thomalla et al., Reference Thomalla, Downing, Spanger-Siegfried, Han and Rockström2006), drawing on an “ecosystem approach” – a cornerstone of ecosystem-based management (Slocombe, Reference Slocombe1993). The ecosystem-based management framework was established alongside the growing scientific interest in ecosystem services in the 1970s (IUCN, Reference Cohen-Shacham, Janzen, Maginnis and Walters2016). It is “the process of managing and understanding the interaction of the biophysical and socioeconomic environments within a self-similar, self-maintaining regional or larger system,” focusing on the whole ecosystem, rather than smaller management units (Slocombe, Reference Slocombe1998). Almost half a century later, the DRR community developed an ‘ecosystem approach’ to disaster risk reduction, referred to as Ecosystem-based Disaster Risk Reduction (Eco-DRR), as a result of the 2004 Indian Ocean Tsunami and the Hyogo Framework for Action (HFA) 2005–2015 (Gupta and Nair, Reference Gupta and Nair2012; Estrella and Saalismaa, Reference Estrella, Saalismaa, Renaud, Sudmeier-Rieux and Estrella2013). This concept later informed the Sendai Framework for Disaster Risk Reduction (2015–2030). Eco-DRR is “the sustainable management, conservation and restoration of ecosystems to reduce disaster risk, to achieve sustainable and resilient development” (Estrella and Saalismaa, Reference Estrella, Saalismaa, Renaud, Sudmeier-Rieux and Estrella2013). Of particular importance is that Eco-DRR considers humans – and thus human action – as an integral part of ecosystems (in reference to the Millennium Ecosystem Assessment, 2005).

Besides “Building with Nature” and Eco-DRR, another concept is EbA, putting a bigger emphasis on “slow-onset” hazards (Sudmeier-Rieux et al., Reference Sudmeier-Rieux, Nehren, Sandholz and Doswald2019), such as sea-level rise. EbA was coined by the Convention on Biological Diversity (CBD) in 2009 (CBD, 2009), despite already circulating in the negotiations prior to the fourteenth session of the Conference of the Parties (COP 14) of the United Nations Framework Convention on Climate Change (UNFCCC) negotiations in 2008 (Lo, Reference Lo2016). Beyond the ecosystem services focus, EbA adopts a vulnerability perspective rather than emphasising hazards, exposure, and technical solutions. This reflects its grounding in climate change adaptation rather than disaster risk reduction, prioritising transformative resilience over reactive risk management. With an early focus on beneficial ecosystem services and a widespread involvement of local communities, private companies, non-government organisations as well as a diverse set of both regional governments and supra-regional countries (for example, the European Union or the United Nations regional group African Group), EbA became soon relevant for science, practitioners and decision-makers alike (Vignola et al., Reference Vignola, Locatelli, Martinez and Imbach2009). Meanwhile, EbA was recognised as one of six response options to sea-level rise in the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) of the United Nations’ Intergovernmental Panel on Climate Change (IPCC). The others are no response, advance, retreat, accommodation and protection (IPCC, Reference Portner, Roberts, Masson-Delmotte, Zhai, Tignor, Poloczanska, Mintenbeck, Alegria, Nicolai, Okem, Petzold, Rama and Weyer2019). In fact, protection within the IPCC classification is again split up into three parts: (a) hard protection – the equivalent of engineered coastal protection here – (b) sediment-based protection, such as beach and shore nourishments which are also referred to as “soft” measures and (c) EbA to account for hybrid forms between engineered and NbS, although it constitutes a separate class in its own right. This overlapping classification elucidates the importance of establishing a clear and unambiguous taxonomy for climate change adaptation categories.

Scrutinising the differences between “Building with Nature,” Eco-DRR and EbA reveals a clear distinction between these concepts, despite all of them considering the inherent requirements of the coastal ecosystem while aiming to support and harness ecosystem services to mitigate risk from marine hazards and extreme events:

Evolving within different communities, more or less in the form of silos, led to an unnecessary and artificial divide between EbA and Eco-DRR (Doswald and Estrella, Reference Doswald and Estrella2015; Sudmeier-Rieux et al., Reference Sudmeier-Rieux, Nehren, Sandholz and Doswald2019). A considerable overlap exists in both concepts (Thomalla et al., Reference Thomalla, Downing, Spanger-Siegfried, Han and Rockström2006; CBD, 2009; Estrella and Saalismaa, Reference Estrella, Saalismaa, Renaud, Sudmeier-Rieux and Estrella2013; Doswald and Estrella, Reference Doswald and Estrella2015; Sudmeier-Rieux et al., Reference Sudmeier-Rieux, Nehren, Sandholz and Doswald2019), with EbA emerging as the more widely recognised terminology. This may be particularly facilitated through the integration of EbA in the IPCC’s SROCC (IPCC, Reference Portner, Roberts, Masson-Delmotte, Zhai, Tignor, Poloczanska, Mintenbeck, Alegria, Nicolai, Okem, Petzold, Rama and Weyer2019).

