Beyond ‘Business as Usual’: A Research Agenda for the Operationalisation of Nature-Based Solutions in Flood Risk Management in The Netherlands

Abstract

The Netherlands is widely recognised as the global leader in water management, with its flood risk management (FRM) infrastructure lauded as being of the best in the world. This status notwithstanding, Dutch FRM primarily maintains established infrastructural practices and experimental applications of NBSs remain less frequent than established structural projects. This paper details and examines the challenges associated with the prevailing ‘business-as-usual’ approach to FRM in The Netherlands, in which traditional ‘grey’ infrastructural techniques are prioritised over innovative ‘green’ nature-based solutions (NBSs). In line with emerging international trends, such as the EU Water Resilience Strategy, NBSs are increasingly advocated as a strategic, complementary layer to enhance the resilience of existing safety frameworks rather than a self-evident replacement for them. Contrary to grey infrastructure, NBSs provide a number of environmental and social co-benefits extending beyond their flood and drought protection utility. The literature on NBSs details the design, effectiveness, and positive socio-economic impact of the operationalisation of such projects for FRM. This notwithstanding, the uptake and practical implementation of NBSs have been slow in The Netherlands. From a legal and policy perspective, this has been attributed to a lack of political will and the corresponding failure to include NBSs in long term FRM planning. Given the long planning horizons associated with FRM (50–100 years), the failure to incorporate NBSs can lead to policy lock-in that blocks future adaptations. Against this backdrop, this paper employs a semi-systematic literature review to clarify the obstacles to implementing NBSs in Dutch FRM and sets a research agenda that charts a course to mainstreaming NBSs in Dutch FRM. Seven core focus areas for future research are identified. The paper concludes by drawing on these identified focus areas to construct a research agenda aimed at systematically addressing each barrier to the practical operationalisation of NBSs in Dutch FRM, emphasising a hybrid green–grey approach which may serve to inspire similar research in other jurisdictions.

1. Introduction

1.1. ‘Business as Usual’ in Dutch Flood Risk Management

Across the globe, countries are facing a broad range of water-related challenges, ranging from the depletion of ecosystems and abiotic assets [1] to pressures on water security and human health [2,3]. Climate change further exacerbates these phenomena. Global warming intensifies the hydrological cycle, thereby causing an increase in the severity of extreme events [4]. Of distinct concern to low-lying deltaic regions, such as The Netherlands, is the compounding threat of anticipated sea-level rise [5] interacting with changing inland hydrology. Specifically, the Dutch river region (Rivierengebied) faces growing risks from projected increases in peak discharges across the delta’s transboundary river systems. Consequently, the liveability of these interconnected areas is threatened by ‘coastal erosion, salt intrusion, reduced river outflow capacities, and increased flood risk’ [6]. The predicted rise in sea levels is dependent on the CO₂ emission scenario, but could range from 0.28 to 2.0 m by 2100 relative to 1995, and will continue to rise beyond 2100 [5]. Uncertainty arising from the large range in these projections, coupled with the long lead times associated with planning and implementation of defence projects, poses significant challenges for investment planning [7]. In dealing with these combined projections of uncertain sea-level rise and river discharge variability, the Dutch government pursues both a risk-based and an adaptive approach to flood risk management (FRM) [8].

The risk-based approach pursued in The Netherlands is operationalised through safety standards for primary and non-primary flood defence structures, as well as through water storage capacity standards for regional water systems [8]. Flood defence structures include dykes, storm surge barriers, sea walls, and reinforced coastal dunes that protect Dutch land regions from flooding by open water bodies, including the sea, major rivers, and large lakes. These physical structures are referred to as grey infrastructure. Grey infrastructure is thus defined as the established engineering practices—such as dykes, sea walls, and storm surge barriers—constructed from “hard” materials, like concrete and steel, which rely on purely technological strategies in conjunction with strict statutory safety standards to resist or divert water [9,10]. Their minimum required strength and height are defined by strict statutory safety standards, accounting for relevant conditions and potential failure mechanisms [8]. The applicable law in this regard is the Environment and Planning Act—‘Omgevingswet’ (2024) [11]—and associated regulations. The Omgevingswet and its associated regulations require a twelve-year monitoring and reporting cycle to ensure that these structures continue to meet legal safety requirements. Where a defence no longer meets—or is expected to fall short of—these standards, the responsible water authority must undertake reinforcement works. Such measures often necessitate new investment, supported by the Dutch Flood Protection Programme (‘Hoogwaterbeschermingsprogramma’) and the Delta Fund [8]. Furthermore, reinforcement of primary and non-primary flood defence structures can only be executed on the basis of a comprehensive project plan describing the measures in detail, and must be accompanied by all requisite spatial planning, construction, and nature conservation permits and decisions [11]. These administrative requirements are executed via ‘an accelerated and coordinated decision-making procedure in coordination with the competent authorities in the adjacent policy fields’ [8]. While this historically technical regime has been a benchmark for safety, the reliance on statutory safety standards can inadvertently maintain a preference for traditional engineering over more flexible, nature-based alternatives. However, this preference is not without issue: a continued prioritisation of reinforcing “grey” infrastructure necessitates escalating long-term investments while potentially increasing “residual risk”—such risk here refers to the catastrophic damage that occurs if traditional defences fail. Conversely, NBSs are increasingly recognised for providing diverse co-benefits, such as nature restoration and recreation, while offering a more cost-effective and adaptive safety layer over long planning horizons [12,13,14]

