What does it take to renature cities? An expert-based analysis of barriers and strategies for the implementation of nature-based solutions

Journal of Environmental Management 354 (2024) 120385 Available online 20 February 2024 0301-4797/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Research article What does it take to renature cities? An expert-based analysis obarriers and strategies or the implementation onature-based solutions Joana A.C. Castellar a , b , c , 1 , Lucia Alexandra Popartan d , 1 , Bernhard Pucher e , * , Rocío PinedaMartos  , Katharina Hecht g , Evina Katsou h , Chrysanthi Elisabeth Nika h , Ranka Junge i , Günter Langergraber e , Nataˇ sa Atanasova j , Joaquim Comas b , d , H` ector Monclús d , Josep PueyoRos a , b a University oGirona, Girona, Catalonia, Spain b Catalan Institute or Water Research (ICRA-CERCA), Emili Grahit 101, 17003, Girona, Catalonia, Spain c CETAQUA, Water Technology Centre, Crta. Esplugues, 75 08940, Cornell` a de Llobregat, Barcelona, Spain d LEQUIA, Institute othe Environment, Universitat de Girona, c/ Maria Aur` elia Capmany 69, 17003, Girona, Catalonia, Spain e University oNatural Resources and Lie Sciences, Vienna (BOKU), Department oWater, Atmosphere and Environment, Institute oSanitary Engineering and Water Pollution Control, Muthgasse 18, 1190, Vienna, Austria  University oSeville, School oAgricultural Engineering (ETSIA), Department oAerospace Engineering and Fluid Mechanics, Urban Greening and Biosystems Engineering Research Group (NatUrIB), Ctra. de Utrera, km.1, 41013, Seville, Spain g Department oBiology/Department oReal Estate &Campus, Utrecht University, Padualaan 8, 3584 CH, Utrecht, Netherlands h Department oCivil &Environmental Engineering, Institute oEnvironment, Health and Societies, Brunel University London, Uxbridge Campus, Middlesex, UB8 3PH, Uxbridge, UK i Institute oNatural Resource Sciences, ZHAW Zurich University oApplied Sciences, Grüntal, 8820, W¨ adenswil, Switzerland j University oLjubljana, Faculty oCivil and Geodetic Engineering, Jamova 2, 1000, Ljubljana, Slovenia A R T I C L E I N F O Handling Editor: Jason Michael Evans A B S T R A C T This paper uses an expert-based methodology to survey the barriers and strategies related to the implementation onature-based solutions (NBS). The ambition othe paper is to oer a bird’s eye overview othe diculties encountered by NBS deployment and ways to overcome them. With a wide participation o80 experts rom COST Action Circular City, we identiy barriers specic to 35 pre-dened NBS othe ollowing our categories: Vertical Greening Systems and Green Roos; Food and Biomass Production; Rainwater Management; and Remediation, Treatment, and Recovery. The research sheds light on how a major interdisciplinary –yet predominantly technically-oriented - community oscientists and practitioners views this important topic. Overall, the most relevant barriers are related to technological complexity, lack oskilled sta and training programs and the lack oawareness that NBS is an option. Our results highlight concerns related to post implementation issues, especially operation and maintenance, which subsequently aect social acceptance. The paper identies a “chain” eect across barriers, meaning that one barrier can aect the existence or the relevance oother barriers. In terms ostrategies, most othem target governance, inormation, and education aspects, despite the predominantly technical expertise othe participants. The study innovates with respect to state-o-the-art research by showing a ne-grained connection between barriers, strategies and individual NBS and categories, a level o detail which is not encountered in any other study to date. 1. Introduction Nature-based solutions (NBS) are stand-alone green technologies, green urban spaces or techniques to support natural processes. In 2021, the European Commission reported that its Horizon 2020 Research and Innovation Program had invested over 400 million euros in NBS projects (European Commission, 2022), while the United Nations called or doubling the investment in NBS by 2025 (United Nations Environment * Corresponding author. E-mail address: bernhard.puch[email protected] (B. Pucher). 1 Equally contributing authors. Contents lists available at ScienceDirect Journal oEnvironmental Management journal homepage: www.elsevier.com/locate/jenvman https://doi.org/10.1016/j.jenvman.2024.120385 Received 30 October 2023; Received in revised orm 26 January 2024; Accepted 9 February 2024 Journal of Environmental Management 354 (2024) 120385 2 Programme, 2023). Despite this avourable policy context, the implementation oNBS on a large scale is lacking (Elmqvist et al., 2013; Kuban et al., 2019;The Nature Conservancy and Gobierno de Espa˜ na, 2019). While there is an emerging literature researching the barriers to NBS implementation, only ew articles oer a wide overview othe topic (e.g. Kabisch et al., 2016;Sarabi et al., 2020), while others ocus on specic categories oNBS, such as food management (Raˇ ska et al., 2022) or on specic purposes and concepts such as circular economy (CE) (Wirth et al., 2022). Others have identied barriers and strategies related to specic ecological domains (green buildings and roos, urban green space, community gardens and urban agriculture, green-blue inrastructure) but mainly rom a specic angle such as nancing (see or instance Toxopeus and Polzin, 2021). Systematic studies on strategies to overcome barriers or implementing NBS at urban scale are even more scarce. The goal othis paper aims to oer a broad overview othe di- culties encountered by NBS deployment and ways to overcome them, while bringing the level oanalysis down to individual NBS. For this, we used a mixed method oexpert workshops and qualitative interpretative analysis, with the participation o80 members othe COST Action CA17133 “Circular City”. We analysed barriers and strategies across a set o35 NBS identied by Castellar et al. (2021) and Langergraber et al. (2021a) and our NBS categories: Vertical Greening Systems and Green Roos; Remediation, Treatment, and Recovery; Rainwater Management; and Food and Biomass Production, which represent the working groups othe COST Action (Langergraber et al., 2020). The paper is guided by the ollowing research questions: 1) What are the most relevant barriers and strategies or the implementation oNBS in cities? 2) Which barriers are specic to some categories oNBS and which, by contrast, are more transversal? 3) What are the most generalizable strategies across dierent barriers to NBS implementation? The results othis article are expected to have a practical application or the guidance ourban planners and practitioners on the process oplanning and implementing NBS or circular cities. To our knowledge, this is the rst study obarriers and strategies or NBS implementation which combines a ne-grained approach to individual NBS with a more general assessment oNBS categories. Finally, he paper provides a data illustration tool, which allows the visualisation othe relationship between the individual NBS, their categories and then linking barriers and strategies. 