As NbS are increasingly being mainstreamed (as exemplified by the global standard for NbS; IUCN, 2020) and integrated into policy and decision-making (see, for instance, the EU Biodiversity Strategy for 2030, EU Strategy on Adaptation to Climate Change and report on NbS in Europe policy; European Commission, 2020, 2021; European Environment Agency et al., Reference Castellari, Zandersen, Davis, Veerkamp, Förster, Marttunen, Mysiak, Vandewalle, Medri and Picatoste2021), a precise understanding of “Building with Nature,” Eco-DRR and EbA is crucial. The difference between these frameworks becomes particularly evident when using them as epitomes for policy-driven and engineering-focused approaches, respectively. The distinction manifests most visibly in their contrasting positions on sediment-based or “soft” measures, such as sand or beach nourishment. While sand nourishments exploit “natural” abiotic drivers (waves and currents) to redistribute sediment (Schipper et al., Reference Schipper, Ludka, Raubenheimer, Luijendijk and Schlacher2020), they simultaneously constitute “disturbances of the environment” (Staudt et al., Reference Staudt, Gijsman, Ganal, Mielck, Johanna Wolbring, Goseberg, Schüttrumpf, Schlurmann and Schimmels2021) where ecological benefits are affected by multiple factors (Speybroeck et al., Reference Speybroeck, Bonte, Courtens, Gheskiere, Grootaert, Maelfait, Mathys, Provoost, Sabbe, Eric, Van Lancker, Vincx and Degraer2006; Staudt et al., Reference Staudt, Gijsman, Ganal, Mielck, Johanna Wolbring, Goseberg, Schüttrumpf, Schlurmann and Schimmels2021). Consequently, the IPCC-endorsed concept distinguishes between (a) soft, sediment-based measures – thereby acknowledging the engineering origin of sand nourishments – and (b) ecosystem-based measures. In contrast, “Building with Nature” embraces sediment-based interventions like sand nourishments as integral components, defining them as dynamic and sustainable engineering solutions, leveraging natural redistribution mechanisms for coastal management and protection (Slobbe et al., Reference Slobbe, Vriend, Aarninkhof, Lulofs, de Vries and Dircke2012).

A differentiated understanding facilitates skillful use and modification of these concepts for the appropriate framing of coastal actions and effective coastal management. Other prominent examples building on such an understanding are “ecosystem-based coastal defence” or “nature-based coastal defence” (see Temmerman et al., Reference Temmerman, Meire, Bouma, Peter, Ysebaert and de Vriend2013; Morris et al., Reference Morris, Bilkovic, Walles and Elisabeth2022; Duvat et al., Reference Duvat, Inès Hatton, Burban, Jacobée, Vendé-Leclerc and Stahl2025) focusing on natural habitats like mangroves, salt marshes, coral reefs and dunes as living infrastructure that provides coastal defence services while delivering additional ecological and economic benefits; “ecosystem engineers” as designation for plants like Spartina and Zostera that autogenically engineer coastal environments by structurally modifying water flow through their physical presence (Bouma et al., Reference Bouma, De Vries, Low, Peralta, Tánczos, van de Koppel and Herman2005); or “ecosystem design” describing restoration approaches, where rather than attempting to restore ecosystems to their historical states, this concept promotes deliberately constructing functioning ecosystems tailored to meet specific human needs and local requirements (Zimmer, Reference Zimmer, Makowski and Finkl2018).

Conversely, a basic or less informed approach to NbS can lead to criticism (Alva, Reference Alva2022) or adverse impacts on site – so-called maladaptation or in a broader sociopolitical context maldevelopment (Schipper, Reference Schipper2020; David et al., Reference David, Hennig, Beate, Roeber, Zahid and Schlurmann2021a). In this context, terms like “Natural-,” “Green-,” or “Green-Blue Infrastructure” (United Nations Office for Disaster Risk Reduction, 2017; Schoonees et al., Reference Schoonees, Mancheño, Scheres, Bouma, Silva, Schlurmann and Schüttrumpf2019; Monteiro et al., Reference Monteiro, Ferreira and Antunes2020) are pertinent but should be carefully considered in coastal settings. These terms encompass a wide range of ecosystem services, including air quality, temperature regulation, waste and wastewater treatment as well as urban benefits like recreation and aesthetics (Veerkamp et al., Reference Veerkamp, Schipper, Hedlund, Lazarova, Nordin and Hanson2021). The concept “Green-Blue Infrastructure” is employed to deliberately create a contrast between “grey” or “hard” infrastructure (United Nations Office for Disaster Risk Reduction, 2017). Despite their prominence in water and urban sciences, architecture and spatial planning (Lamond and Everett, Reference Lamond and Everett2019; Langeveld et al., Reference Langeveld, Cherqui, Tscheikner-Gratl, Muthanna, Juarez, Leitão, Roghani, Kerres, do Céu Almeida, Werey and Rulleau2022), the semantics of green-blue infrastructures in coastal climate change adaptation remains ambiguous when contrasted with the evolution and widespread support of previously outlined ecosystem and nature-based counterparts or their comprehensive derivatives.

Nature-based Solutions in coastal engineering

NbS in coastal engineering exploit natural ecosystems to protect the coastline from wave impacts, sedimentation and erosion and enhance biodiversity. Typical NbS are mangroves (see Figure 2a), coastal dunes (see Figure 2b), salt marshes, reefs (coral, shellfish, oyster) and seagrass meadows or other natural elements of ecosystems, creating habitats for various species while protecting coastal communities (David et al., Reference David, Schulz, Schlurmann, Renaud, Sudmeier-Rieux, Estrella and Nehren2016; Jordan and Fröhle, Reference Jordan and Fröhle2022; Hitzegrad et al., Reference Hitzegrad, Brohmann, Herding, Pfennings, Jonischkies, Scharnbeck, Mainka, Mai, Windt, Kloft, Wehrmann, Lowke and Goseberg2024). The increased biodiversity enhances the variability of flora and fauna – a benefit cascading down to coastal dwellers (IUCN, Reference Cohen-Shacham, Janzen, Maginnis and Walters2016), for example, via improved food provisioning and economic utilisation (Kuwae and Crooks, Reference Kuwae and Crooks2021). These co-benefits are commonly referred to as ecosystem services. The most important ecosystem service for coastal protection is wave attenuation and – where required – sediment accumulation (Narayan et al., Reference Narayan, Beck, Reguero, Losada, Van Wesenbeeck, Pontee, Sanchirico, Ingram, Lange and Burks-Copes2016). In addition to ecosystem services, natural elements have an intrinsic capacity to regulate and maintain themselves and (seasonal) nearshore dynamics (Kench, Reference Kench2012; Spalding et al., Reference Spalding, McIvor, Beck, Koch, Möller, Reed, Rubinoff, Spencer, Tolhurst, Wamsley, van Wesenbeeck, Wolanski and Woodroffe2013; David and Schlurmann, Reference David and Schlurmann2020; David, Reference David2021). Thus, NbS potentially requires lower maintenance, possessing self-repairing capabilities that enable recovery from moderate storm impacts (Morris et al., Reference Morris, Boxshall and Swearer2020; Waryszak et al., Reference Waryszak, Gavoille, Whitt, Kelvin and Macreadie2021).

