A Framework to Evaluate the SDG Contribution of Fluvial Nature-Based Solutions

: Nature-based solutions (NBSs) are measures reﬂecting the ‘cooperation with nature’ approach: mitigating ﬂuvial ﬂood risk while being cost-effective, resource-efﬁcient, and providing numerous environmental, social, and economic beneﬁts. Since 2015, the United Nations (UN) 2030 Agenda has provided UN member states with goals, targets, and indicators to facilitate an integrated approach focusing on economic, environmental, and social improvements simultaneously. The aim of this study is to evaluate the contribution of ﬂuvial NBSs to the UN 2030 Agenda, using all its components: Sustainable Development Goals (SDGs), targets, and indicators. We propose a four-step framework with inputs from the UN 2030 Agenda, scientiﬁc literature, and case studies. The framework provides a set of ﬂuvial ﬂooding indicators that are linked to SDG indicators of the UN 2030 Agenda. Finally, the ﬂuvial ﬂooding indicators are tested by applying them to a case study, the Eddleston Water Project, aiming to examine its contribution to the UN 2030 Agenda. This reveals that the Eddleston Water Project contributes to 9 SDGs and 33 SDG targets from environmental, economic, societal, policy, and technical perspectives. Our framework aims to enhance the systematic considerations of the SDG indicators, adjust their notion to the system of interest, and thereby enhance the link between the sustainability performance of NBSs and the UN 2030 Agenda.


Introduction
According to the United Nations (UN) 2015 report, 'The Human Cost of Weather-Related Disasters' [1], flooding has negatively affected 2.3 billion people over the last 20 years. This accounts for 56% of all those negatively affected by weather-related disasters such as droughts, storms, landslides, and extreme temperatures (64.4 million/year). Especially for fluvial floods, the number of affected people under the most extreme river flooding scenario and without further adaptation may rise from 39 million people per year to 134 million people per year by 2050. Approximately two-thirds of this increase can be attributed to increases in the severity and frequency of flooding due to climate change and the remainder due to population growth in flooding-prone areas [2].
Rapid development combined with the expansion of infrastructure, agricultural intensification, transport, and other linked socioeconomic systems has increased society's vulnerability to environmental disasters, especially in floodplain areas [3]. At the same time, climate change is an important driver for implementing sustainable practices in protecting and managing river ecosystems. In this context, the UN 2030 Agenda [4] has provided international and national governments with goals, targets, and indicators to facilitate an integrated approach focusing on economic, environmental, and social improvements simultaneously. Since 2015, all UN member states are expected to pursue these Sustainable Development Goals (SDGs), tailoring a path towards a peaceful and prosperous planet.
Nature-based solutions (NBSs) can help in addressing many of the SDGs as established in the UN 2030 Agenda. The inclusion of natural elements could create manifold benefits for all the three pillars -'People', 'Planet', and 'Prosperity' which reflect the three sustainability principles (society, environment, economy) and are adopted by the UN 2030 Agenda. From a societal perspective, they could provide access to nature and recreation while adding cultural and heritage value to the landscape. From ecological and environmental perspectives, they could enhance biodiversity and contribute to water and air purification. From an economic viewpoint, they could promote sustainable and responsible resource management, resulting in cost-effective practices. In Europe, nature-based protection measures (green-blue-hybrid) have already gained increasing prominence in application [5][6][7][8]. Green/Blue infrastructure indicates a strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services. It incorporates green spaces (or blue if aquatic ecosystems are concerned) and other physical features in ter-restrial (including coastal) and ma-rine areas. Hybrid solutions mix hard infrastructure with ecosystem-based infrastructure (https://portals.iucn.org/library/sites/library/files/documents/2016-036.pdf, accessed on 10 October 2021). In global relevance, the effort to learn, implement and promote NBSs is worldwide and supported by many programs and data pools [9].
Sustainability is addressed in recent NBSs-related frameworks either with the inclusion of the three pillars in the assessment of the NBSs' performance or their (co-)benefits or by measuring the sustainability performance according to the components of the UN 2030 Agenda. Initially, studies such as those by ones of Artmann et al. [22], Pakzad et al. [23], Raymond et al. [20] showed that NBSs interact across and within society, economy, and environment. Building on that, subsequent studies (e.g., [18,21,24]) examined the potential contributions of NBSs to the UN 2030 Agenda by examining the SDGs and/or their targets. Schipper et al. [21] developed the Sustainability Impact Score (SIS) Assessment Framework, which uses a selection of SDGs and SDG targets to score the sustainability performance of coastal management projects. Whilst it is apparent that some of the recent frameworks address the SDGs and/or SDG targets from the UN 2030 Agenda, they omit consideration of the SDG indicators.
However, in scrutinizing the UN 2030 Agenda, it is noticeable that (i) often, SDG targets refer to multiple elements which are broken down into SDG indicators, and (ii) 12 SDG indicators are repeated (some with slight amendments) under different SDG targets. The SDG indicators (rather than the SDG targets) seem to have the right abstraction level to serve as a connection between the NBSs and the UN 2030 Agenda. Although reaching the SDGs in itself is a promising achievement in preserving our planet, as stated by the United Nations, using the SDG indicators could bring a new perspective to the effort of linking the NBSs to SDGs and hence to assessing the contribution of NBSs to the achievement of the SDGs.
To bridge the gap identified in the research to date, the aim of this study is to evaluate the sustainability performance of NBSs projects with respect to the UN 2030 Agenda, involving all its three components: SDGs, SDG targets, and SDG indicators. In other words, we seek the SDGs, SDG targets, and SDG indicators to which NBSs projects could contribute. We focus on NBSs projects for fluvial flood risk mitigation (FFRM) implemented in riverine ecosystems up to 100 km 2 , which is a smaller scale than that examined in riverine environments to date. Specifically, we aim to evaluate the sustainability performance of NBSs projects for FFRM by: (a) Creating a set of fluvial flooding indicators that reflect the interactions of NBSs for FFRM projects with societal, environmental, economic, policy, and technical perspectives; (b) Establishing a link between the set of fluvial flooding indicators and the SDG indicators; (c) Testing the fluvial flooding indicators by selecting a specific case study with the necessary project metadata.
We consider case studies from countries with high-income economies only, as NBSs projects in countries with upper-middle, lower-middle, and low-income economies typically aim to cover more fundamental needs, such as water quality and scarcity, and flood mitigation is seldom the main driver for NBSs implementation. Interaction of the river ecosystem with the coastal environment is out of the scope of this research.