2. Materials and methods The study was carried out in the ollowing our blocks: (i) Identi- cation obarriers and strategies or NBS implementation, developed through two elicitation expert workshops within the COST Action CA17133 Circular City (Section 2.1); (ii) Data curation using a Microsot Excel-based database, and text analysis via coding (Section 2.2); (iii) Data analysis and statistics (Section 2.3); (iv) Literature review (Section 2.4); and (v) Development oa data illustration tool (Section 2.5). 2.1. Expert-based systematic approach or identication obarriers and strategies or NBS implementation Two virtual elicitation workshops – adapted rom the IDEA protocol (“Investigate”,“Discuss”,“Estimate”,“Aggregate” (Hemming et al., 2018)– were organised. The rst workshop was ocused on the identi- cation obarriers or the implementation oNBS at city scale; while the second workshop aimed to identiy strategies to overcome the identied barriers. In addition, 3 barriers identied by Sarabi et al. (2019,2020), were also included in the discussions. The workshops were conducted online on February 17th, 2021, and March 18th, 2021, with a total o80 and 78 experts, mainly comprising researchers and urban planners ocused on NBS rom across Europe. The majority othe experts had a background on civil, sanitary, environmental, and agricultural engineering (59%), ollowed by chemistry, biotechnology, biology, geosciences, and geology experts (22%), while architecture, urban, landscape, and rural planners accounted or 15%, and social sciences represented 4% othe attendees The participation in both workshops was highly consistent, with almost all experts present in the rst workshop also attending the second one. Only three experts participated in just one workshop. By presenting the results othe rst workshop in the second workshop, participants had the opportunity to build upon the collective knowledge and engage in productive discussions. This sequential approach allowed or a more comprehensive exploration othe topic and encouraged in-depth discussions among the participants. Both workshops were organised in our topic-specic parallel sessions.: (1) Vertical Greening Systems and Green Roos; (2) Remediation, Treatment, and Recovery; (3) Rainwater Management; and (4) Food and Biomass Production. Those categories represent the our working groups othe COST Action “Circular City” (Langergraber et al., 2020). Barriers and related strategies were identied or a total o35 NBS (Langergraber et al., 2021a). In the rst workshop, experts identied barriers or specic NBS and assessed their “relevance levels”, representing the signi- icance othe barriers. In the second workshop, they tailored strategies to overcome these barriers and expanded the scope to address additional challenges. For this the metric “level odiculty” is used, representing how dicult the implementation oa strategy is perceived. Notably, experts also discovered new links between barriers and NBS, supplement the results rom the rst workshop. 2.2. Data curation The results obtained rom both workshops were organised in a Microsot®Excel-based database. Each unit oanalysis (expert statement/post-it rom the workshops) was coded, using an inductive method by assigning meaning (codes) to each statement (Chandra and Shang, 2019). The collected data was analysed in our consecutive coding sessions. A set ocodes was developed by an interdisciplinary team or both barriers and strategies. Another team revised the proposed list ocodes. During this second session, some codes were merged to avoid content overlapping. Both teams kept notes during the interpretation process and disagreements were discussed in a third session. The experts’ team agreed on the interpretation odata, and the nal list ocodes or both barriers and strategies. Each data entry (one Excel line) contained the ollowing inormation: NBS category, NBS, expert statement (post-it), barrier and respective level orelevance, strategy and respective level odiculty and related examples (also codied rom the experts’ statements). Likert-scale (low =1, medium =2, high =3) was used, or both relevance, and diculty scoring. Finally, barriers and strategies were described based on the experts’ statements during the workshops and authors’ expertise. 2.3. Data analysis and statistics The data analysis included the development o2 indicators or both, barriers (eqs. (1) and (2)) and strategies (eqs. (3) and (4)), to, provide a detailed understanding or the implementation oNBS in cities. Barrier Weighted Relevance (BWR): refects the signicance o each barrier, as assessed by experts. It is derived by averaging the relevance scores oequal combinations (NBS - Barriers) while considering the number olinked NBS categories as weights. In cases where NBS - Barrier combinations were not scored, we utilised the average score othe corresponding NBS category. BWR = 1 Nj∑ n j=1 Rij (1) where, R ij is the relevance obarrier ior category j; and Nis the total number oNBS categories. Barrier Transversality Index (BTI): reers to the requency oa J.A.C. Castellar et al. Journal of Environmental Management 354 (2024) 120385 3 certain barrier across dierent NBS categories. The index otransversality is calculated as the number oNBS categories obarrier i, divided by the total number oNBS categories. BTI = 1 Nj∑ n i=1 ji(2) where jdenotes category; and idenotes barrier; and N j is the total number ocategories. Strategy Transversality Index (STI): reveals the requency o strategies across dierent NBS categories and barriers. Thus, a higher STI value indicates a greater number odierent barriers and NBS categories that can be addressed simultaneously. The index otransversality is calculated as the number oNBS categories ostrategy i, multiplied by the number obarriers ostrategy i, divided by the total number oNBS categories multiplied by the total number obarriers: STI = 1 NjNk∑ n i=1 jiki(3) where jdenotes NBS category; kdenotes barrier; and idenotes strategy. Strategy level odifculty (SLD): stands or the level odiculty associated with implementing a certain strategy, as assessed by the experts. The workshops’ participants reported the diculty oimplementing a given strategy with a Likert-scale rom 1 to 3. Thereore, the diculty oimplementing a strategy is calculated as the mean diculty reported by the participants. We scaled the values rom 0 to 1 to allow comparisons with the STI: SLD = 1 5N∑ n i=1 di(4) where d i is the diculty stated by strategy i; and Nis the number o times that the diculty or that strategy was reported in the second workshop. 