Figure 2. Schematic representations of Nature-based Solutions (NbS) depicted as a collection of mangroves for wave attenuation (Temmerman et al., Reference Temmerman, Horstman, Krauss, Mullarney, Pelckmans and Schoutens2023) and a coastal dune (Mehrtens et al., Reference Mehrtens, Lojek, Kosmalla, Bölker and Goseberg2023; Dang et al., Reference Dang, Pham, Nguyen, Giang, Pham, Nghiem, Nguyen, Vu, Bui, Pham, Nguyen and Ngo2023). All renders are done with Blender version 4.3.1.

Coastal NbS provide another co-benefit: blue carbon sequestration and storage. Blue carbon sequestration and storage mitigates the main driver of global heating – carbon dioxide, or $ {\mathrm{CO}}_2 $ – because coastal ecosystems like mangroves, seagrass meadows and salt marshes capture atmospheric $ {\mathrm{CO}}_2 $ through photosynthesis and store this carbon in plant biomass and sediments. Vegetated coastal habitats demonstrate substantial sequestration capacity despite covering only 0.2% of the ocean surface, yet contributing 45.7% of total oceanic carbon burial at rates 30–50 fold higher than terrestrial forest soils (Duarte et al., Reference Duarte, Middelburg and Caraco2005; Duarte et al., Reference Duarte, Losada, Hendriks, Mazarrasa and Marbà2013). These ecosystems maintain dual functionality through simultaneous carbon storage and vertical sediment accretion, with average rates that currently keep pace with contemporary – but probably not mid-century – global mean sea-level rise (estimated average accretion rates for coastal ecosystems are $ 6.73\;\mathrm{mm}\;{\mathrm{yr}}^{-1} $ for salt marshes, $ 5.47\;\mathrm{mm}\;{\mathrm{yr}}^{-1} $ for mangroves and $ 2.02\;\mathrm{mm}\;{\mathrm{yr}}^{-1} $ for seagrasses, see Duarte et al., Reference Duarte, Losada, Hendriks, Mazarrasa and Marbà2013; current estimated sea-level rise rates are $ 4.5\;\mathrm{mm}\;{\mathrm{yr}}^{-1} $ for today; $ 6.5\pm 2.6\;\mathrm{mm}\;{\mathrm{yr}}^{-1} $ for 2050, according to Hamlington et al., Reference Hamlington, Bellas-Manley, Willis, Fournier, Vinogradova, Nerem, Piecuch, Thompson and Kopp2024). Globally, blue carbon ecosystems have a collective restoration potential to address $ \sim 3 $ % of global emissions by 2030 (Macreadie et al., Reference Macreadie, Micheli, Atwood, Friess, Kelleway, Kennedy, Lovelock, Serrano and Duarte2021).

However, a fundamental distinction exists between blue carbon projects and NbS for coastal management: Blue carbon projects prioritise the conservation, restoration and sustainable management of coastal ecosystems to maximise carbon sequestration and storage. NbS in coastal engineering primarily address risk mitigation from marine hazards while providing secondary carbon storage and sequestration benefits. Although both approaches offer complementary advantages, they represent distinct strategies with respective objectives and implementation methods.

NbS offer many benefits for coastal protection but face limitations in specific safety aspects, such as handling consistently rising water levels, space constraints, seasonality (died-off parts of plants or extensive flooding in storm surge seasons) and urgency (Morris et al., Reference Morris, Boxshall and Swearer2020). “Urgency” in this context refers to the need for immediate effectiveness, that is, to effectively safeguard coastal areas against marine hazards – such as strong sea states or storm surges – upon completion of construction (Narayan et al., Reference Narayan, Beck, Reguero, Losada, Van Wesenbeeck, Pontee, Sanchirico, Ingram, Lange and Burks-Copes2016; Pontee et al., Reference Pontee, Narayan, Beck and Hosking2016). In contrast to engineered structures, NbS often provide limited immediate ecosystem services, like wave attenuation, following their implementation, as they need time to develop naturally (Morris et al., Reference Morris, Boxshall and Swearer2020; Jordan and Fröhle, Reference Jordan and Fröhle2022). Growth is a fundamental aspect of their evolution and maturation, which is a negative aspect concerning immediate protection but comes with the ability of self-repair over time (Morris et al., Reference Morris, Boxshall and Swearer2020). In this context, it is important to understand that growth is an inherently gradual process (Pontee, Reference Pontee2023). It can take several years for NbS to achieve sufficient effectiveness, particularly in accumulating new sediment layers or protecting against wave impacts (Gómez Martín et al., Reference Martín, Eulalia and Máñez2020). Moreover, while NbS are effective in attenuating wave impacts, they are not designed to counter sea-level rise intentionally. Unlike engineered structures, NbS generally do not possess the structural properties needed to hold back rising sea levels (Hobbie and Grimm, Reference Hobbie and Grimm2020). Furthermore, NbS only attenuates wave impact effectively if sufficient space is available (Gómez Martín et al., Reference Martín, Eulalia and Máñez2020). This space allows the development of friction and turbulence, dissipating wave energy (Narayan et al., Reference Narayan, Beck, Reguero, Losada, Van Wesenbeeck, Pontee, Sanchirico, Ingram, Lange and Burks-Copes2016). However, the need for additional space – either land- or seaward – is a critical aspect to consider with NbS for natural dynamics to evolve or ecosystem services for coastal protection to unfold. Another big disadvantage of NbS in contrast to engineered solutions is that they are also susceptible to marine biological hazards (for example, disease outbreaks, invasive species, harmful algal blooms) on top of physical oceanographic hazards (for example waves, storms and storm surges, sea-level rise, coastal morphology changes).