The adaptive approach pursued in The Netherlands is operationalised through the Delta Programme, through which a balance is sought between ‘too little too late’ and ‘too much too early’ [15]. This policy approach involves scenario planning, flexible strategies, and periodic reviews every 6 years [16]. An independent advisory body, the ‘Signal Group’, provides the Delta Commissioner with annual guidance on emerging developments and knowledge relevant to the programme’s objectives and implementation [7]. To specifically address long-term uncertainties regarding sea-level rise and its compounding impact on river drainage, the Knowledge Programme Sea Level Rise explores fundamental strategic directions [17,18]. These range from ‘protecting’ (via open or closed systems) and expanding ‘seaward’, to ‘moving along’ (meebewegen). Of distinct relevance to the Southwestern Delta is the emerging strategy of ‘growing along’ (meegroeien) [19,20], which positions NBSs as potential mechanisms for sediment accumulation and adaptive system resilience. Consequently, the Delta Programme is widely considered as a blueprint for a successful approach to integrated FRM, which tackles the emerging issue of droughts alongside traditional concerns of flooding through an emphasis on ‘urgency, inclusive participation, idea incubation, and consensus-building to inform long-term adaptation strategies’ [21].

Against the backdrop of the Delta Programme and the newly implemented Omgevingswet, a shift is identifiable in Dutch FRM over the last two decades [8]. Instead of pursuing a purely protectionist strategy implemented by specialised public water management authorities, Dutch FRM has moved towards a new era characterised by a strong drive for innovation, diversification, and public participation. This shift suggests a potential alignment between emerging policy trends and the integration of NBSs into Dutch FRM planning. This is particularly so in light of the increasing recognition of the utility of NBSs at the international [22], European Union [23,24], and even Dutch national policy levels [25]. This trend further aligns with broader policy frameworks such as the EU Water Resilience Strategy, which advocates for nature-restorative measures to bolster regional water security [26]. Also, the Delta Program recognises the importance and need of NBSs, given its recently presented ‘grow along’ (meegroeien) strategy [20]. This notwithstanding, an investigation into practice reveals that traditional grey infrastructural solutions continue to be prioritised in FRM in The Netherlands, with a ‘business-as-usual’ approach being pursued over the experimentation and operationalisation of the use of NBSs as a complementary strategic layer for these essential traditional defences [8].

1.2. Benefits of Mainstreaming Nature-Based Solutions to Flood Risk Management in The Netherlands

The International Union for Conservation of Nature defines NBSs as

‘actions to protect, sustainably manage, and restore natural and modified ecosystems that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits’ [27].

Similarly, the preamble to the European Nature Restoration Law defines NBSs as

‘solutions that are inspired and supported by nature, that are cost-effective, and that simultaneously provide environmental, social and economic benefits and help build resilience’… Nature-based solutions need to therefore benefit biodiversity and support the delivery of a range of ecosystem services [28].