2.4. Literature review In order to systematically compare our results with the current scientic literature, a comprehensive systematic review in the Scopus database as o20th July 2023 was carried out. Two parallel searches, one ocusing on articles discussing barriers or NBS implementation in cities and one related to strategies was conducted. The search was limited to title, abstract, and keywords. For both searches, we used the keywords “nature-based solution*" OR “nature-based solution*" AND “implementation” AND “urban” OR “city” OR “cities.” For barriers and strategies, we employed the ollowing keywords, respectively: barrier* OR constraint* OR limitation* OR challenge*; opportunit* OR enabler* OR strategy*. The was narrowed to articles and reviews in English published between 2016 and 2023. To establish the baseline or 2016, we reerred to the article with the highest citation count, which was 587 (Kabisch et al., 2016). Next, we calculated the average citation count o articles and reviews, using this average as a benchmark, we excluded articles with ewer citations than the calculated average. As the next step, the authors reviewed the abstracts and ull text (ineeded) o preselected articles. A control procedure was implemented, whereby 10% othe articles were randomly read and annotated by two othe authors to ensure consistent choices by the team. To assess the relevance oan article, we considered whether its results identiy and/or discuss barriers/strategies or the implementation oNBS – within our our categories – in cities. Ithe article addresses the barriers/strategies related to NBS in any othese categories, we selected it or urther consideration. This rigorous process ensured that we include only the most relevant and impactul articles in our systematic review. To analyze the literature data, we systematically extracted and organised the barriers and strategies identied by other authors and compared them to our results. Our aim was to detect and discuss both shared and distinctive aspects. 2.5. Development odata illustration tool The gathered results were imported to Kumu. io (https://kumu.io/p rivacy) as a csv-le ater being organised in a tabular ormat in Microsot Excel. The baseline table consists o280 lines and our columns representing (1) individual labels containing the collected statements rom the workshops each categorised into types oinormation (2) ‘Annotation’, ‘Barrier’, ‘Category’, ‘Example’, ‘Nature-based solution’, ‘Strategy’, (3) tags with a urther theme-based categorization othe barriers and nature-based solutions, and (4) a description obarriers, categories, nature-based solutions, and strategies. Each label is presented as an element in the data illustration tool coloured by its respective type allowing or easy identication. The size othe elements containing barriers and strategies is adjusted according to their BTI and STI scores. When hovering over an element’surther inormation, including the tags, descriptions, BTI, BWR, SLD, STI, and reerences, can be retrieved rom a sidebar. For the ull description readers are directed to the supplementary material. 2.6. Potential limitations This paper uses an expert-based approach, which has been used in the past to perorm analysis obarriers and enablers oNBS (Raˇ ska et al., 2022). The value othis approach does not reside in its ability to produce ‘objective’ results – which may be seen as a limitation - but in oering a consistent account oexpert knowledge, which can be used as a valuable guidance or policy-making and urther research. While the results do emerge rom subjective opinion othe participants, this opinion is nonetheless based on expertise. To compensate or the potential bias o the method, several measures were undertaken: (i) a careul selection o Circular City experts as participants and moderators; (ii) the manuscript has at least one expert author per NBS category, possessing a vast knowledge in the specicities oeach category, while including social scientists or a holistic perspective across. Hence, it is assumed that the analysis supported by this constellation oexpertise allowed or a su- cient capacity to critically interpret, and account or potential biases when discussing the results othe workshops; (iii) With the incorporation othe literature review potential limitations due to the biases o experts are addressed. 3. Results and discussion 3.1. The data illustration tool The data illustration tool contains a dataset with over 4000 entries. Through a 5-level relationship diagram, our tool systematically showcases the interconnectedness between NBS, their NBS categories, barriers, strategies, and examples. The 5-level relationships diagram consists o275 elements, creating a total o815 connections across the ve levels ointeraction. The 35 nature–based solutions had a total o 430 connections to the 24 barriers, while the barriers were linked via a total o152 connections to potential strategies to overcome the barriers. 196 links were identied between 192 examples and a total o19 strategies. This result indicates that several barriers can be encountered or a NBS and more than one strategy can be applied to overcome a barrier. Our tool oers valuable insights on the topic to non-academic end-users, including urban planners and decision-makers, by providing a visual and structured representation opotential barriers and strategies to oster NBS implementation. A ull description is provided in the supplementary material. 3.2. Description and analysis obarriers Based on the results, 24 barriers or the implementation oNBS at the J.A.C. Castellar et al. Journal of Environmental Management 354 (2024) 120385 4 urban scale were identied (B1) High land ootprint: The high land ootprint oa certain NBS is perceived as an important limitation or their implementation in cities, especially in highly densied ones and oten leads to avouring ogrey inrastructure with lower ootprint. (B2) Competition or space: The implementation oNBS, regardless o their land ootprint, can be perceived as competing or space with other grey inrastructures and other land uses in general. This barrier applies or instance in existing areas/buildings (public or private) and during the planning onew areas othe city. (B3) Potential high costs: The costs related to implementation and maintenance oNBS are perceived as generally high. Depending on the type oNBS, these costs reer to rent, land prices, production, supplies/equipment, saety requirements, maintenance, and specialized proessional sta. (B4) Environmental conditions &local context: The local context (i.e., decision makers, political parties, citizens, organized social groups, historical/political situation) and environmental conditions (i.e., local climate, water, materials, and light availability) may exclude the implementation ospecic NBS. (B5) Adaptation oexisting inrastructure: Concerns related to the potential modications that might be required in the implementation site oa NBS, as or example, adjustment oslopes and adaptation ohydraulic/electric inrastructure. (B6) Lack oawareness that NBS is an option: Oten, ‘grey’ solutions are implemented simply because olack opublic awareness about the existence onature-based