Yet, the biggest contemporary issue for practical implementation of NbS is both legal and societal issues: NbS, being a relatively new approach, lacks long-term experience and is in early stages of technical guidelines (Schoonees et al., Reference Schoonees, Mancheño, Scheres, Bouma, Silva, Schlurmann and Schüttrumpf2019), despite the first standards emerged recently (e.g., the “IUCN Global Standard for Nature-based Solutions,” IUCN, 2020). Standards constitute a critical framework for engineering practice; compliance establishes legal defensibility against liability claims when structural failures arise from events beyond foreseeable design conditions. In storm-prone, densely populated areas, coastal communities may favour engineered coastal protection over NbS due to limited trust and lack of information as to secondary benefits (i.e., enhanced ecology; Frantzeskaki, Reference Frantzeskaki2019; Anderson et al., Reference Anderson, Renaud, Hanscomb and Gonzalez-Ollauri2022). Increasing implementation of NbS for flood protection requires overcoming critical barriers, including insufficient financial incentives and political will, inadequate institutional frameworks, challenges in land acquisition, uncertainties about their effectiveness and a lack of trust among stakeholders, which collectively form cascading obstacles amplified by disciplinary and regional disparities (Raška et al., Reference Raška, Bezak, Carla, Kalantari, Banasik, Bertola, Bourke, Cerdà, Davids, de Brito, Evans, Finger, Halbac-Cotoara-Zamfir, Housh, Hysa, Jakubínský, Kapović, Solomun, Kaufmann, Seifollahi-Aghmiuni, Schindelegger, Šraj, Stankunavicius, Stolte, Stričević, Szolgay, Zupanc, Slavíková and Hartmann2022).

Hybrid Nature-based Strategies

Hybrid Nature-based Strategies are characterised by measures configured in series, where engineered structures (such as breakwaters, sea walls and groynes) are sequentially arranged with NbS (including salt marshes, seagrass meadows, coral and oyster or shellfish reefs; Sutton-Grier et al., Reference Sutton-Grier, Wowk and Bamford2015; David et al., Reference David, Schulz, Schlurmann, Renaud, Sudmeier-Rieux, Estrella and Nehren2016). The aim is to enhance technical systems with ecosystem services including provisioning services (e.g., habitat and food provision) and regulating services, such as the improvement of water quality (Sutton-Grier et al., Reference Sutton-Grier, Wowk and Bamford2015). Consequently, HNbS strategies offer the opportunity for conserving and restoring biodiversity by including nature-based or natural features, aiming to reduce the adverse ecological impact of engineered structures (David et al., Reference David, Schulz, Schlurmann, Renaud, Sudmeier-Rieux, Estrella and Nehren2016; Pontee et al., Reference Pontee, Narayan, Beck and Hosking2016; Narayan et al., Reference Narayan, Beck, Reguero, Losada, Van Wesenbeeck, Pontee, Sanchirico, Ingram, Lange and Burks-Copes2016; Schoonees et al., Reference Schoonees, Mancheño, Scheres, Bouma, Silva, Schlurmann and Schüttrumpf2019; Morris et al., Reference Morris, Boxshall and Swearer2020). However, NbS-related shortcomings persist, such as the time-delayed unfolding of their effects due to growth time and development phases. Engineered structures can mitigate these challenges of NbS, as the engineered components of hybrid adaptation strategies offer their complete protection service immediately after implementation (see Figure 3). For example, artificially strengthened dunes can bridge the time needed for vegetation cover to develop, where an engineered dune core offers full safety levels in case of exceptional storm surges and associated erosion (Nordstrom, Reference Nordstrom2018; Mehrtens et al., Reference Mehrtens, Lojek, Kosmalla, Bölker and Goseberg2023; Mehrtens et al., Reference Mehrtens, Lojek, Ahrenbeck, Schweiger, Kosmalla, Schürenkamp and Goseberg2025a). Similarly, dike-salt marsh combinations demonstrate this principle, where the dike provides protection during storm surge events, while salt marshes accumulate sediment over time to mitigate adverse sea-level rise impacts (Kirwan et al., Reference Kirwan, Temmerman, Skeehan, Guntenspergen and Fagherazzi2016; Waryszak et al., Reference Waryszak, Gavoille, Whitt, Kelvin and Macreadie2021). Other vegetation growth support strategies employ artificial mimics like biodegradable seagrass structures or protective fences to facilitate marine restoration in hydrodynamically demanding conditions (Carus et al., Reference Carus, Arndt, Schröder, Thom, Villanueva and Paul2021; Kamperdicks et al., Reference Kamperdicks, Lattuada, Corcora, Schlurmann and Paul2025). Similarly, implementing (temporary) breakwater or floodgates to create specific growth conditions for mangroves and salt marshes reduces wave impact and controls inundation rates, creating protected areas supporting the vegetation’s growth phase (Sutton-Grier et al., Reference Sutton-Grier, Wowk and Bamford2015; Morris et al., Reference Morris, Boxshall and Swearer2020; Waryszak et al., Reference Waryszak, Gavoille, Whitt, Kelvin and Macreadie2021; Jordan and Fröhle, Reference Jordan and Fröhle2022). A (more) immediate unfolding of protection services not only compensates the temporal drawbacks of NbS but also increases the acceptance for NbS in high-risk areas by addressing key implementation barriers: providing immediate visual evidence of effectiveness, reducing uncertainty about performance and rapidly building the evidence base needed for decision-making. Thus, HNbS Strategies can lower the barriers, build confidence and strengthen the acceptance of NbS (for more information on implementation barriers and increasing acceptance, see Sutton-Grier et al., Reference Sutton-Grier, Wowk and Bamford2015; Anderson et al., Reference Anderson, Renaud, Hanscomb and Gonzalez-Ollauri2022). In addition to mitigating negative aspects, components of HNbS strategies can also have protective effects on each other. The wave attenuation by NbS reduces loads before these waves reach engineered structures and consequently lowers maintenance costs.