Methodology
The methodology of this study uses the SIS Assessment Framework in a way that looks to build on the systematic methodology introduced by Schipper et al. [21], but with focus on river ecosystems, recognizing and introducing new elements reflecting the scope of the research. With the UN 2030 Agenda as a starting point, four steps are considered that eventually lead to the formation of the framework. Subsequently, the framework is presented along with the four steps through which our aim is accomplished and, ultimately, a case study to test its applicability.

The Sustainability Performance Evaluation Framework
The Sustainability Performance Evaluation Framework presented here is derived from the SIS Assessment Framework with the necessary alterations. It encompasses a systematic methodology for creating a set of fluvial flooding indicators, linking them to the SDG indicators, and evaluating the sustainability performance of an FFRM NBSs project through four steps. Starting with the components of the UN 2030 Agenda as input (Figure 1), Step I defines the fluvial flooding indicators which relate to NBSs for FFRM. Subsequently (Step II), the SDGs relevant to NBSs for FFRM, along with the respective SDG targets and SDG indicators, are selected from the UN 2030 Agenda. In Step III, Step I and Step II are brought together, creating a set of NBSs fluvial flooding indicators that demonstrate the FFRM NBSs' contribution to the UN 2030 Agenda. The definition of fluvial flooding indicators and their subsequent connection to the SDG indicators (instead of directly using the SDG indicators) makes it possible to overcome an apparent lack of conceptual clarity inherent in some of the SDG indicators due to their universal nature. Finally, Step IV consists of the assessment of the fluvial flooding indicators based on their application to a case study with available project metadata, with the sustainability performance of the project as the main output.

Step I-Definition of Dimensions and Fluvial Flooding Indicators
The scope of the sustainability evaluation is defined through the identification of the NBSs dimensions. The word 'dimension' is chosen above terminology such as 'property' or 'aspect' to emphasize the broadness of the NBSs (co-)benefits. The NBSs dimensions express sectors that are affected by FFRM NBSs projects in river ecosystems. For instance, floodplain ponds will, in addition to temporarily storing water during floods, provide habitats for wildlife and support biodiversity. Therefore, wildlife and biodiversity are two sectors that are affected by floodplain ponds and are expressed by the Environmental dimension in our study. The identification of the dimensions is based upon an analysis of 7 existing NBSs frameworks or reviews ( [14,15,20,22,[25][26][27]), complemented by the examination of three case studies (Table 1). Case studies are used in order to validate that the framework/review findings are realistic and to add any relevant findings that might have been neglected in the literature. The selection of the case studies was based on the following five independent criteria. The criteria are not prioritized; the order is indicative.