alternatives. This, along with the lack osense ourgency regarding environmental issues among policymakers, might limit the uptake oNBS. (B7) Functional &perormance uncertainties: Detailed and accessible inormation on the perormance oNBS in real ull-scale environments are still perceived as missing. Moreover, data regarding the environmental benets is ound to be more accessible than data regarding the social and economic benets and disservices o NBS. (B8) Lack oclear guidelines: The lack ostandards, guidelines or recommendations or adequate design, implementation, operation, maintenance, and monitoring oNBS, especially those which have been less studied, represents a undamental gap or the successul expansion othese technologies. (B9) Bureaucratic burden: A complicated administrative process prior to the implementation might work as a deterrent to NBS implementation. This is mainly related to obtaining permits on saety requirements, sanitary concerns or human health, and to changes concerning the implementation site (e.g., historical acades, protected areas, etc.). (B10) Lack osupportive policies &conficting legal rameworks: The lack oa clear regulatory ramework, coherent policies, as well as overlapping among existing legal rameworks are perceived as a limitation or the implementation oNBS in cities. (B11) Heritage value obuildings: There is a general concern about potential negative impacts oNBS on the built heritage. Some examples include biodeterioration oheritage materials and complicating the works or conservation and maintenance. Moreover, the NBS implementation might be disincentivised by the expectation olegal barriers. (B12) Water quality standards: The eectiveness otreating urban wastewaters with NBS can vary depending on the design, operational mode, and seasonal conditions. Existing knowledge gaps on the perormance oNBS under dierent conditions might inhibit their employment or specic water reuse applications that need to comply with legal standards. (B13) Lack otime or maintenance: The lack otime or maintaining NBS can restrict their long-term operation. Isolutions or this problem are not envisaged rom the outset, this may deter planners to decide or an NBS. (B14) Lack oskilled sta and training programmes: There is a perceived lack ocompetent personnel able to carry on tasks related to implementation, operations, and maintenance oNBS. This can be due to the lack otraining programmes. (B15) Potential negative environmental impacts: Concerns related to potential environmental harming impacts (e.g., leaching osoluble pollutants, greenhouse gas emissions, etc.). These impacts have been mainly related to design, planning and maintenance issues. (B16) Saety threats: Concerns related to public health/physical integrity, such as ingestion opollutants (rom water or ood), risk o overheating, explosions and re. (B17) Potential odours: Certain NBS, especially the ones related to water treatment, are perceived as a potential source ounpleasant odour. Ithis impacts the local population, it can lead to resistance to accept NBS. (B18) Propagation oinsects and pests: NBS are perceived as a natural environment, thus having the potential to oment the propagation oinsects (e.g., mosquitoes); and in extreme cases, also pests (e.g., rodents). (B19) Combination with other solutions: Concerns related to the practical implications ocombining NBS with other solutions (naturebased or not). This might increase the complexity odesign, operation, and maintenance, thus inhibiting the widespread uptake oNBS systems that cannot be implemented on their own. (B20) Technological complexity: The perceived complexity oNBS as environmental technologies is mostly derived rom the intricacy onatural processes involved therein. This means dealing simultaneously with multiple challenges (multiunctionality), which has direct impact on the need ospecialized sta to operate NBS. (B21) Potentially resources intensive: Concerns related to the potentially excessive requirement oresources such as energy, water, and nutrients, which can lead to an increment ocosts and decrease o sustainably (depending on type and origin oresource). Additionally, as mentioned in the methodology, we considered three added barriers identied by Sarabi et al. (2020), not identied in our rst workshop, to urther discuss during the second workshop (strategies identication). We present a short description adapted rom Sarabi et al. (2020): (B22) Property ownership complexities: NBS implementation may inter- ere with ownership arrangements oboth land and real estate in cities. This barrier covers the legal complexities and as well the perception, acceptance and in general, the behaviour oindividual owners with regards to these solutions. (B23) Lack o nancial resources/lack o nancial incentives: From public, citizen and private perspective, there are limited options to invest in NBS as public unds may prioritize other key urban sectors (health, transportation, etc). Meanwhile, citizens and entrepreneurs are less willing to invest since they perceive this as a responsibility olocal government. Moreover, participants maniested a concern about the very ew incentives to encourage citizens and entrepreneurs to implement NBS. (B24) Lack opolitical will and long-term commitment: Politicians oten prioritize short-term projects directly aecting the quality olie ocitizens (job creation, housing, etc.) with more immediate and tangible outcomes than the ones provided by NBS which are more long-term related. We observed that 18 barriers (85% othe total barriers identied) displayed a dierence between their BTI and BWR scores oless than 0.25 (Fig. 1), meaning that the most relevant barriers are those that display more transversality. Thereore, identiying and addressing J.A.C. Castellar et al. Journal of Environmental Management 354 (2024) 120385 5 transversal barriers together with tailored and context-sensitive approaches becomes indispensable or eectively tackling these challenges (Connop et al., 2016). Technological complexity (B20), the lack o skilled sta and training programs (B14), unctional perormance uncertainties (B7), as well as the lack oawareness that NBS is an option (B6), emerged as the most transversal and relevant barriers in our workshops. This can be attributed to the novelty othe NBS topic, leading to a lack oclear career paths or those interested in specialising in this eld. Additionally, it indicates a deciency in communication mechanisms to raise awareness among citizens, urban planners, and decision-makers about the benets and potential oNBS (or an overview, see Utkarsh, 2023). The barriers identied in our workshops show considerable Fig. 1. Weighted relevance and transversality or identied barriers in Circular City rst workshop. In dark green: Barriers Weighted Relevance (BWR); In light green: Barrier Transversability Index (BTI). (For interpretation othe reerences to colour in this gure legend, the reader is reerred to the Web version o this article.) J.A.C. Castellar et al. Journal of Environmental Management 354 (2024) 120385 6 similarities to those ound in the literature. 