Nature-based solutions in the urban context: terminology, classification and scoring for urban challenges and ecosystem services

• Researchers developed an integrative list from overlaps among NBS in 27 projects, formalized terminology through web-based surveys, and clustered NBS hierarchically.Source: Graphical abstract 18 · p. 2
• They assessed NBS performance using qualitative project evaluations, then applied PCA and the evenness index to explore patterns among NBS.Source: Graphical abstract 18 · p. 2
• NBS units (NBSu) are more versatile than NBS interventions (NBSi) across urban challenges and ecosystem services.NBSu are stand-alone green technologies or urban spaces that can be combined with other solutions, whereas NBSi involve intervening in existing ecosystems and NBSu through techniques supporting natural processes.Source: Graphical abstract 18 · pp. 2–3
• The integrative assessment finds greater agreement for environmental urban challenges and cultural and regulating ecosystem services than for socioeconomic challenges and supporting and provisioning services.Source: Graphical abstract 18 · p. 3
• The study identifies the “green factor” and replication of non-intensive natural practices as key criteria practitioners use to recognize an NBS.Source: Graphical abstract 18 · p. 3

Conceptual background

• NBS definitions differ in scope: the European Commission definition takes a broad perspective centered on economic, environmental, and social sustainability, while the IUCN definition emphasizes actions (European Commission, 2015; Cohen-Shacham et al., 2016).Source: §1 · p. 4

Problem statement

• Lack of agreement on characteristics unique to NBS has caused confusion about what qualifies as NBS, because the concept does not always distinguish it from established environmental concepts.Source: §1 · p. 4

Research aim

• The paper aims to advance a common understanding of NBS typologies, terminology, classification, and evaluation by integrating knowledge from recently completed or ongoing European H2020 projects.The need arises because one intervention may receive several names, such as “green façade with climbing plants” and “climber green wall.”Source: §1 · p. 5

Methodology

• A mixed quantitative-qualitative methodology combines dedicated workshops, expert interviews, surveys, and statistics to consolidate knowledge across the NBS community.The approach supports communication and knowledge-sharing through a common NBS list with nomenclature based on surveyed practitioner preferences.Source: §1 · p. 5

Objectives

• The study develops a user-friendly classification scheme, examines what defines an NBS, and provides an integrative qualitative assessment of how European projects evaluate NBS performance through UC and ES.These objectives are intended to support NBS conceptualization and mainstreaming.Source: §1 · p. 5

2. Methods

• The methods comprised three parts: developing a common NBS list through expert workshops, reaching terminology and classification consensus through two surveys, and integrating UC and ES scores statistically.The integrative assessment included crossed-score calculation and visualization plus multivariate analysis.Source: §2 · p. 5, §2.3 · p. 9

2.1 Common list of NBS

• A common NBS list was derived from four H2020 projects, retaining NBS assessed for UC or ES and consolidating similar measures shared by at least two projects.Similarity was judged by nature’s role, technical or design requirements, implementation scale, and benefits.Source: §2.1 · p. 6

2.2 NBS common terminology and classification

• Terminology was established through two web-based surveys of NBS expert preferences, selecting names receiving at least 50% of votes and applying a one-sample chi-square test at 0.05 significance.A three-level hierarchical classification scheme was proposed, with its first level built on three existing categories from Eggermont et al. (2015).Source: §2.2 · pp. 7–8

2.3 Integrative assessment framework for urban challenges and ecosystem services

• The integrative framework standardized differing project lists by mapping UC and ES to common baselines from Eklipse, TEEB, and Millennium ecosystem assessments before crossed-score calculation and multivariate analysis.The framework linked project-specific challenges and services to shared categories for comparable assessment.Source: §2.3.1 · p. 9, §2.3 · p. 9

2.3.1 Calculation and visualization of crossed scores

• Normalized raw scores for each NBS used binary logic: 1 when it addressed a UC or ES, and 0 otherwise, based on public project documents.The qualitative approaches included ecosystem-services assessments, expert panels or consultations, and literature reviews.Source: §2.3.1 · p. 9
• Crossed urban-challenge and ecosystem-service scores were visualized per nature-based solution using ggplot2, with the tool’s source code publicly available.The source code is available at https://github.com/icra/nbs_list.Source: §2.3.1 · p. 10

2.3.2 Multivariate analysis

• PCA explored similarities among nature-based solutions and identified the urban challenges and ecosystem services driving them, grouping services into regulation, provision, cultural, and supporting types.UC with more than 1 missing value were excluded from analysis, and missing-value treatment used missMDA in R; PCA used FactoMineR in R.Source: §2.3.2 · p. 10
• Pielou’s evenness index measured which nature-based solutions were most balanced across urban challenges and ecosystem-service types, using positive crossed-score counts and zero-imputed missing values.The index used Shannon diversity and the number of crossed-score variables exceeding zero; calculations used the Vegan package in R.Source: §2.3.2 · p. 10

3.1 Common list of NBS

• A participatory elicitation workshop consolidated 107 NBS from four H2020 projects into a common list of 32 NBS.For judging similarity or equality among NBS, design and implementation scale were more relevant than benefits or replicating natural processes.Source: §3.1 · p. 12