1.
The main objective of the NBSs should be fluvial flood risk mitigation; 2.
Coverage of different geographical regions and scales; 3.
Accessibility to relevant data, information, documentation; 5.
Availability of grey literature relevant to the case studies, to be used as an additional source of information, including published articles and videos.  [36][37][38][39][40][41][42][43][44][45][46][47] Each dimension consists of fluvial flooding indicators, as shown in Figure 2, that act as a metric that condenses complexity and provides relevant information [48]. The aim of the fluvial flooding indicators is to list specific effects that might occur in a dimension when implementing an NBSs project for FFRM. For instance, biodiversity abundance is a fluvial flooding indicator that can be found under the Environmental dimension ( Figure 2). A preliminary list of fluvial flooding indicators was created by: (i) Collecting existing indicators from literature. The collection of indicators comes from the 7 frameworks reviewed for the dimensions. However, starting with the already identified frameworks and using snowballing techniques, three additional frameworks were identified that also revealed additional indicators [23,49,50]. (ii) Using the indicators derived from (i) in analyzing the three case studies (Table 1), chosen to reflect a reasonable geographical coverage, spread in surface and in geomorphological aspects and with enough information at hand to quantify the indicators. In this process, the case studies gave rise to several new indicators that were not included in Step (i).

Step II-Selection of Relevant SDG Targets and SDG Indicators
The Sustainable Development Goals, targets, and indicators constitute very broad but versatile milestones that users may need to adapt depending on the context and their precise area of interest. As the goals themselves are very broad, the starting point for examination in this study is the 169 targets, followed by the 247 indicators. In reviewing these, the aim is to establish what they address and then select those SDG targets and SDG indicators that are relevant to NBSs for FFRM. For this purpose, a screening process has been developed ( Figure 3) to help select relevant SDG targets and SDG indicators according to (i) the boundary conditions (high-income economies, river ecosystem) and (ii) potential NBSs' contribution to them for FFRM. The latter, in particular, has been developed from insights gained from the literature review and case studies. The selection process is presented in Figure 3:

Step III-Connection of the SDG Indicators with the Fluvial Flooding Indicators
Having selected the relevant SDG targets and SDG indicators from the UN 2030 Agenda (Step II), the SDG indicators were connected with the preliminary list of fluvial flooding indicators, as formed in Step I. The connection was made at a conceptual level: matching the description of the fluvial flooding indicator with the SDG indicators. For instance, the biodiversity abundance fluvial flooding indicator was connected to an SDG indicator that addresses the presence and diversity of species.

Step IV-Assessment of the Fluvial Flooding Indicators Based on Project Metadata
Finally, the connected fluvial flooding indicators (Step III) are applied to a selected case study using project metadata as input to them. Through examination of the fluvial flooding indicators, the sustainability performance of a selected FFRM NBSs project with respect to the UN 2030 Agenda is evaluated.

Step I-Definition of Dimensions and Fluvial Flooding Indicators
Five dimensions are defined: Environment, Economy, Society, Policy-Procedural, and Technical. The Environment, Economy, and Society dimensions represent the three pillars of sustainability. These three pillars, along with the Policy-Procedural dimension, can all be found as broad divisions within the UN 2030 Agenda [51]. The Environment, Economy, and Society dimensions are also used by other frameworks addressing either NBSs sustainability or the additional benefits that NBSs bring [17,[19][20][21]52]. The Technical dimension is a new addition that is considered highly relevant because it refers to the fulfillment of the objective of the intervention (flood protection) and to characteristics that the intervention should comply with, including structural integrity, reliability, ease of implementation, adaptability, and resilience. The Technical dimension has recently been introduced in the literature. The study of Pugliese et al. [19] uses the framework introduced by PHUSICOS [53], where the Technical dimension is used as an ambit to examine the NBSs' technical and economic feasibility aspects. The Policy-Procedural dimension is usually found in the Society dimension. This is the case both in the EC Handbook for Practitioners [52], which places the 'Participatory Planning and Governance' under the People pillar of Sustainable Development, and in the PHUSICOS framework [53] that includes the 'Community Involvement and Governance', in the Society ambit. In the Sustainability Performance Evaluation Framework, we distinguished the Policy-Procedural dimension from the Society one, aligning with the UN 2030 Agenda [4], which devotes entire goals to partnerships (Goal 17) and inclusive collaboration (Goal 16).
A preliminary list of 32 fluvial flooding indicators was identified spread across the five dimensions (Environment 8, Society 5, Economy 5, Technical 6, Policy-Procedural 8) to describe the effects of NBSs for FFRM. Overall, 24 of the fluvial flooding indicators were collected from the NBSs frameworks and reviews examined; three emerged from the case studies, and five technical fluvial flooding indicators were introduced by the authors adjusted after Slinger, J.H. [54]. A detailed table with all the dimensions, their fluvial flooding indicators, and their use is presented in the description of Step III to avoid repetition. Table 2 shows the selection of 10 SDGs, 42 SDG targets, and 51 SDG indicators as relevant to NBSs for FFRM, as derived from the screening process. The selection starts by examining, one by one, all the SDG targets following the screening process ( Figure 3). For each SDG target that was considered relevant to the FFRM NBSs, at least one of its SDG indicators also had to be FFRM-NBSs-relevant. The explanation as to which SDG indicator is considered relevant is shown in the fourth column of Table 2 and is derived from our examination of the literature and case studies.