2 Our results highlight concerns related to post-implementation issues, especially in NBS or water pollution control included in the categories oVertical Greening Systems and Green Roos,Rainwater Management and Remediation, Treatment and Recovery also indicated by other authors (e.g.,O’Donnell et al., 2017; Ortega et al., 2023). Concerns include (B11) potential disturbance o heritage value obuildings, (B17) propagation obad odours and (B18) insects/pests, (B15) generation onegative environmental impacts, and (B21) excessive use oresources (Cross et al., 2021). These ndings provide valuable insights into the actors that may infuence the social acceptance oNBS or water pollution control. Additionally, we identi- ed that (B8) excessive bureaucratic burden in obtaining permits or water reuse or interventions in existing buildings, along with absence o(B12) water quality standards (Kisser et al., 2020;Nika et al., 2020), urther hinder the uptake and mainstreaming oNBS. As also others describe (Tompkins et al., 2019), the acceptance oNBS is a key barrier and explained with variety. There are concerns about ensuring a proper design and unctioning oNBS and their long-term eectiveness in terms osaety, perormance, cost-benet in comparison with other solutions. The lack oinormation and technical uncertainties oNBS (B14, B7), identied with a high BWR (Fig. 1), contribute to a lack ouptake by decision-makers. Additionally, the absence olong-term monitoring reinorces this reluctance (Wamsler et al., 2020). Moreover, the lack o clear processes or regulating NBS multiunctionality (B10) in many jurisdictions restricts their uptake (Zuniga-Teran et al., 2020). Also, although NBS may be more cost-eective, existing regulations can oset this economic advantage, especially when the design parameters have not been institutionalised (Zuniga-Teran et al., 2020). Furthermore, the multiunctionality oNBS poses a challenge to current ragmented urban governance models, necessitating a shit towards a more collaborative and integrated approach (Dorst et al., 2019). Ultimately, these actors culminate in a lack opolitical will and long-term commitment (B24), resulting in NBS not being seen as a priority in urban planning (Duaut et al., 2022;Faivre et al., 2017;Sarabi et al., 2020). The barrier (B7) unctional and perormance uncertainty coincides with the literature signalling “limited evidence on long-term perormance oNBS” (Blackwood and Renaud, 2022;Duaut et al., 2022). The accent placed on uncertainty both in our results and in the reviewed literature is surprising. Even though NBS concept is relatively “new” (in use ater 2015), they build on existing concepts such as ecological engineering, green inrastructure, and ecosystem-based adaptation (Almenar et al., 2021;Sarabi et al., 2019). Moreover, in the last eight years, the European Union has invested millions in producing knowledge on NBS through I +D projects, including living labs and pilots all over Europe. Moreover, despite valuable attempts to develop impact assessment guidelines or NBS (European Commission, 2021;Raymond, 2017;Raymond et al., 2017), their wider adoption remains lacking. According to Dushkova and Haase (2020) and Bayulken et al. (2021), there is still a lack oa common impact assessment ramework to accurately describe the positive impacts oNBS. This allows us to assert that the main issue lies not in the lack oevidence, but rather in the absence oreplicable evidence-based assessments and comparable benchmarks (Bayulken et al., 2021). Furthermore, the way in which evidence is communicated may not bridge the gap between academic production and practice, and thus limit political and societal changes. As a result, there is a general lack oknowledge, appreciation, and interest rom urban residents on NBS (Blackwood and Renaud, 2022;Boateng et al., 2023). The review oliterature revealed a consistent set obarriers which were not considered by our experts. For instance, the intricate relationship between NBS and the paradigm oeconomic growth (Kabisch et al., 2016) and the vital role oequity implications in avoiding gentrication processes (Zuniga-Teran et al., 2020) were not explicitly discussed. Furthermore, in terms opolitical and institutional barriers, participants did not identiy certain challenges, such as the disconnection between short-term actions and long-term goals (Kabisch et al., 2016;Sarabi et al., 2020), sectoral silos or ragmented governance (Dorst et al., 2019;Kabisch et al., 2016;Sarabi et al., 2019,2020), and the translation oevidence-based knowledge into policy and planning (Frantzeskaki, 2019;Young et al., 2019). Additionally, there was no mention oa lack osense ourgency among policymakers (Sarabi et al., 2020) or the need or top-down guidance (Wamsler et al., 2020). In terms oshared perceptions, concerns, and behaviours, the participants did not identiy certain barriers, such as ear othe unknown (Kabisch et al., 2016), path dependency (Sarabi et al., 2019), silo mentality, and risk aversion and resistance to change (Sarabi et al., 2020), along with conusion due to the multiplicity oterms, as well as the cultural importance obiodiversity (Duaut et al., 2022), and potential negative perceptions (Connop et al., 2016;Zuniga-Teran et al., 2020). Furthermore, while our study aligns with the ndings oexisting literature, we have also identied intriguing nuances that warrant urther exploration. For example, we did identiy potential high costs or NBS implementation and maintenance as a major constraint (Boateng et al., 2023;Sarabi et al., 2020;Toxopeus and Polzin, 2021), however we did not explicitly identiynancial constraints such as a insucient unding/incentives or implementation (Dushkova and Haase, 2020; Faivre et al., 2017;Ferreira et al., 2020;Ortega et al., 2023;Sarabi et al., 2020) or maintenance (Dushkova and Haase, 2020;Ortega et al., 2023). Comparable cost-benet analyses (Blackwood and Renaud, 2022;Young et al., 2019), valuation methods, and accounting or the multiple benets ourban NBS (Toxopeus and Polzin, 2021;Zuniga-Teran et al., 2020) or cost/benet quantication (Connop et al., 2016) were not explicitly addressed in our workshops. Additionally, nancial mechanisms such as the monetization oecosystem services provided by NBS were not identied either or the establishment oa market ready to support the economic valuation othese services (Zuniga-Teran et al., 2020). Finally, while in our study NBS high land ootprint is perceived only as a spatial constraint, Duaut et al. (2022) and Sarabi et al. (2019) highlight the importance oconsidering as well potential high land prices in cities. In terms oNBS adaptation to the local context, Duaut et al. (2022) restricts it to local climate-related barriers or long-term uptake oNBS, while our study widens the denition owhat ‘local context’ (B4) means: Who are the stakeholders aected by the NBS implementation? Who are the stakeholders that need to be involved or ensuring a long-term unctioning othe NBS? What is the political/- historical moment in the city? Additionally, our results suggest that the primary barrier related to the maintenance oNBS is the time needed to perorm the required tasks. However, it is important to acknowledge that Connop et al. (2016) highlight additional aspects such as costs, knowledge, and data requirements as equally important maintenance barriers. By considering these broader actors, a more holistic understanding othe challenges associated with NBS maintenance can be achieved. 