Applied to FRM, NBSs pursue an ecosystem-based approach by employing nature’s own capacity to absorb excess water to prevent flooding and promote resilience [29]. The appeal of NBSs is the extensive range of co-benefits that extend well beyond FRM. The term ‘co-benefits’ is used in the NBS literature to refer to a situation in which a measure with a specific objective (such as flood prevention) simultaneously contributes to other objectives (such as biodiversity restoration) [30,31,32]. In a comprehensive study in this regard, Moraes et al. (2022) identified 156 co-benefits in the NBS projects examined [33]. Most prevalent were the reduction in the impact of flooding, reduction in coastal erosion, promotion of biodiversity conservation and/or restoration, establishment of blue-green recreational spaces supporting mental and physical wellbeing, and increased tourism [33]. Furthermore, such measures can enhance spatial quality, as demonstrated in Figure 1 below—this additionally being a cornerstone for Room for the River and other large programmes—and offer potential for green agri-aquaculture, such as sustainable mussel and oyster breeding in residual polders [34,35]. Successful integration of such co-benefits has been evidenced in large-scale evaluations of the Room for the River project, where participation, clear binding goals, and a multi-objective approach served as critical success factors [13,36].

In the Dutch context, where availability of land is limited, the operationalisation of NBSs is best achieved in combination with, rather than the replacement of, (existing) technological and engineering solutions [37]. This hybrid approach avoids the undesirable outcome of abandoning existing flood defence systems—which would return protected land to its natural state—and instead builds upon existing infrastructure to enhance long-term safety and resilience. The precise type of NBS to be utilised for FRM depends on local socio-economic and environmental factors [38]. As such, each NBS implementation should be individually considered. Effective implementation may require policy makers to account for the fact that the successful application of NBSs in one location does not mean its application in another context constitutes best practice [38,39]. Examples of successfully applied NBSs in the context of coastal FRM include the expansion of saltmarshes with high vegetation [40], dune protection and stabilisation [38], mangrove restoration and afforestation [41], beach nourishment/replenishment [42], coastal lagoon restoration and conservation [43], the reconstruction of seagrass meadows [42], and ‘double-dike systems’ or ‘residual polders’ [44].

Beyond the coastal context, urban ‘rainproof’ initiatives in Amsterdam, Rotterdam, and Utrecht have demonstrated the utility of NBSs in diverse spatial contexts [45], while international examples, such as the Sponge City projects in China, provide further evidence of technical viability [46].

There is, thus, sufficient empirical evidence demonstrating the effectiveness of NBSs for FRM, including within the Dutch context. Nevertheless, their uptake in practice remains limited, suggesting that various barriers continue to hinder the operationalisation of NBSs in practice. In what follows, this paper outlines these barriers and identifies the research needed to address them. We conducted a semi-systematic literature review (SSLR) to clarify the obstacles to implementing NBSs in Dutch FRM and to identify remaining research gaps. The SSLR suits broad, multidimensional subjects like NBSs because it maps prevailing theories and themes while highlighting areas that remain underexplored [47]. Using this approach, we examined existing research on NBSs in Dutch FRM to address our core research question:

Which knowledge gaps hinder the operationalisation of NBSs in Dutch FRM, and what further research is required to address them?

This research thus seeks to identify and analyse the gaps preventing a more integrated green–grey flood management strategies in The Netherlands, and is structured as follows: Section 2 presents the methodology; Section 3 sets out the results of the SSLR and proposes the research required to address the barriers identified within the literature; Section 4 draws on the results to propose a research agenda aimed at systematically addressing each barrier to the practical operationalisation of NBSs in Dutch FRM; and finally, Section 5 summarises the main conclusions.

2. Methodology

This section outlines the methodology employed in the literature review, which served as the foundation for the identification of the primary barriers frustrating the implementation of NBSs in FRM, which was then applied to the Dutch context. There is insufficient literature on the application of NBSs to Delta regions of The Netherlands. As such, the primary aim of the literature review was to identify relevant case studies and explore the broader academic discourse on NBSs across various contexts, such that the lessons learnt could be adapted and applied to the Dutch context.

The review process involved a semi-systematic literature review (SSLR) approach, which is particularly suited for broad, multidimensional subjects like NBSs, as it allows for the mapping of prevailing theories and themes while highlighting underexplored research gaps. The process involved searching the Google Scholar database using tailored search terms, followed by the evaluation of abstracts and full texts against predefined inclusion criteria. Google Scholar was selected in preference to conventional academic databases, such as Web of Science and Scopus, as it provides access to grey literature, including reports and other non-peer reviewed sources, which offer valuable insights into the practical dimensions of NBS planning and implementation in Delta regions. This approach enabled the mapping of the research field, the synthesis of the state-of-the-art, and the creation of an agenda for further research [47].