Step III-Connection of the SDG Indicators with the Fluvial Flooding Indicators
The preliminary list of fluvial flooding indicators was coupled with the relevant SDG indicators, resulting in 21 out of 32 fluvial flooding indicators being linked to various of the 51 SDG indicators. It was expected that not all the fluvial flooding indicators would be linked to the relevant SDG indicators since the intention was to match a targeted-to fluvial flooding-list specifically derived from the authors' examination of literature and practice to a universal solid agenda. However, since the UN 2030 Agenda is a universally recognized policy framework and the aim of this study is to examine the FFRM NBSs's contribution to it, the preliminary list was extended to cover all the relevant SDG indicators. A total of 12 fluvial flooding indicators were added in the Policy-Procedural dimension, which can be seen in Table 3, rows #29-34 and #39-44. Therefore, the final list comprises 33 fluvial flooding indicators coupled with all the 51 relevant SDG indicators.        Table 3 presents the five dimensions with their respective fluvial flooding indicators (first four columns) and their connection with the SDG indicators (last two columns).

Case Study: The Eddleston Water Project
The Eddleston Water Project was selected as a representative case study to test the use of the fluvial flooding indicators. Importantly for our assessment, the Eddleston Water Project adopts a multi-benefit approach to the use of NBSs aiming at (i) exploring whether flood risk can be reduced by means of NBSs, (ii) the use of NBSs for improving the ecological condition of the river, and (iii) working with landowners and communities to maintain and enhance sustainable land management practices and farm businesses. Furthermore, since the measures were implemented in 2013, preliminary outcomes from the monitoring campaigns are already available. Finally, it is also part of the EU North Sea Region (NSR) Interreg Building with Nature (BwN) program (https://northsearegion. eu/building-with-nature/), providing good links with experts if further consultation was needed (see also Appendix A).
The details of the Eddleston Water Project are summarized in the first column of Figure 4. A full description is available on the project website (https://tweedforum.org/ our-work/projects/the-eddleston-water-project/). The Eddleston Water Project started as a learning-by-doing project, which is successfully evolving and revealing valuable insights as to how a catchment approach reduces flood risk, involving both structural measures and natural flood management (NFM), and may help improve resilience to climate change. A key element throughout the project has been close stakeholder consultation because uptake of NBSs measures is voluntary, and all the locations for NBSs measures within the project catchment are privately owned. Local land managers and the wider community had been engaged from the very beginning of the project, and these and other stakeholders are still actively involved through regular meetings and surveys, ensuring productive continuation and uptake of the project. The present year (2021) was the end of the 5 years of matched funding for the Eddleston Water Project from the EU NSR Interreg BwN program. The Interreg program focused on assessing the costs and benefits of implementing NFM through improved monitoring and modeling. With ongoing support for the current phase (2021-2024) from the Scottish Government, and the participation of local farmers and landowners, the study continues with the implementation of different types of NFM measures across the catchment, alongside detailed hydrological and ecological monitoring. Some of the headline outcomes from the Eddleston Water Project so far are summarized in the second column of Figure 4. For a more extended description of the project, reference is made to the Eddleston Water Project Report [55]; to the paper on flood risk reduction [56]; and to the Tweed Forum website (see above) where all the reports, including those from the Building with Nature program, are made publicly available.

Step IV-Assessment of the Fluvial Flooding Indicators Based on Project Metadata
To assess the utility and effectiveness of the fluvial flooding indicators, they were applied to the Eddleston Water Project to examine its sustainability performance in terms of:

•
Whether the Eddleston Water Project contributes to the attainment of the SDGs, and, if so; • To which SDGs; • How and why.
The application was performed with input metadata from the Eddleston Water Project to the fluvial flooding indicators. The metadata for the Eddleston Water Project in Table 4 were collected from the Project Reports and [56][57][58][59]. The output of the evaluation is presented in Table 4, with three columns and the following format: each fluvial flooding indicator (Column I) contributes to none/one/or more SDGs and SDG targets (Columns II and III) as justified by the Eddleston Water Project metadata (Column IV). The contribution of the Eddleston Water Project to the UN 2030 Agenda is presented in Table 4 in terms of SDGs because this is more practical and easier to remember as a take-home message. However, by referring back to Table 3, it is possible to see the derivation and connection between the relevant SDG targets and respective SDG indicators. For instance, in Table 4, it can be seen that the Eddleston Water Project contributes to SDG 15 and SDG target 15.1, as assessed by the biodiversity abundance (fluvial flooding) indicator according to available project metadata. By referring back to Table 3   An increase in overall physical diversity of habitats within re-meandered sections and an increase in habitat area, both greater where there has been a greater degree of re-meandering Cultural heritage/educational value 12 12.8 The project works as a living laboratory, open to public and to schools for raising awareness of flooding in the area and encouraging pupils and teachers to take an active part in the project and learn about their catchment. Additionally, interpretation boards enhance the commercial use of the area. Finally, as a publicly funded Research Platform, the river is the location for many research projects from universities and academic institutions 10 Recreation/leisure value 11 11.7 Soon, a multi-use track (biking, walking) will be constructed on the old railway line, which will attract even more people for recreation 11 Enhance attractiveness 11 11.7 Interpretation boards to be produced along this new path will improve the recreational side of the Eddleston Full details of the economic costs and benefits of the implementation of NBSs measures have been analyzed as they impact farm income and profitability in the case study area based on land use data, agricultural and environmental support subsidies, and foregone farm income The salmon fishery of the Tweed is worth a total of over 24 million GBP a year to the local economy and supports over 500 jobs, so any improvement to fish habitat is important. Although salmon fishing is predominantly on the main Tweed River, Eddleston Water and similar tributaries are vital as breeding and nursery locations for salmon Modeling for a range of climate change scenarios shows a positive net present value from NFM tree planting, indicating that the riparian woodland is worth implementing. Annual benefits of c. 80 k GBP per year were estimated, with a high average benefit-cost ratio for the riparian woodland, though full benefits will not be realized for some 15 years after implementation Direct measurements on the ground of the value of a range of ecosystem services/multiple benefits already delivered as part of the NFM measures is an additional GBP 4. Where possible, interventions are made of local timber from recently felled trees in the forest. An exception was for the rocks protecting the meander where it approaches the road, which were imported Large woody structures: on the Middle Burn, nearby conifers were felled and pinned across the channel Woodland and riparian woodland planting with native trees: species included oak, ash, willow, birch, aspen, and hazel 17 Adaptability 11, 13 11.a, 13.2 Although measures put in are seen as permanent, they are all subject to natural ecological and hydrological processes, and thus they will eventually need replacement. Potential change in the land of the area must be feasible, and project managers must be willing to facilitate and work with the landowners for land-use changes 18 Different stakeholders/disciplines involved 11, 12 11.3, 12.7 Landowners are key, and to date, 25 farmers and landowners have been involved, and 19 have hosted measures on their land.
The Tweed Forum acts as project managers with Scottish Government, SEPA, Scottish Borders Council, Dundee University, and British Geological Survey. Others include Peebles Community Council, Forest Commission Scotland, Environment Agency, Scottish Natural Heritage, and National Farmers Union (Scotland) 19 Planning/participatory processes 6, 12 6.b, 12.7 Shared policy development and implementation: as a 'Trusted Intermediary', Tweed Forum spent significant time and effort informing and engaging with the local community and landowners in framing the project prior to implementation; regular meetings and presentations with the Peebles Community Council; interviews with landowners; leaflet to locals outlining and explaining aims of the project before the start of it; hands-on participatory engagement at local shows; questionnaire survey for the implemented measures Integrated water resources management 6 6.5 The Eddleston Project adopted an integrated catchment approach across all aspects of water resource management since this underpins the project approach to address the 'sources-pathways and receptors' contributing to flood risks 22 Transboundary water cooperation --Not a transboundary water project 23 Capacity-building for development actions 13 13.3 Eddleston is a small catchment where a specialized focus and strengthening of locals' interest and involvement was needed for the realization of the project. This was achieved through participatory processes and engagement strategies Generally, a partnership approach has been followed, and Tweed Forum has brought together the landowners, the community, and the project experts Overall, the Eddleston Water Project contributes to 9 SDGs: 1, 6,8,9,11,12,13,15, and 17, and to 33 SDG targets, as can be seen graphically in Figure 5. Figure 5 complements Table 4 since it shows all the relevant SDGs and SDG targets (as established from Step II). The SDG targets in bold black color are the ones that the Eddleston Water Project contributes to, while in red, the ones that it does not. We showed that the Sustainability Performance Evaluation Framework follows a systematic methodology that allows to identify the interactions of the Eddleston Water Project within the five dimensions and define fluvial flooding indicators, which showed the Eddleston Water Project's contribution to the UN 2030 Agenda. Table 5 presents the Eddleston Water Project's SDGs under their respective dimensions. As expected, most of the Eddleston Water Project's SDGs contribute to multiple dimensions at the same time due to the repetition of some of the SDG indicators under several SDG targets. Figure 5. NBSs for FFRM could address 10 SDGs (legend) and 42 SDG targets (circle), as set by the UN 2030 Agenda. The framework application to the Eddleston Water Project revealed that the Eddleston contributes to 9 out of these 10 SDGs (not to the third) and to 33 SDG targets (not to the ones in bold red color in the outer circle). The Sustainability Performance Evaluation Framework was developed from literature review, insights from three case studies, and then applied to one independent case study. On the one hand, this enabled us to assess the performance of the fluvial flooding indicators in depth, as we tried to find and access metadata for each fluvial flooding indicator. On the other hand, application to more case studies would have provided more insights that could extend or alter the NBSs-SDG coupling. For instance, the research showed that SDG seven, which assesses energy resources, could potentially be linked to the fluvial flooding indicators. More specifically, the Noordwaard polder project provided some clues about energy production from biomass, but the other case studies did not (https: //www.ecoshape.org/en/cases/wave-attenuating-willow-forest-noordwaard-nl/), and thus SDG seven was not included in the link between NBSs and SDGs. A broader case study examination will provide further insights regarding the potential of energy production in NBSs for FFRM interventions.
For data-dependant frameworks, such as the Sustainability Performance Evaluation Framework, data accessibility and/or method availability are key factors. Literature indicators such as 'population viability', although meaningful in the context of NBSs, are difficult to measure in practice. Similarly, within the Tier Classification for the Global SDG Indicators [60], Tier three includes the need for new or re-examination of existing measuring methods. Shah et al. [12] recognize the need for making the level of information even more local and specific alongside primary data collection for local NBSs or their indicators. From a more general point of view, Kumar et al. [10] state that the challenge of inadequate or insufficient data hinders the acceptance, assessment, and potentially successful operationalization of NBSs. Recent studies have addressed this challenge; Schröter et al. [9] provide extensive lists of online data pools on NBSs, and the EC's Handbook for Practitioners [52] devotes a chapter to types of data, data sources, and data generation techniques for NBSs monitoring and impact assessment. Therefore, current sources seem to allow for the effective use of such data-based frameworks while acknowledging the need for new or enhanced measuring methodologies.
The output of the Sustainability Performance Evaluation Framework is qualitative. We acknowledge, however, the potential to extend it to a quantitative one. Quantitative outputs, such as scored evaluation against pre-defined targets, enhance the evidence base of NBSs effectiveness [13]. To date, several studies provide quantitative results regarding effectiveness, co-benefits, and NBSs' sustainability contribution. Schipper et al. [21] provide a methodology for scoring the sustainability performance of coastal management projects using numeric data. Pugliese et al. [19] apply a multi-criteria tool to assess the effectiveness of NBSs for a specific case study compared to a grey alternative. Martín et al. [18] use qualitative analysis of Fuzzy Cognitive Maps (FCM) alongside semi-quantitative analysis of the co-benefits to examine the effectiveness of different NBSs and their co-benefits. Liquete et al. [17] perform an ex-post assessment of the environmental, social, and economic benefits of multi-purpose NBSs for water pollution control based on Multi-Criteria Analysis (MCA). The proposed framework is used to couple to the SDGs but can (with minor changes) also be used as an independent framework for the ex-post evaluation of individual NBSs projects, as a tool to compare grey-green alternatives for an NBSs project, or even as a tool to compare different NBSs projects. Andrikopoulou [61] describes the development of such a framework. We speculate that application of such a framework to a 'Room for the River' project, where pre-defined targets for the Rhine's conveyance capacity were set (https: //www.tandfonline.com/doi/abs/10.1080/02508060508691839), would have resulted in a better understanding regarding the potentials of the Sustainability Performance Evaluation Framework to derive a scored evaluation. A Multi-Criteria Analysis would both bring the framework output closer to reality and better assist decision makers in prioritizing river management options. In this research, flood safety was the primary river function under consideration, while ecosystem development, water quality, and recreation issues were also examined. However, rivers typically have a larger number of functions (e.g., water supply, navigation, water quality, nature development), the importance of which may have greater or lesser weighting at any one time and location, and to different stakeholders, depending on the focus of the project. Therefore, prioritization of the fluvial indicators based on the primary river functions of a specific case would greatly benefit the framework output.