2 These similar barriers are mainly ocusing on technical concerns about design, implementation, operation, and maintenance – (B1), (B2), (B4), (B8), (B13), (B19) (Artmann et al., 2020;Blackwood and Renaud, 2022;Boateng et al., 2023;Duaut et al., 2022;Ortega et al., 2023;Sarabi et al., 2019,2020; Zuniga-Teran et al., 2020), capacity building – (B14) (Artmann et al., 2020; Boateng et al., 2023;Duaut et al., 2022;Ferreira et al., 2020;Sarabi et al., 2020;Wamsler et al., 2020), shared perceptions, concerns, and behaviours – (B3), (B6), (B7), (B16), (B20) (Artmann et al., 2020;Blackwood and Renaud, 2022;Boateng et al., 2023;Connop et al., 2016;Dushkova and Haase, 2020; Ferreira et al., 2020;Sarabi et al., 2019;Sarabi et al., 2020;Wamsler et al., 2020) and organisational and political/legal issues – (B10) and (B9) (Artmann et al., 2020;Connop et al., 2016;Duaut et al., 2022;Dushkova and Haase, 2020;Ferreira et al., 2021;Sarabi et al., 2019,2020;Wamsler et al., 2020; Zuniga-Teran et al., 2020). J.A.C. Castellar et al. Journal of Environmental Management 354 (2024) 120385 7 3.3. Description and analysis ostrategies During the second workshop, 19 strategies were identied or overcoming the listed barriers (Section 3.1). As highlighted previously, some examples are mentioned or each identied strategy. (S1) Adaptive design &operation techniques: To consider environmental/social/economic conditions during the design and planning process along with proper techniques or urther operation and maintenance. Examples: Climate adaptive design; Inclusive co-design &planning; Proper selection oplants, materials and components; Develop adaptation strategies according to building types; Include skilled sta or maintenance. (S2) Selection olocation: This strategy is mostly related to the planning phase when there is some fexibility regarding the selection othe implementation site. In this case, it is recommended to consider potential trade-os related to targeted sites, to ensure a successul implementation and long-term operation othe NBS ointerest. Examples: Implementation in green areas to avoid competition with existing grey inrastructures; Implementation in less densely built-up areas. (S3) Selection oNBS: This strategy applies when there is some fexibility regarding the decision regarding which NBS to use or a given purpose – e.g., water treatment. In this case, NBS must be selected in accordance with the local context, in terms otechnical requirements, space availability, potential issues regarding social acceptance or permits. Examples: Prioritize NBS with less potential to produce unpleasant odours; Promote vertical NBS in highly dense areas; Use omobile NBS to reduce potential heritage damages. (S4) Automation and sensors: To use automation and sensors to enhance the eciency ooperation and to reduce the need or maintenance. Example: Sensors to regulate the resources applied (water, energy) and or to monitor inputs and outputs rom the NBS systems. (S5) Standards and guidelines: To develop easily understandable and open access guidelines and standards or a proper design, operation, and maintenance oNBS. Such guidelines should consider specic characteristics opotential implementation sites, as well as a wide variety oclimate conditions. This strategy does not apply to those NBS that already have clear standards and guidelines – i.e., treatment wetlands. Examples: Categorize NBS systems according to building types/uses, orientation/structural conditions; Standard operating procedures (SOP). (S6) Coupled/hybrid solutions: To consider more holistic approaches by combining NBS with other solutions (non-NBS or NBS). This can enhance perormance, reduce potential negative impacts and to promote a more circular management oresources. Examples: Solar panels; Supporting unit or removal omicropollutants; Combine NBS with natural habitats; Combine NBS with disin- ection process (UV) to ensure removal opathogens. (S7) Stakeholder collaboration and communication: To acilitate stakeholder communication and collaboration in all phases or the establishment oNBS, rom design and planning to implementation and maintenance oNBS. Examples: Early involvement; Develop riendly relations with local decision makers; Lobbying; Set-up stable working groups; Enhance involvement via innovation and scientic networks and projects; Public and private partnerships; Facilitate cross-sectoral work. (S8) Community involvement: To put in place mechanisms to enhance the involvement othe community aected by the implementation oNBS. Such mechanisms should be based on the overall understanding othe needs and demands othe people involved. Examples: Bottom-up pressure; Grassroots movements; Include people in decision-making with citizen councils and similar structures (e.g., citizen science); Interaction between communities; Favour the participation olocal schools, vocational training institutions, and universities. (S9) Models and methods or ownership management: To set up mechanisms which can lower the resistance to NBS given property arrangements. Examples: Agreements through purchase or exchange; Develop new operating models (renting out) or the NBS; shitrom land ownership to land stewardship; Stakeholders’ mapping; Specialized oces in the municipality or dealing with property ownership issues. (S10) Capacity building: To develop capacity building programs targeting, among others, maintenance sta, decision makers, public servants, and practitioners. Examples: Training activities; Targeting decision makers; Certied training; Unemployment training programmes. (S11) Education: To avour a better inclusion oNBS in education programmes at all levels (elementary, vocational, bachelor, master, technical, doctoral), including practical and theoretical activities. Examples: Training and demonstration actions; Field trips; Seminars; Apprenticeship; Hand-on courses; International diploma; Internships; Incentive new (NBS-related) proessional paths; Vocational trainings; Online tutorials. (S12) Interdisciplinarity: To promote interdisciplinarity across working teams and in terms oindividual skills, ensuring multiple stakeholders’ participation to acilitate the implementation and operation omultiunctional eatures oNBS in a diverse range o sites. (S13) Market