These examples demonstrate that engineering and natural methods offer distinct yet complementary temporal dynamics. Engineering structures, achieving peak safety immediately post-construction, experience gradual degradation. Conversely, natural features, such as dynamically growing vegetation, typically enhance resistance, durability and effectiveness over time. On the one hand, such a temporal complementarity increases the interest for biodegradable engineered measures, such as natural fibre geotextiles, which obviate maintenance and recycling needs as natural features progressively dominate structural integrity (David et al., Reference David, Schulz, Schlurmann, Renaud, Sudmeier-Rieux, Estrella and Nehren2016; Mehrtens et al., Reference Mehrtens, Lojek, Ahrenbeck, Schweiger, Kosmalla, Schürenkamp and Goseberg2025b). On the other hand, this synergy can also encounter practical challenges: Vegetation roots can compromise engineered structures, requiring specialised maintenance (Schüttrumpf and Scheres, Reference Schüttrumpf and Scheres2020). Similarly, floating debris and driftwood from nearby salt marshes may damage the protective grass cover of dikes, causing failure during storm surges. Acknowledging these interactions is crucial in designing hybrid coastal defence systems, where proactive planning can mitigate potential conflicts between natural growth and engineered stability.

Hybrid Nature-based Modules

The key distinction between hybrid strategies and Hybrid Nature-based Modules for coastal protection is that a strategy can consist of a series of engineered and nature-based structures. In contrast, Hybrid Nature-based Modules are structures composed of both natural and engineered features within the same structure (this becomes especially obvious in Figure 5, where Hybrid Nature-based Strategies on the left, Figure 5a, are depicted next to Hybrid Nature-based Modules in the centre, Figure 5b). Confined urban spaces often face tradeoffs between structural integrity and ecological impact (Charlier et al., Reference Charlier, Chaineux and Morcos2005; Spalding et al., Reference Spalding, McIvor, Beck, Koch, Möller, Reed, Rubinoff, Spencer, Tolhurst, Wamsley, van Wesenbeeck, Wolanski and Woodroffe2013). But Hybrid Nature-based Modules can improve the ecological quality of built environments, as they offer sheltering and provisioning services for ecosystems by design (Schoonees et al., Reference Schoonees, Mancheño, Scheres, Bouma, Silva, Schlurmann and Schüttrumpf2019). Various methods are used to enhance the ecological function of coastal structures: combining natural and engineered elements within a single structure, such as gabion breakwaters filled with oyster shells to foster reef development and shoreline protection (Safak et al., Reference Safak, Norby, Dix, Grizzle, Southwell, Veenstra, Acevedo, Cooper-Kolb, Massey, Sheremet and Angelini2020), and, more recently, installing hybrid mangrove shorelines with rock fillets to facilitate mangrove growth at eroding coasts, enhancing habitat structure and faunal abundance while modifying community composition (Chan et al., Reference Chan, Hsiung, Swearer and Morris2025). Together, these approaches epitomise a progression towards integrating ecological considerations in engineered solutions to actively design structures that restore and support marine habitats. By combining structural innovation, material adaptation and ecological facilitation, hybrid modules can address both the physical and biological needs of coastal environments as well as reducing risks of suffering from marine extreme events for coastal dwellers.

Figure 5. Illustration of Hybrid Nature-based Solutions (HNbS) across different scales. The top left panel (a) shows a Hybrid Nature-based Solutions Strategy, where engineering and NbS are combined in a coastal area. The top right panel (b) presents one Hybrid Nature-based Solutions Module, where the module consists of engineered and nature-based components. The bottom panel (c) shows a Confluent Hybrid Nature-based Solutions, where on the material level, biotic and abiotic materials amalgamate to a Hybrid Nature-based Solution, for example, through biomineralisation. Biomineralisation refers to the natural formation of minerals by living organisms through biological processes, where cyanobacteria facilitate the deposition of calcium carbonate minerals on surface structures (Reinhardt et al., Reference Reinhardt, Ihmann, Ahlhelm and Gelinsky2023). The biomineralised material appears green because it incorporates living cyanobacteria containing chlorophyll. The coral in subfigure (c) is a digitised Astraea (Orbicella) coronata, provided by the Smithsonian Institution’s 3D collection under the Creative Commons Zero (CC0) license. The scan is maintained by the National Museum of Natural History (NMNH) – Invertebrate Zoology Dept. and can be found by the record ID: nmnhinvertebratezoology_31148. The coral is produced by a mobile robotic additive manufacturing (AM) system, using an extrusion-based printing procedure (depicted by the grey printer next to the beach) to fabricate the coral with biomineralised material (for more information on AM using mobile robots, see Dörfler et al., Reference Dörfler, Dielemans, Lachmayer, Recker, Raatz, Lowke and Gerke2022). All renders are done with Blender version 4.3.1.