How Do the Outcomes of Our Framework Relate to Other Relevant Studies?
We have set up a framework to create a set of fluvial flooding indicators to evaluate the ex-post contribution of the Eddleston Water Project to the UN 2030 Agenda. Ligtvoet [2] and Ge et al. [62] have also explored the relationship between rivers and the SDGs. Ge et al. [62] have defined SDGs for river basin scale in terms of water, ecosystems, and socioeconomic capabilities. Ligtvoet [2] has identified those SDGs related to people and the economy that are negatively affected by river flooding.
Framework application to the Eddleston Water Project shows that it contributes to six out of the nine SDGs mentioned in Ligtvoet [2]. In the study of Ligtvoet [2], Ward and Winsemius have identified the negative effects of fluvial flood risk on SDGs 1, 2, 3, 6, 8, 9, 10, 13, and 16 for people and the economy (specifically on agriculture). Although Ward and Winsemius established a broader link (river flood risk and SDGs) than ours (NBSs for FFRM and SDGs), the outcomes of our framework application show that the Eddleston Water Project can ameliorate most of the negatively affected SDGs by river flooding. Exceptions constitute SDGs 2, 10, and 16. SDG two addresses food security issues and agricultural practices, which were considered out of scope for the functions of fluvial flood risk mitigation NBSs considered in this research. SDG 10 refers to the reduction of inequalities between countries in terms of providing the same means of coping with flood risk between high-income, upper-middle, lower-middle and low-income economies. NBSs could address such an aspect; however, it needs examination in a broader context combined with geopolitical considerations. Similarly, SDG 16 talks about justice and inclusivity in societies, which can be promoted as part of the general NBSs conceptual framing but are out of scope for this study. Leaving these aside then, it is apparent that the Eddleston Project positively affects the following 6 SDGs: 1, 3, 6, 8, 9, 13. The SDGs to which the Eddleston Water Project contributes align with seven out of eight SDGs proposed by Ge et al. [62]. Ge et al. [62] link river basins (e.g., Amazon, Nile, and Heihe river basins) to SDGs in terms of water, ecosystem, socioeconomic, and ability-related issues, and they find all SDGs relevant apart from SDG seven. The water-related SDGs (6,11,12,14) and the ecosystem-related SDGs (14,15) coincide with the SDGs derived from the present Eddleston Water study, apart from SDG 14 (which was out of scope for the current research context because it focuses on the coastal environment). The socioeconomic-related SDGs were omitted because they focus mostly on food security, justice, and inequalities which, whilst they might be added in other situations, were not within the scope of the impacts of the fluvial NBSs examined for the present research. However, although they were not specifically considered, it could be argued that by including and elaborating on the third aim in the Eddleston Water Project's objectives (working with landowners and communities to maintain and enhance sustainable land management practices and farm businesses), the project contributes to an element of this SDG. The ability-related SDGs (9,11,13,17) coincide with the ones derived from our research because they refer to structural actions and strategies for conserving and protecting the rivers. Hence, SDGs 6,9,11,12,13,15,17 are shared between Ge et al. [62] and the Eddleston Water Project with respect to water, ecosystem, and ability-related issues.