and Business: To carry out activities capable oincreasing market and business opportunities or NBS in cities. Examples: Business models; Advertising; Business orums and hubs; Economies oscale; Create new markets. (S14) Financial instruments and incentives: To acilitate the access/ awareness regarding existing nancial incentives/instruments along with the development onew incentives. Indeed, ensuring a proper allocation o unding or NBS implementation by restructuring priorities at national/continental levels. Examples: Crowdunding; Donation; Payments or Ecosystem Services; Subsidies; Increase taxation on conventional solutions; Tax exemption; Diverse nancing options or smaller communities; Employment incentives or workorce depending on certied trainings; Donation. (S15) Legal and policy instruments: To develop/adapt legal and policy instruments or acilitating the NBS implementation in cities. Such instruments should help reducing legislative obstacles and bureaucratic burden, avour the implementation oNBS instead oother non NBS solutions and put in place NBS regulations and certications. Examples: Establish a minimum percentage o space or NBS in buildings and city; Develop cross-cutting policies (Energy/Water/Climate/Food/Ecosystems nexuses); Develop an EU legal ramework or NBS or harmonize existing. (S16) Governance, planning and political agendas: To include NBS in planning and political agendas to augment NBS deployment in urban planning. Examples: Shared responsibility and risks. (S17) Inormation and dissemination: To promote inormative campaigns and dissemination activities to raise awareness on NBS purposes, benets and multiunctionality. Moreover, such campaigns should help to demystiy concerns related to potential negative eects oNBS such as odours, insects, dubious quality ogoods produced – e.g., water and ood. Examples: Audio-visual and written press; Workshops, ocus groups, open days; Marketing campaigns; Itinerant exhibitions; Showcase best practices; Case studies; Catalogues; Database; Demonstration actions/sites; Promote the use oexisting ree tools/platorms to illustrate benets; Increase use olayman (non-technical) language; Public Helpdesk service. (S18) Research and demonstration: To encourage research and demonstration activities (i.e., case ostudies, pilots, catalogues) on J.A.C. Castellar et al. Journal of Environmental Management 354 (2024) 120385 8 topics that avour the mainstreaming, uptake and social acceptance oNBS in cities. This can overcome existing mindsets dominated by path dependency and silo thinking. To develop unding schemes along with a better diusion osuch opportunities among researchers. Examples: Studies on additives or materials to reduce odours; comparative studies (NBS vs. NBS; NBS vs. non-NBS); Models to simulate combinations between technologies; Methodologies or assessment oimpacts and perormance; Models or scenario/benets simulation; Assessment o non-monetary benets; Research to improve guidelines; Assessment opotential trade-os (e.g., disservices). (S19) Compensation mechanisms and positive narratives: To create compensation mechanisms (nancial or conceptual) and encourage positive narratives to acilitate the implementation o NBS instead oother solutions; – or instance using ewer pharmaceutical products to reduce contamination owater bodies. Examples: Promote awareness on NBS multiunctionality, purposes, benets, and cost-eectiveness; Placing the NBS on degraded areas to help revitalising it; Monetization oco-benets; Tax exemption according to benets provided; Communicate that NBS are held to higher standards (risk, health) than conventional, because they are new. In spite othe predominantly technical expertise othe COST experts, only 5 out o18 strategies ocus on technological solutions (S1, S2, S3, S4, S6). Instead, the vast majority othe strategies target governance, inormation and education aspects. As the literature conrms (e.g. Cohen-Shacham et al., 2019;Dumitru et al., 2020;European Commission, 2020a,b) it is the multiunctional character oNBS and their aspiration to achieve impact on both social and ecological terrains, that orients strategies beyond the mere technical xes. We ranked these strategies according to two indicators: diculty oimplementation and transversality across barriers (Fig. 2). Thus, (S17) Inormation and dissemination; (S18) Research and innovation and (S7) Stakeholders collaboration and communication that are deemed as most transversal. It is also possible to assert that participants put the weight othis trans- ormation on inormal rather than ormal mechanisms: (S7) Stakeholders collaboration, (S19) narrative change, (S17) inormation and to a certain extent (S18) research are deemed as better equipped to enhance acceptability oNBS, than proound political transormations (Sekulova Fig. 2. Transversality index and mean diculty or identied strategies in Circular City second workshop. In dark green: Strategy Level oDiculty (SLD); In light green: Strategy Transversality Index (STI). (For interpretation othe reerences to colour in this gure legend, the reader is reerred to the Web version othis article.) J.A.C. Castellar et al. Journal of Environmental Management 354 (2024) 120385 9 and Anguelovski, 2017). In terms odiculty, the experts tend to be cautious: most strategies are considered between moderately and very dicult to implement. In this sense, it appears that this community oexperts settled or targeted strategies which require specialisation and thereore are more complex. This result can also be attributed to the act that the participants were asked to ocus on individual NBS, not NBS in general. In general, the strategies are consistent within the state-o-the art literature. 3 Another block ostrategies identied in the literature coincide with our results but there are nuances and additional ndings. Since it is impossible to discuss all these aspects, we ocus here on two othe most widely mentioned categories, namely related research and stakeholder involvement strategies. As expected, (S18) Research and demonstration, considered both dicult to implement and transversal (Fig. 2), is widely mentioned in the literature. Some othe topics which were not present in our study would include: investigation in NBS benets demonstration and quantication (Blackwood and Renaud, 2022;Du- aut et al., 2022;Kabisch et al., 2016;Sarabi et al., 2019), open innovation and experimentation (Bayulken et al., 2021;Kabisch et al., 2016; Sarabi et al., 2019), pilots and open-air laboratories (Blackwood and Renaud, 2022;Boateng et al., 2023); coupled-hybrid solutions - yet, unlike our study, limited to combining NBS only with grey in- rastructures - (Kabisch et al., 2016;Sarabi et al., 2019); evidence-based evaluations, assessments and monitoring, and the denition oeasily measurable indicators (Bayulken et al., 2021); incrementing case-based knowledge on the adaptation to local conditions, including urthering ‘ecomimicry’ strategy, plant