To begin, the parameters of the comprehensive review were set. To ensure only the most up-to-date information was included, the time frame of the review was restricted to include only records published in the past 10 years (2015–2025). To ensure a high quality of the literature, the source type was restricted to records published within recognised academic journals, and the document types were restricted to published books, articles, reviews, scientific reports, and conference papers. To ensure accessibility, only records published in their final publication stage were included. The key words selected were “nature-based solution” and “flood-risk management” in all possible combinations.

The flow of information through the different phases of the selection process is summarised in the flow diagram below (Figure 2), following the structure of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) to ensure methodological transparency.

The abstracts of identified records were screened to include only those addressing, directly or indirectly, the barriers to the operationalisation of NBSs in FRM contexts. The initial screening and identification of barriers were conducted by the primary author (N.H.), while the final determination of study eligibility and the thematic grouping into the seven research focus areas were verified and refined through consultation with the co-authors (H.K.G. and M.v.R.) to ensure consistency. Post-screening, 81 records were deemed eligible and included within this review (see Appendix A). Each record was reviewed in full, with a catalogue of identified barriers recorded, organised, and synthesised. These individual barriers were then grouped based on their primary disciplinary nature—such as legal, economic, or sociocultural—resulting in the identification of recurring clusters of knowledge gaps. Finally, these clusters were cross-referenced with the specificities of the Dutch FRM context to identify where current law, policy, and practice lack empirical or theoretical guidance. This iterative process (summarized in Appendix B) ensured that the resulting research agenda directly addresses the systemic obstacles identified within the broader international and national literature. In this way, the researchers identified the seven key focus areas requiring further research, and extrapolated the requisite research required to address the identified barriers within these seven focus areas, the results for which are set out in what follows below.

3. Results: Barriers to the Operationalisation of NBSs in Dutch FRM and the Research Required to Address Them

This article has demonstrated thus far that (1) although the Dutch approach to FRM is undergoing a shift toward diversification, it remains predominantly reliant on grey infrastructure, with little to no practical implementation of NBSs; and (2) this remains the case despite high-level frameworks, like the EU Water Resilience Strategy, which advocate for these measures [26] and ample evidence in existing research supporting the effectiveness of NBSs—not only as a strategy for FRM but also for delivering a range of environmental and social benefits. The failure to mainstream NBSs in Dutch FRM has been examined extensively in the existing literature, and several potential contributing factors, hereafter called ‘barriers’, have been identified. The following section synthesises these insights into a summary of seven key areas requiring attention to enable the first steps toward the operationalisation of NBSs as a complementary strategic layer within a hybrid grey–green Dutch FRM framework. This approach recognises that an NBS is intended to build upon and enhance existing grey infrastructure defences rather than replace them, ensuring that statutory safety standards are maintained while long-term resilience is bolstered and co-benefits are fully realised.

A.

Assessment of the adequacy of prevailing law and policy for the further operationalisation and mainstreaming of NBSs in Dutch FRM

The literature identifies the need for a supporting regulatory environment to operationalise and mainstream NBSs in FRM. It further identifies the existing regulatory framework as one of the primary barriers in this regard. This conclusion is reached, however, in the absence of a detailed doctrinal review into the existing law and policy. The mere fact that the existing law is not being utilised to pursue a particular policy objective does not necessitate the conclusion that there exists no space within such law to do so [49]. A doctrinal review is therefore required to properly assess the prevailing law and policy with regard to three primary topics (see, for example, [50]).

Firstly, the operationalisation and mainstreaming of NBSs for Dutch FRM is only possible when supported by an enabling regulatory environment. Though the need is identified in the majority of the literature examining the barriers and enablers of NBSs [51,52,53,54], the content of an enabling regulatory environment is not yet established. As such, further research is required to identify the characteristics of a regulatory environment that enables the operationalisation of NBSs. Inspiration may be drawn from the methodology utilised to answer the same question in relation to different innovative resource governance approaches, such as the water, energy, and food nexus [49], panarchy theory [55,56], ‘bridging mechanisms’ [57], and the concept of ‘solution space’ [17,58,59].