Standardization and Scale of Sustainability Assessments
As mentioned by Pohle et al. [63], standards, in general, have a twofold role: they can serve as a source of information and enabler for the development and transfer of technology. The EU Research and Innovation program Horizon 2020 has recognized standardization as a measure that underpins innovation since it bridges the gap between research and the market but also facilitates the propagation of research outcomes to the European and international markets [64]. That said, standardization of sustainability assessments could provide harmonization in indicators, reliability and transparency in calculation methods, and comparability of results [65]. To date, standardization for sustainability assessments has already been discussed in the literature [66][67][68]. Although most of the studies recognize the aforementioned benefits of standardization, they also recognize the risk of compromising agendas, contexts, and needs when treating larger scales. Similarly, in the present study, although the aim was to derive indicators as widely applicable as possible, given the scope of the study, we acknowledge that most of the indicators would need further consideration when used in projects of different scales and contexts. Some of the indicators, e.g., the environmental and technical indicators, could relatively easily be standardized -some with slight amendments-for larger scales. For others, such as the Social, Policy-Procedural, and Economic indicators, this is more difficult because they are geopolitically dependent. For instance, in a transboundary water body apart from the international legislation, the in-between the country-member treaties and arrangements should also be considered. Such a view was out of scope in our research and thus requires further research.
Sustainability assessments should be able to be carried out at any scale. However, the necessary data are not always available or accessible at any scale. For instance, it is likely that the Sustainability Performance Evaluation Framework cannot be applied globally to any NBSs project due to the lack of data. Most of the data collected for such projects either suit national aims, which do not always align with the global SDG indicators, or come from private sources. Therefore, it seems that currently, the main challenge lies in finding adequate, available, and accessible data to upscale the sustainability assessments, and although there is still a lot to accomplish in this direction, the EC's Handbook for Practitioners [52] and Schröter et al. [9] have made promising steps (as discussed in paragraph 4.1).