diversity and habitat structure (Connop et al., 2016). In the same category, a widely discussed strategy which is also key in our ndings is (S7) Stakeholders’ communication and collaboration, also displaying a high transversality. This is not surprising since NBS are designed to deal with a wide variety ochallenges which in turn are aected by or aect multiple stakeholders, making partnerships crucial or these arrangements to work (Frantzeskaki et al., 2020;Giordano et al., 2020). However, the literature mentions specically the utility o integrating dierent value systems and stakeholders’ viewpoints to support environmental decision-making and allow or the integration o dierent policies relevant into territorial planning (Liquete et al., 2016; Zwierzchowska et al., 2019). Another important nuance in the literature, not present in our workshops, is that local residents and community groups might either welcome or contest NBS interventions in their neighbourhoods, depending on how they perceive the distribution o socio-ecological benets but also decision-making power across dierent - cultural, gendered, or ethnical groups - is ound to be key to implementation (Dushkova and Haase, 2020;Kotsila et al., 2020;Melanidis and Hagerman, 2022). In this sense, there is a call or meaningul participation ocommunities, where the citizens can have an actual say in the design and overall implementation strategy (Raymond et al., 2017). Democratising NBS, oering incentives and “nudging” stakeholders (Poch et al., 2023) into adopting sustainable practices remain key strategies which decision-makers employ to promote NBS (Bayulken et al., 2021). Finally, there are strategies which did not come out in our workshop’s discussions. While the discussion on strategies bears more interest when coupled with specic barriers (see section 3.4), we provide here a nonexhaustive list ogenerally applicable strategies not encountered in our empirical results. They include: targeted recommendations such the importance oaesthetically appealing solutions (Bayulken et al., 2021; Hoyle et al., 2017); alteration ointernal working conditions by the municipalities implementing NBS and the systematic science–policy integration to help progressively mainstream NBS into inormal/ormal planning regulations and mechanisms/tools (Wamsler et al., 2020); developing long-term, shared visions or the city, specically relying on public-private partnerships (Dushkova and Haase, 2020); taking into consideration a distributive justice perspective (Anguelovski and Corbera, 2023;Bayulken et al., 2021); including the trade-os and cost implications oimplementation oNBS and/or the lack osuch improvements (Alves et al., 2020); acknowledging inherent conficts between uses and political/economic interests odierent groups, while overcoming neoliberal, technocratic perspectives and allowing or a politicisation oNBS (Kotsila et al., 2020); putting the communities at the centre ochange, avouring new green urban commons, building trust between the city and its citizens, creating dierent co-creation ora, collaborative governance approaches, and an inclusive narrative (Frantzeskaki et al., 2020). Chausson et al. (2023) highlight interesting nancial mechanisms, such as positive economic incentives rewarding sustainable land management, increased scal collection and spending or delivering public goods, taxing environmentally harmul activities or unding, and exploring decolonial nance mechanisms like unconditional cash transers or debt relieschemes. Given that the strategy (S14) concerning nancial instruments and incentives was considered one othe most challenging to implement, the latter-mentioned nancial aspects carry signicant importance. 3.4. Analysis ostrategies and barriers The connection between barriers and strategies is illustrated by the transversality indicator, namely how many and which barriers are addressed by which strategy (Fig. 3). The most transversal strategy is (S17) Inormation and dissemination which addresses 17 barriers, while (S8) Community involvement is considered the least transversal, addressing only 3 barriers. (S7) Stakeholders collaboration and communication can address a great variety obarriers, rom technical (i.e., (B7) Functional and perormance uncertainties), to political (i.e., (B10) Lack o supportive policies &conficting legal ramework) administrative (i.e., (B9) Bureaucratic burden) and nancial barriers (i.e., (B23) Lack o nancial resources and incentives - B23). Hence, the strategy (S7) can acilitate exchange otechnical knowledge and play an important role in producing changes in dierent spheres othe society. In contrast, (S8) Community involvement is limited to addressing political (B24) and educational issues (B14) and the Lack oawareness that NBS is an option (B6). According to our participants, the involvement ocitizens and stakeholders can change the short-term mindsets opolicymakers, oten determined by election cycles. Interestingly, the level odiculty to implement both strategies is similar (Fig. 2. Score 0.4–0.6) and mostly related to challenges regarding lack oapproaches to ensure long-term engagement. Surprisingly, experts did not identiy a direct correlation between (S18) Research and demonstration and (B20) Technological complexity. In our ramework, this barrier is related to the lack o experience, guidance and standards. Consequently, the strategies linked to this barrier ocus on education, capacity building, interdisciplinary approaches, and collaboration. 3.5. Barriers and strategies per NBS category The identied barriers are more specic to certain categories oNBS than the strategies (Fig. 4). Out o24 barriers identied only six (B6, B7, B14, B22, B23, B24 were reerred to all our categories (Fig. 3). 3 The technical strategies S1, S2, S3, S4, S6 are also mentioned in Alves et al., (2020);Boateng et al. (2023);Connop et al. (2016);Hoyle et al. (2017); and Liquete et al. (2016). Governance, political and legislation-related strategies - S5, S8, S9, S15, S16 are also proposed by Bayulken et al. (2021);Boateng et al. (2023);Dushkova and Haase (2020);Hoyle et al. (2017);Kabisch et al. (2016); Sarabi et al. (2019);Sarabi et al. (2020);Toxopeus and Polzin (2021);Wamsler et al. (2020) and Zwierzchowska et al. (2019). The nancial and market-related strategies - S13 and S14 - are studied in Kabisch et al. (2016);Sarabi et al. (2019,2020) and Toxopeus and Polzin (2021). As or knowledge production and dissemination, including education and capacity.building strategies, S12, S17, S18 S10, S11, they are discussed by Bayulken et al. (2021);Boateng et al. (2023);Dushkova and Haase (2020);Kabisch et al. (2016);Sarabi et al. (2019); and Sarabi et al. (2020). J.A.C. Castellar et al. Journal of Environmental Management 354 (2024) 120385 16 urban waters with nature-based solutions in circular cities – exemplied through seven urban circularity challenges. 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