Secondly, a doctrinal review of existing applicable laws and policies within the Dutch context is required to identify whether there exists untapped capacity for the promotion of NBSs. If such untapped capacity exists, it can then be utilised to avoid the need for extensive legal reform. This is desirable given that ‘legal reform requires a lengthy procedural process often thwarted by layered bureaucracy and insufficient political will, and is thus not likely to happen as swiftly as impending environmental issues require’ [60]. Recent developments within Dutch law and policy, such as the coming into force of the Omgevingswet in January 2024 [11,61], as well as the first Room for the River program, the follow up program Room for the River 2.0, and Delta Programme, indicate a shift towards innovative and collaborative responses to environmental and water management. A detailed investigation, viewed through the lens of the NBS objectives, may thus reveal the possibility to interpret existing water safety and management obligations in a manner that can promote the mainstreaming of NBSs. A clear legal and policy basis to pursue NBSs is necessary, as the literature has identified a lack of trust by stakeholders in NBSs, given the limited information available on the long-term effects thereof [52,54,62,63].

Finally, beyond trust, the legal system and the constitution of property rights influence the extent to which an NBS can be operationalised where the requisite land for such implementation is privately owned. As such, the legal review should additionally seek to clarify public and private responsibilities and, furthermore, answer the question of whether there exists a need for strategic land policy to enable the mainstreaming of NBSs in Dutch FRM [52,64]—more on this below.

B.

Insight into appropriate incentive structures that may trigger public actors to move beyond a preference for the ‘business-as-usual’ approach to FRM

Closely related to the first focus area is the need for research into the design of incentive structures that will motivate interested public authorities (water management authorities, provinces, and municipalities) and relevant stakeholders to pursue NBSs and thereby move beyond the prevailing ‘business-as-usual’ approach to Dutch FRM [52]. The pursuance of incentive-based policies has been recommended over ‘punitive’ measures (for example, mandatory legal requirements for the implementation of NBSs and correlating grounds for state liability for failure to do so) to encourage the adoption of NBSs [55].

C.

Clear requirements for coordination across the highly fragmented departments that will be required to operationalise NBSs

NBSs broaden the scope of FRM by engaging a wider range of stakeholders from diverse sectors in the decision-making process, compared to traditional grey infrastructure approaches [65,66,67]. The broader scope of NBSs thus requires coordination across multiple policy domains. This is directly challenged by existing institutional fragmentation or ‘sectoral silos’ resulting from diversification, (conflicting) sectoral regulatory frameworks, and the distribution of mandates across departments, leading to differing priorities within the decision-making process [8,53,57,67,68]. The new Omgevingswet increases participation opportunities at an early stage for projects concerning the construction or reinforcement of primary flood defence structures [8]. However, there is, at present, insufficient guidance within the literature on the coordination and participation requirements of operationalising NBSs for FRM, particularly in light of the recently enforceable obligations under the Omgevingswet. Further research is thus required in this regard.

D.

Solutions to overcome the limitation that private land ownership poses in obtaining the additional space required to implement NBSs in FRM in The Netherlands

This article has already identified the need for doctrinal research clarifying public and private responsibilities in relation to enabling the acquisition of the land required to operationalise NBSs.

Compared to traditional grey infrastructure, NBSs generally require more land to derive expected benefits [53,69]. This is particularly so for the large-scale NBSs that aim to implement floodplains, wetlands, changes in land-use management practices, and river restoration [53]. This poses a significant challenge in The Netherlands, where space is severely limited and where the vast majority of land bordering water is subject to private ownership. Additionally, protecting farmland is a primary objective of the current FRM system, and utilising this land for NBSs is often seen as a highly sensitive political issue.

The central tenets of the prevailing FRM regime are the dual objectives of flood prevention and the protection of high-value agricultural land. The implementation of NBSs thus involves balancing these competing public interests through enhanced stakeholder engagement and transparent decision making [45,70,71]. Recent scholarship concerning “voorlanden” (forelands) and “wisselpolders” (exchange polders) indicates that NBSs can be integrated within the Southwestern Delta as a complementary safety layer without necessitating the wholesale abandonment of existing land-use functions [12,72]. By operationalizing innovative configurations, such as double-dike systems, it is possible to provide the requisite space for sediment accumulation and ecological restoration while simultaneously upholding the primary safety standards for the hinterland [12,73].

In certain limited situations, however, the protection of public interests may necessitate the expropriation of private property. Authorities wanting to implement an NBS on privately owned land therefore require requisite legal competence, including the authority to impose obligations permitting the construction of public water infrastructure [8]. Under the new Omgevingswet, the same authority responsible for approving the project or spatial development also has the power to initiate expropriation [11]. However, this decision must be formally confirmed through an administrative court procedure. The role of the civil court is limited to determining whether private parties are entitled to full compensation [8]. There has been additional success in The Netherlands in utilising ‘poldering’ to reach voluntary agreements over the loss of agricultural land, mitigating the need for formal expropriation procedures that can result in tensions along the public–private divide [74].