Surface modifications of engineered structures to enhance their ecological integration are an essential consideration, especially for digital fabrication methods (see Digital fabrication section). The deliberate and controlled design of surfaces and surface features has been demonstrated to significantly influence settlement patterns and promote colonisation by local organisms and species:

For surface designs, depending on the target species and body size, as well as other biotic and abiotic target parameters, several considerations arise: For smaller species, surface alterations at the millimetre scale primarily provide a substrate for settlement while hydrodynamically influencing surface roughness. Smaller surface adjustments have a lower impact on the flow conditions but can additionally provide exposure or shelter to light for small species (Perkol-Finkel and Sella, Reference Perkol-Finkel and Sella2015; Evans et al., Reference Evans, Lawrence, Natanzi, Moore, Davies, Crowe, McNally, Thompson, Dozier and Brooks2021). On a centimetre to decimetre scale, structural elements for water retention or shelter play a crucial role in offering protection against currents and maintaining favourable environmental conditions. Following the food chain in a habitat, the cultivation and colonisation of vegetation and species on smaller scales will attract larger species (Firth et al., Reference Firth, Schofield, White, Skov and Hawkins2014; Perkol-Finkel and Sella, Reference Perkol-Finkel and Sella2015; MacArthur et al., Reference MacArthur, Naylor, Hansom, Burrows, Lynette and Boyd2019). With increasing surface complexity and thus attracting diverse species with varying body sizes, the highest level of colonisation is achieved (Morris et al., Reference Morris, Chapman, Firth and Coleman2017; MacArthur et al., Reference MacArthur, Naylor, Hansom, Burrows, Lynette and Boyd2019). While surface and form complexity are primary design variables (next to material selection), ecological success depends less on complexity itself than on the resulting species variety, which drives greater abundance and richness (Loke et al., Reference Loke, Heery, Lai, Bouma and Todd2019).

Beyond passive surface modifications, active electrochemical approaches – as introduced by the BioRock technology (Goreau and Trench, Reference Goreau and Trench2012) or seacrete concept (Johra et al., Reference Johra, Margheritini, Antonov, Frandsen, Simonsen, Møldrup and Jensen2021) – offer a transformative pathway for coastal protection structures. Electrochemical methods apply low-voltage electrical currents to submerged metal frameworks, inducing the crystallisation of dissolved minerals from seawater into a robust white limestone coating (Hilbertz, Reference Hilbertz1979). This mineral accretion material, composed primarily of calcium carbonate and magnesium hydroxide, exhibits mechanical strength comparable to that of conventional concrete (Hilbertz, Reference Hilbertz1979; Johra et al., Reference Johra, Margheritini, Antonov, Frandsen, Simonsen, Møldrup and Jensen2021). Unlike traditional concrete, which is cast and static, the continuously electrochemically created material grows and strengthens continuously over time, possesses properties to repair damaged sections and provides a dynamic living foundation for rapid colonisation by corals, oysters and other marine organisms (Goreau, Reference Goreau2022). Thus, electrochemical mineral accretion processes achieve structural resilience and ecological enhancement on an organism or material scale.

Another relevant design factor for coastal structures is material selection. While engineered structures typically rely on synthetic construction materials, such as concrete and steel, adapting materials to incorporate natural components has demonstrated benefits for species colonisation and biodiversity (Ly et al., Reference Ly, Yoris-Nobile, Sebaibi, Blanco-Fernandez, Boutouil, Castro-Fresno, Hall, Roger, Deboucha, Reis, Franco, Borges, Sousa-Pinto, van der Linden and Stafford2021). Research into alternative substrates, such as limestone sand, grass, seashells and coral rubble, shows that these natural additives further promote colonisation (Ly et al., Reference Ly, Yoris-Nobile, Sebaibi, Blanco-Fernandez, Boutouil, Castro-Fresno, Hall, Roger, Deboucha, Reis, Franco, Borges, Sousa-Pinto, van der Linden and Stafford2021; Dodds et al., Reference Dodds, Schaefer, Bishop, Nakagawa, Brooks, Knights and Elisabeth2022). For species particularly sensitive to specific cultivation conditions, such as coral reefs, additional factors like the presence or abundance of marine algae and other benthic organisms are crucial for forming symbiotic relationships (Petersen et al., Reference Petersen, Laterveer and Schuhmacher2004).

Concrete will remain essential for urban coastal protection due to its strength, resistance against marine impacts and versatility in shaping (through precast or additive manufacturing). Additionally, as a composite material, concrete can be engineered to minimise its ecological impact, making it more attractive for marine species (Georges et al., Reference Georges, Bourguiba, Chateigner, Sebaibi and Boutouil2021; Natanzi et al., Reference Natanzi, Thompson, Brooks, Crowe and McNally2021). Research indicates that material composition influences the abundance, though not the diversity, of organisms colonising marine structures (Dodds et al., Reference Dodds, Schaefer, Bishop, Nakagawa, Brooks, Knights and Elisabeth2022). Thus, integrating complex surfaces in concrete marine structures can help reduce their ecological footprint. Australia’s living sea wall serves as an exemplary case (Bishop et al., Reference Bishop, Vozzo, Mayer-Pinto and Dafforn2022; Firth et al., Reference Firth, Bone, Bartholomew, Bishop, Bugnot, Bulleri, Chee, Claassens, Dafforn, Fairchild, Hall, Hanley, Komyakova, Lemasson, Lynette, Mayer-Pinto, Morris, Naylor, Perkins, Pioch, Porri, O’Shaughnessy, Schaefer, Strain, Toft, Waltham, Aguilera, Airoldi, Bauer, Brooks, Burt, Clubley, Cordell, Espinosa, Evans, Farrugia-Drakard, Froneman, Griffin, Hawkins, Heery and Knights2024): These hexagon seawall tiles feature nature-inspired designs, such as honeycomb or oyster surface structures and different-sized water-retaining rock pools, promoting biodiversity with various complex surface structures for settlements.

Digitalisation and digitisation

The revolution in digital fabrication began in 1952 when engineers first coupled milling machines with numerical control units, creating computerised numerical control (CNC) technology resulting in the digitisation of the entire industrial sector (Gershenfeld, Reference Gershenfeld2012). In this domain, a distinction is made between two forms of transformations: digitalisation and digitisation. Digitisation refers to the conversion of data, signals, images, audio and other forms into a format readable by computers. Notable examples in the field of coastal engineering include numerical wave models (see, for example, Roeber and Cheung, Reference Roeber and Cheung2012; Crespo et al., Reference Crespo, Domínguez, Rogers, Gómez-Gesteira, Longshaw, Canelas, Vacondio, Barreiro and García-Feal2015; Bihs et al., Reference Bihs, Kamath, Chella, Aggarwal and Arntsen2016) and, in the realm of design, CAD programs. On the other hand, digitalisation considers the utilisation of digitised data, for example, in the post-processing of field campaigns or pre-processing of live data for numerical models or other computer programs (David et al., Reference David, Kohl, Casella, Rovere, Ballesteros and Schlurmann2021b; Carlow et al., Reference Carlow, Mumm, Neumann, Schneider, Schröder, Sedrez and Zeringue2022). Digital fabrication, succinctly defined as “turning things into data” (as digitising transfers analogue information into data) “and data into things,” (and using the digitised data to create new data or objects) underscores its transformative nature (Gershenfeld, Reference Gershenfeld2012).