Conclusions
The aim of this study is to propose a ready-to-use methodology to evaluate the sustainability performance of Nature-Based Solutions (NBSs) with respect to the United Nations (UN) 2030 Agenda, involving all its components (i.e., Sustainable Development Goals (SDGs), targets and indicators). This was achieved by building on the Schipper et al. [21] systematic framework and adjusting it to fit our needs. The focus is on NBSs for Fluvial Flood Risk Mitigation (FFRM) in river basins of sizes up to 100 km 2 . The derived framework is called the Sustainability Performance Evaluation Framework. It encompasses four steps through which the end-user creates a set of fluvial flooding indicators that can then be linked to the SDG indicators, and by applying the fluvial flooding indicators to a specific FFRM NBSs project, it is possible to ascertain the project's contribution to the UN 2030 Agenda. The Eddleston Water Project was used as a case study to test the effectiveness of the fluvial flooding indicators. Application to the Eddleston Water Project has shown that it contributes to 9 SDGs and 33 SDG targets. In developing the Sustainability Performance Evaluation Framework and testing its fluvial flooding indicators, the findings are: 1.
The Sustainability Performance Evaluation Framework can systematically consider SDG indicators by exploring potential interactions of NBSs for FFRM projects within five chosen dimensions: economy, environment, society, policy, and technical.

2.
Through the Sustainability Performance Evaluation Framework, it is possible to adjust the SDG concept to the system of interest and qualitatively measure its alignment with and progress towards the SDGs.

3.
Data availability and accessibility play a crucial role in the Sustainability Performance Evaluation Framework. Although potentially challenging in some situations, many NBSs programs and projects have been funded by the European Union (EU) or national governments and agencies, and data are typically available either publicly or upon request.
To further develop the Sustainability Performance Evaluation Framework, a key recommendation is its trial application in other areas and by different end-users. This should focus on three aspects:

•
Application to projects where quantified targets pre-exist would help enable the derivation of some form of scored evaluation.

•
Application to different case studies in terms of scale, location, and type of measures (e.g., projects in upland rivers and transboundary projects) is suggested.

•
Application to case studies in countries with upper-middle, lower-middle, and lowincome economies, with different cultural contexts, legal frameworks, governance structures, challenges of environmental justice, and data scarcity would add value. This recommendation, combined with the previous one, would also shed light on the potentials of the proposed indicators to be standardized.

•
Application with end-users, stakeholders, or even people unfamiliar with NBSs and SDGs, to examine whether the framework would yield the same indicators and/or the same result regarding the evaluation of the NBSs project (regarding its contribution to the SDGs).
We recognize that the more the framework is reviewed and applied, the more insights will be gained with respect to its biases, limitations, and gaps, including opportunities that could extend or alter the NBSs-SDG coupling. Data Availability Statement: For the framework application, data were mostly collected from publicly available sources, with a few of them from expert consultation. Details regarding data availability and accessibility for the Eddleston Water Project are available in Appendix A.
Acknowledgments: The research behind this paper was a collaboration of TU Delft and Rijkswaterstaat in The Netherlands. It was funded by Rijkswaterstaat and greatly benefited from discussions with TU Delft and RWS colleagues, examining nature-based projects and their potentials with respect to the UN 2030 Agenda. The EU Interreg Building with Nature program's projects were an inspiration and key evidence-contributor to the paper. We would also like to thank the Tweed Forum that provided all the information needed for the Eddleston Water Project, as well as the external reviewers for their time devoted to this manuscript since we feel their comments helped to improve the quality of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A. Data Availability and Accessibility for the Eddleston Water Project
One of the main reasons for choosing the Eddleston Water Project was the data availability and accessibility. Indeed, most of the fluvial flooding indicators (85%) were filled in with data publicly available online, while only 15% of the indicators needed a project specialist -either to verify data found online or to provide additional information. Expert consultation was needed for the following five fluvial flooding indicators: CO 2 emissions, recreation/leisure value, enhance attractiveness, employment, and adaptability. Both qualitative and quantitative data were gathered since not all indicators required a numerical value -for example, those covering the Policy-Procedural dimension. On the contrary, fluvial flooding indicators such as 'well-being' or 'extent of water-related ecosystems' could be filled in with quantitative data.
During the framework application, a few overlaps of the project metadata per fluvial flooding indicator were observed. For example, in looking at the 'recreation/leisure value' and 'enhance attractiveness', similar data were used for both fluvial flooding indicators. Although the attractiveness of the area has been enhanced and it is being used by the public, new plans for a cycleway will further increase its recreational value. To this end, there is a limited extent of data for these two indicators, leading to their current overlap. However, for a case study where all the interventions had been finalized, these two indicators might provide different information.