The Netherlands thus possesses an institutional framework for carrying out expropriation and compensation schemes, as well as demonstrating success in reaching voluntary agreements. This notwithstanding, practice indicates that a low frequency of resort to these techniques due to administrative and social complexities [13,45,46,75]. This is due to a combination of factors, including the burden of conducting formal administrative procedures and negotiating with landowners, as well as concerns about negative public perception and potential backlash [76,77]. This orients public actors towards favouring a ‘business-as-usual’ approach over innovation. This is because public opinion generally recognises the importance of the grey infrastructural defences and supports the continued investment into their construction and maintenance. The procedures for pursing these technical approaches have a decades-long administrative history and are thus well established routine practices for public actors. Contrastingly, the benefits of NBSs are less widely known (making public perception difficult to anticipate), as are the procedures required for their operationalisation [52,55].

With this in mind, there is a need for research to investigate solutions for overcoming the limitations posed by private land ownership that extend beyond expropriation and compensation schemes and the necessity for wide consultation [78]. A central component of this research is the evaluation of targeted education campaigns to clarify that NBSs are designed as a complementary layer to existing grey infrastructure, rather than a replacement for it. Addressing this distinction is necessary to respond to bureaucratic and public concerns that NBSs might involve the abandonment of traditional safety systems, which would return protected areas to their natural state—an outcome generally considered undesirable for safety and land-use objectives. This research trajectory may include assessing the potential role of education campaigns and the arts in shifting public perception toward hybrid green–grey strategies, identifying the administrative bodies responsible for execution, and evaluating a redistribution of responsibility to include private obligations for public good. By framing an NBS as an additional safety layer that provides documented co-benefits—such as nature restoration and recreation—while mitigating the escalating long-term costs and residual risks of purely grey systems, educational efforts may enhance the social and political legitimacy of integrated FRM strategies [12,13].

E.

Need for a clear toolbox for policy makers of the various NBS options available and in what physical and geographic contexts these may be applied

NBSs are numerous and can take various different forms along the coastline. For example, the International Guidelines on Natural and Nature-Based Features for Flood Risk Management has presented the use of beaches and dunes, coastal wetlands, islands, reefs, and submerged aquatic vegetation as possible NBSs for FRM [9]. Each form of NBS comes with different spatial, temporal, technical, and institutional requirements [51]. Presently, there is no consensus within the literature on which form of NBS to apply in which FRM context [51,52]. Further variation exists in the optimal design of the various forms as well as uncertainty relating to the (long-term) effects and maintenance requirements of NBSs in each particular FRM context [52]. This uncertainty and absence of data has been identified as one of the primary barriers preventing the uptake of NBSs by decision makers [53,79]. It is therefore important that cross-cutting research be conducted that consolidates the existing empirical results on the practical implementation of NBSs and identifies precisely which gaps in the knowledge remain for later research. In this way, the NBS literature may begin to move towards developing a common language and consolidate findings into a toolbox that can guide policy makers in their decision making of which forms of NBS to investigate in each particular context [51].

F.

Clarity on the long-term cost-effectiveness of NBS for FRM

The initial implementation of an NBS can often involve higher costs than the maintenance and reinforcement of existing grey infrastructure. There is little to no research quantifying the long-term benefits, and potential cost-effectiveness, of NBSs. As such, the high cost does account for the co-benefits realised in the longer term, such as the increased liveability of water lying areas and promotion of biodiversity [53]. Given the importance of economic considerations in public decision making, a clear understanding of the financial implications and returns associated with such large-scale initiatives is required [80]. Cost–benefit considerations of NBSs are essential for ‘bridging the gap between theoretical feasibility and the practical implementation’ [10,80]. Additionally, as identified by Kok et al. [81], ‘research analysing the mechanisms for leveraging public investment in a larger sample of cases, in various ecosystems and under various institutional (e.g., capacity of public budget, institutional arrangements for flood protection) and socio-economic settings (e.g., supply and demand for co-benefits) around the world would promote insights in their applicability and sustainability under various circumstances’.

G.