In the construction sector, digital fabrication entails integrating various digitisation and digitalisation processes into an automated, digitised procedure (Gershenfeld, Reference Gershenfeld2012; Lowke et al., Reference Lowke, Dini, Perrot, Weger, Gehlen and Dillenburger2018; Buswell et al., Reference Buswell, da Silva, Bos, Schipper, Lowke, Hack, Kloft, Mechtcherine, Wangler and Roussel2020; Kloft et al., Reference Kloft, Krauss, Hack, Herrmann, Neudecker, Varady and Lowke2020; Lowke et al., Reference Lowke, Talke, Dressler, Weger, Gehlen, Ostertag and Rael2020; Dörfler et al., Reference Dörfler, Dielemans, Lachmayer, Recker, Raatz, Lowke and Gerke2022; Rennen et al., Reference Rennen, Gantner, Dielemans, Bleker, Christidi, Dörrie, Hojjat, Mai, Mawas, Lowke, D’Acunto, Dörfler, Hack and Popescu2023). Within automation, there is a distinction between software and industrial automation of processes. Software automation describes the utilisation of computational forces to execute processes. An example of software automation is the calculation of force flows in a structure using numerical programs. The procedure is known as topology optimisation, where a numerical model calculates the force flow resulting from a set of loads. This force flow is used to adjust and optimise a structure’s or component’s design (Zhu et al., Reference Zhu, Zhou, Wang, Zhou, Yuan and Zhang2021). Depending on loads from permanent and variable actions, an algorithm precisely determines where materials need to be applied to carry these loads, leading to less orthogonal and more adaptive, topology-optimised forms (Vantyghem et al., Reference Vantyghem, De Corte, Steeman and Boel2019). These models serve as a foundation for determining the impact and, consequently, the force flow within the structure to be manufactured. Parametrisation transits force flows into the design process by coupling numerical models and CAD programs.

Fabrication methods

The utilisation of various fabrication methods for the creation of a final product is referred to as manufacturing. In fabrication, three major methods exist: formative manufacturing, subtractive manufacturing and additive manufacturing (AM).

Formative manufacturing involves altering shape without material addition or removal, typically with constant volume. Examples include concrete molding or casting steel into premade molds (Mainka et al., Reference Mainka, Kloft, Baron, Hoffmeister and Dröder2016). This method often incurs high resource consumption due to the prefabrication of molds (Mainka et al., Reference Mainka, Kloft, Baron, Hoffmeister and Dröder2016).

A more flexible process than formative manufacturing is subtractive manufacturing, where objects are shaped through material removal (Francis, Reference Francis2016). Despite its precision and flexibility in shape, subtractive manufacturing is spatially constrained and entails relatively long process times and high costs (Delgado Camacho et al., Reference Camacho, Daniel, O’Brien, Ferron, Juenger, Salamone and Seepersad2017). New research on combining milling devices with robotic arms alleviates these restrictions and enhances spatial reach, degrees of freedom and flexibility in this method. Another drawback of subtractive material is the associated removal of material. This is more resource consuming than successively adding layers of materials, such as in AM (Sathish et al., Reference Sathish, Kumar, Magal, Selvaraj, Narasimharaj, Karthikeyan, Sabarinathan, Tiwari and Kassa2022).

AM, commonly labelled as 3D printing, is a fabrication method, where a specific volume of material is deposited or solidified layer by layer (Bos et al., Reference Bos, Wolfs, Ahmed and Salet2016). Modern, computer-aided extrusion techniques use nozzles with high fabrication speed for large structures (Buswell et al., Reference Buswell, de Silva, Jones and Dirrenberger2018; Mai et al., Reference Mai, Brohmann, Freund, Gantner, Kloft, Lowke and Hack2021). Another form of AM is particle bed printing (Ngo et al., Reference Ngo, Kashani, Imbalzano, Kate and Hui2018), where raw construction material is solidified by activating substances in thin, aggregating layers (Ngo et al., Reference Ngo, Kashani, Imbalzano, Kate and Hui2018; Siddika et al., Reference Siddika, Al Mamun, Ferdous, Saha and Alyousef2019). The remaining, inactivated particles are removed after finalising the printing processes (Lowke et al., Reference Lowke, Dini, Perrot, Weger, Gehlen and Dillenburger2018; Lowke et al., Reference Lowke, Talke, Dressler, Weger, Gehlen, Ostertag and Rael2020; Mai et al., Reference Mai, Brohmann, Freund, Gantner, Kloft, Lowke and Hack2021). While this method is relatively precise and has a good resolution of details, it comes with restrictions of free-form design compared to other approaches like extrusion or injection three-dimensional printing (3DP; Mai et al., Reference Mai, Brohmann, Freund, Gantner, Kloft, Lowke and Hack2021). The injection of concrete into a carrier liquid, referred to as injection 3D concrete printing (I3DCP), combines AM with formative manufacturing, introducing greater flexibility in shaping (Lowke et al., Reference Lowke, Vandenberg, Pierre, Thomas, Kloft and Hack2021). Current research is aiming towards 4D printing, where 3D printing structures perform mechanical work through material properties depending on external stimuli and internal material responses (for example, temperature or changing magnetic conditions, see Zastrow, Reference Zastrow2020).