Deeper investigation into the social dimension of NBSs for FRM

Perhaps one of the most under-researched dimensions of NBSs is the sociocultural one. Current research focuses on investigating existing public perceptions of NBSs [76,82,83,84,85]. Understanding existing perception is important. However, prevailing research has not yet gone deeper to consider the broader dimensions of the social and cultural context, such that we may be informed of how we can promote a positive public perception of, and thereby rally the popular support for, NBSs in FRM. These broader dimensions requiring investigation include the historical context shaping prevailing public perceptions, as well as the national, regional, and local culture(s) and identities (and the interaction thereof) that shape public perception [58]. Through this understanding, research can identify strategies to increase the social acceptability of NBSs, recognising this as a critical factor for their broader feasibility. Methods to achieve this may include advertising and information campaigns, but also the use of the arts as a vehicle for education and societal engagement in this regard.

4. Discussion: Charting the Course to Mainstreaming Nature-Based Solutions in Flood Risk Management in the Dutch Context and Beyond

As this paper advances toward a structured research agenda, it is necessary to briefly examine the factors identified above as supporting a transition toward a hybrid NBS framework. While Dutch water authorities have historically prioritised fulfilling statutory safety standards through technical excellence, a singular focus on reinforcing grey infrastructure alone creates significant “residual risk” [13,75]. This established approach is associated with escalating financial investments for reinforcements and a heightened potential for catastrophic damage should these defences fail [86]. Conversely, a hybrid approach—which supplements grey defences with an NBS—is increasingly recognised as a viable method to enhance long-term resilience through cost-effective and adaptive solutions across extended planning horizons [72]. This transition aligns with high-level policy frameworks, such as the EU Water Resilience Strategy, which advocates for restorative measures to bolster regional water security alongside existing structural defences [26]. In this context, the integration of innovative NBS concepts, including “wisselpolders” and adaptive foreland management, emerges as a potential component promoting a future-proof flood risk strategy [12,73].

The results of this review highlight a need for research that provides actionable guidelines for the operationalization of NBSs, thereby bridging the gap between theory and practice. The proposed research agenda outlined in what follows below is predicated on the recognition that a hybrid green–grey approach does not constitute a “self-evident” replacement for traditional defences. Instead, as far as is reasonably possible, research into NBS implementation should strive to achieve strategic ‘double-use’ of space rather than land sacrifice.

While the Omgevingswet provides water boards with expropriation powers, such measures are often viewed as a last resort due to the potential for significant public backlash and political friction within elected boards. Consequently, there is a clear scholarly need for research focusing on identifying legal and economic “win–win” scenarios where innovative nature-restorative measures simultaneously fulfil statutory safety standards and support the economic interests of landowners. To this end,

Table 1. A research agenda for the mainstreaming of nature-based solutions in Dutch flood risk management.

Research Question	Methodology	Disciplinary Focus
What institutional/legal characteristics require reflection within the regulatory environment to enable the operationalisation of NBSs?	A literature review of the findings of the existing research on NBSs identifying what characteristics enable and constrain the operationalisation of NBSs. This will entail, in particular, characteristics that enable coordination across the various policy domains required to implement NBSs as well as characteristics that promote and enable decision makers to move beyond the ‘business-as-usual’ approach to FRM and dare to explore the potential for NBSs.	Law, political science, public policy, and governance.
What untapped capacity exists within the existing law and policy regulating FRM that can enable decision makers to pursue NBSs in practice in The Netherlands?	A doctrinal review of all applicable laws and policies regulating FRM to identify existing capacity to promote NBSs. This review may be conducted in a multi-level environment, including national as well as regional laws and policies.	Law.
Part 1: What existing public responsibilities are reflected within the prevailing law that require public officials to pursue NBSs for FRM in The Netherlands? Part 2: What existing private responsibilities are reflected within the prevailing law that require individual landowners to yield ownership rights in certain circumstances to enable public actors to pursue NBSs for FRM in The Netherlands? Part 3: In light of the existing responsibilities of public and private actors within the prevailing law, is there a need for a strategic land policy to enable the mainstreaming of NBSs for FRM in The Netherlands, and, if so, what form could this take?	Although Dutch law has clear procedural requirements for land expropriation where required for water management, research indicates that public actors avoid this resort for fear of negative popular opinion. Expropriation and compensation schemes, though necessary, are not the only means of enabling public bodies to access the privately owned land required to implement NBSs. There is potential for solutions, such as the formulation of shared (public/private) responsibilities for water management, construction of legal duties to act in a manner that promotes the interest of the community in the face of climate change, and co-ownership structures that could better enable access to private land while simultaneously ensuring that public opinion does not negatively perceive NBS implementation efforts.	Law.
What incentive structures are required to encourage public actors to look beyond grey infrastructural solutions in Dutch FRM and instead pursue NBSs?	The current ‘business-as-usual’ approach, which prioritises grey infrastructure for flood risk management, is widely regarded—both nationally and internationally—as highly successful. Given its proven effectiveness, long-standing implementation, and the established and familiar administrative procedures it offers public authorities, shifting toward NBSs will require the introduction of targeted incentives within the policy domain to redirect entrenched practices.	Law, political science, public policy, governance, and economics.
Which policy domains are involved in the implementation of NBSs and what existing institutional mechanisms can public actors utilise to enable coordination across the various policy domains required to operationalise NBSs in Dutch FRM?	The broader scope of NBSs thus requires coordination across multiple policy domains, which challenged by existing institutional fragmentation or ‘sectoral silos’. Relevant policy domains include FRM, water quality, nature conservation, nature restoration, recreation/tourism, and spatial planning, amongst others. Public actors require clear guidelines on what their coordination obligations are under prevailing law, and what mechanisms they may use to fulfil these obligations. Research should investigate both formal (law and policy) and informal (standard operating procedures and customs) institutional mechanisms.	Law, political science, public policy, and governance.
Which forms of NBSs present the most potential for successful application for FRM in The Netherlands?	Given the lack of consensus on which FRM to apply in which context, there is a need for research to formulate guidelines for decision makers on which NBS to prioritise in feasibility assessments. Not all forms of NBSs can be considered all the time. Guidelines can be tailored to specific regions, e.g., coastal, estuaries, rivers, and lakes.	Engineering, spatial planning, economics, and governance.
What are the economic implications (societal costs and benefits), both upfront and in the future, of implementing NBSs for FRM in The Netherlands?	Cost–benefit analyses and upfront investment cost estimates, as well as projected maintenance costs and economic quantification of co-benefits is required to better understand the financial implications and returns associated with NBSs.	Economics.
What social factors shape the existing (lack of) tolerance for NBS in The Netherlands, and how may these factors be tapped into to educate and promote a positive perception of NBSs for FRM?	Empirical investigation of the historical context shaping prevailing public perceptions, as well as the national, regional, and local culture(s) and identities (and the interaction thereof) that shape public perception. Empirical results may inform the development of means of promoting a positive public perception of NBSs, including through the use of the arts.	Broad range of social sciences and humanities, including the arts and cultural studies.

, below, proposes an initial research agenda to address these systemic knowledge gaps and serves to direct and inspire future research trajectories in this regard.

The centrality of the disciplinary fields of law, political science, governance, spatial planning, and economics is clearly illustrated in the above table. This is unsurprising given the limited presence of these disciplinary fields in prevailing research. As such, much of the outstanding knowledge gaps relate, in particular, to the regulatory, economic, and social–cultural implications of operationalising NBSs. This paper is intended not only to provide a roadmap for future research into the operationalisation of NBSs in Dutch FRM, both within and beyond The Netherlands, but also to encourage scholars in the aforementioned fields to contribute their disciplinary expertise to the development of the NBS state-of-the-art.

5. Conclusions

Despite its international reputation for excellence in FRM, this paper has shown that a prevailing ‘business-as-usual’ approach continues to impede the mainstreaming of nature-based solutions in Dutch FRM. While this observation applies broadly, effective implementation requires tailored approaches and decisions that account for regional differences, such as those between coastal, riverine, and estuarine systems. Although the past decade has seen a discernible shift in Dutch FRM towards more innovative, participatory, and diversified strategies, traditional grey infrastructure remains the default and dominant approach. This tendency persists despite growing empirical evidence supporting the viability and derivable co-benefits of NBSs.

To traverse the gap between theory and practical operationalisation of NBSs, this paper identifies seven core focus areas for further research, each targeting a specific barrier to operationalisation. These range from legal and policy readiness to economic feasibility and social legitimacy. The prominence of legal, political, economic, governance, and sociocultural dimensions across these research areas emphasises the need for deeper interdisciplinary engagement, with a focus on what feasibility and acceptability mean from these perspectives. While the Dutch case provides a unique starting point, many of the identified barriers—specifically, policy lock-in, fragmented institutional coordination, and the complexities of private land ownership—represent universal challenges. By systematically addressing these systemic obstacles, this research agenda serves not only to inform future research within The Netherlands but also provides a transferable framework for scholars and practitioners in other jurisdictions aiming to advance the operationalisation of NBSs in regional FRM.