AM is already applied to coastal structures like reef balls or artificial mussel and coral reefs (Levy et al., Reference Levy, Berman, Yuval, Loya, Treibitz, Tarazi and Levy2022; Yoris-Nobile et al., Reference Yoris-Nobile, Slebi-Acevedo, Lizasoain-Arteaga, Indacoechea-Vega, Blanco-Fernandez, Castro-Fresno, Alonso-Estebanez, Alonso-Cañon, Real-Gutierrez, Boukhelf, Boutouil, Sebaibi, Hall, Greenhill, Herbert, Stafford, Reis, van der Linden, Gómez, Meyer, Franco, Almada, Borges, Sousa-Pinto, Tuaty-Guerra and Lobo-Arteaga2023; Berman et al., Reference Berman, Weizman, Oren, Neri, Parnas, Shashar and Tarazi2023; Hitzegrad et al., Reference Hitzegrad, Brohmann, Herding, Pfennings, Jonischkies, Scharnbeck, Mainka, Mai, Windt, Kloft, Wehrmann, Lowke and Goseberg2024), with benefits in the form of automated production of individualised “free-form” shapes. In the past, manufacturing complex shapes with irregularly designed surfaces was more expensive than fabricating standardised structures in a traditional, geometric shape (Wise et al., Reference Wise, Pawlyn and Braungart2013). However, automatised processes reduce labour and thus cost-intensive individualisation of structures both in the purposive design and construction, so that today and in the future, more complex structures designed to attract and enhance biodiversity become readily available.

Living and responsive construction materials

Current advances in material research for AM feature, for example, biodegradable construction material (Contardi et al., Reference Contardi, Montano, Galli, Mazzon, Ayyoub, Seveso, Saliu, Maggioni, Athanassiou and Bayer2021) or calcium carbonate photo-initiated ink (Albalawi et al., Reference Albalawi, Khan, Valle-Pérez, Kahin, Hountondji, Alwazani, Schmidt-Roach, Bilalis, Aranda, Duarte and Charlotte2021). Calcium carbonate photo-initiated ink is a specialised 3D-printable material designed for ecological applications, particularly coral reef restoration (coming from medical applications “ink” in this sense is the printed material; Avila-Ramírez et al., Reference Avila-Ramírez, Valle-Pérez, Susapto, Pérez-Pedroza, Briola, Alrashoudi, Khan, Bilalis and Charlotte2024). Calcium carbonate photo-initiated ink combines three key components to create durable, coral-like structures. Calcium carbonate forms the rigid scaffold, mimicking the natural skeleton of corals. Photo-initiators activate under light to rapidly solidify the biopolymer matrix, which holds the structure together during printing. Afterward, seawater triggers additional hardening through ionic interactions with calcium ions, enhancing strength and stability. These structures provide immediate surfaces for coral larvae to settle and grow, reducing the energy corals need to build their skeletons. Made from natural materials, they degrade safely over time while replicating natural reef shapes, supporting marine biodiversity and ecosystem restoration (Albalawi et al., Reference Albalawi, Khan, Valle-Pérez, Kahin, Hountondji, Alwazani, Schmidt-Roach, Bilalis, Aranda, Duarte and Charlotte2021; Avila-Ramírez et al., Reference Avila-Ramírez, Valle-Pérez, Susapto, Pérez-Pedroza, Briola, Alrashoudi, Khan, Bilalis and Charlotte2024). Those skeletons are later deployed in the sea and coated with living coral cells, nonetheless, the skeleton is not living material.

Other advances in AM material research focus on biomineralisation. Biomineralisation builds on mineralising cyanobacteria integrated with hydrogel and sea sand, enabling persistent mineralisation and self-repair capacity (Reinhardt et al., Reference Reinhardt, Ihmann, Ahlhelm and Gelinsky2023). The hydrogel facilitates extrusion-based AM, embedding sand particles that provide structural nucleation sites. The cyanobacteria drive calcium carbonate precipitation via $ {\mathrm{CO}}_2 $ (carbon dioxide) absorption and these mineral bridges between sand particles to enhance structural stability while sequestering atmospheric carbon. The system demonstrates potential for marine applications, including adaptable artificial reefs that combine $ {\mathrm{CO}}_2 $ capture with natural growth, adaptation and repair processes.

Over the past decade, there has been significant advancement and increasing discourse concerning 4D printing as briefly mentioned before. In this context, the fourth dimension refers to time, where objects can evolve in response to various stimuli (Grira et al., Reference Grira, Khalifeh, Alkhedher and Ramadan2023). The primary external stimuli currently considered include thermal, photo, electrical, magnetic or exposure to water, which trigger reactions in the objects (Kuang et al., Reference Kuang, Roach, Wu, Hamel, Ding, Wang, Dunn and Qi2018). Thermal and photon stimuli, for instance, can initiate the folding and unfolding of materials, enabling them to replicate the natural behaviours observed in flowers (Mao et al., Reference Mao, Ding, Yuan, Ai, Isakov, Wu, Wang, Dunn and Qi2016; Yang et al., Reference Yang, Leow, Wang, Wang, Yu, He, Qi, Wan and Chen2017). Triggered by light, objects with floral patterns fold and unfold through photothermal actuation. Other material research shows that, on a small scale, smart reactive materials growing from genetically reprogrammed cells (internal stimuli) are created, developing in a predefined way over time (Caro-Astorga et al., Reference Caro-Astorga, Walker, Herrera, Lee and Ellis2021).

These advancements are not yet available for coastal structure scale AM technologies, but they open a new perspective for hybrid solutions in form and material, bridging the gap between engineered structures and NbS. Once upscaling of bioprinting towards structural scale becomes available, this creates objects, which are made of living cells mimicking behaviours found in nature and may resemble natural forms. As the integration of nature and natural processes over different spatial scales increases, the boundaries between nature and engineered construction increasingly blur.