3.2 NBS common terminology and classification

• Two web-based surveys gathered more than 160 participants from 46 countries and 92 participants from 30 countries to establish common NBS terminology.Participants were mainly researchers, engineers, architects, and urban planners working in the NBS field, with approximately 70% from Europe.Source: §3.2 · pp. 14–15
• The first survey selected names for 20 NBS using more than 50% of total valid votes; 12 NBS received no option with most votes and proceeded to a second survey.In the second survey, seven NBS showed a significant difference at the 0.05 chi-square significance level, while five showed no significant difference.Source: §3.2 · p. 15
• Terminology choices favored names supported by scientific usage, selecting “green façade” for over 236 documents and “Riverbank engineering” for slightly greater publication frequency.“Systems for erosion control” was discarded because it did not specify the intervention site and could be confused with other erosion-control NBS.Source: §3.2 · p. 15
• The 32 common NBS were organized into a three-level hierarchy distinguishing NBS units from NBS interventions, then separating spatial and technological units.The third level groups units by vegetation form and implementation scale, whereas interventions are grouped by purpose or intervention site without a third level.Source: §3.2 · pp. 16–18

3.3 Integrative assessment framework for urban challenges and ecosystem services

• The assessment framework produced NBS performance scores from 0 to 1 for 10 urban challenges and 19 ecosystem services, plus four ecosystem-service categories.The categories were Regulating, Cultural, Support, and Provision.Source: §3.3 · p. 19
• Green space management was the highest-scoring urban challenge and public participation the lowest, while cultural ecosystem services ranked highest and provisioning services lowest.The crossed scores showed no clear overall pattern, with substantial diversity among assessed challenges and services.Source: §3.3 · p. 20
• 49.4% of model variance accumulated in the first two PCA dimensions, with no obvious NBS clustering while the proposed classification categories generally performed properly.The first dimension combined social and environmental urban challenges with cultural ecosystem services; the second contrasted provisioning and supporting services and water management against air quality, climate adaptation, and economic opportunities.Source: §3.3 · p. 21
• All NBS scored between 0.85 and 1 on the evenness index, with NBS units more generalist and NBS interventions better suited to specific challenges and services.Spatial and technological units generally had higher evenness than interventions, indicating broader performance across assessed urban challenges and ecosystem services.Source: §3.3 · p. 22

4.1 To be or not to be an NBS: What does it take?

• Participatory results identify vegetation—the “green factor”—as central to recognizing an NBS, while nature-inspired processes alone are insufficient without employing nature.All NBS units contained vegetation, and approximately 70% used green-related terms; porous asphalt illustrates that replicating infiltration without vegetation may not qualify.Source: §4.1 · p. 24
• Additional criteria require NBS to be non-intensive and occur in nature, excluding intensive or non-natural examples such as smart soil production and small-scale urban livestock.Requiring greenness also excludes natural porous-material solutions such as infiltration trenches, despite their filtration or biofiltration functions.Source: §4.1 · p. 25
• Participants resisted classifying planning and management approaches as NBS, although the authors recommend treating them as supportive actions for preserving natural capital and implementing and monitoring NBS.This resistance lacked a clear participant justification, while the authors associate such approaches with maintaining natural capital and supporting NBS implementation.Source: §4.1 · pp. 25–26

4.2 The path to NBS standardization

• The proposed NBS list advances standardization by identifying a cross-project core set and incorporating worldwide expert views on NBS identity and naming.It is presented as an evolving foundation that can change with future advances.Source: §4.2 · p. 26
• The authors propose a more compact hierarchical classification with only 11 conceptually fine-tuned categories, supported by simple questions for classification.Source: §4.2 · p. 27
• NBS names encode location, scale, ownership, structure, processes, and design requirements, while intervention names generally use action verbs distinguishing them from existing ecosystems.Examples include street trees, pocket parks, green roofs, constructed wetlands, infiltration basins, and intensive or extensive roofs.Source: §4.2 · p. 27
• NBS terminology remains fragmented: surveys proposed more than 250 names despite existing guidelines, because social, ethnic, and cognitive contexts generate different terms for identical concepts and vice versa.Reviewing standards and applying cognitive models could help validate the terminology proposed in this article.Source: §4.2 · p. 28

4.3 Assessment for urban challenges and ecosystem services

• Regulation (Me = 0.37) and cultural services (Me = 0.47) scored above supporting services (Me = 0.25) and provisioning services (Me = 0.08), with most latter services scoring 0 for over 90% of NBS.Only “Habitat for species” and “Fresh water” were exceptions among supporting and provisioning services.Source: §4.3 · pp. 28–29
• NBSsu provided more co-benefits than NBStu and NBSi, possibly because NBSsu have been more extensively studied and their benefits better documented.The PCA and evenness index supported this multivariate finding.Source: §4.3 · p. 29
• NBS classification showed no overlap in urban challenges and ecosystem services, while each NBS addressed distinct situations and the scheme robustly organized delivered benefits.The classification was designed around visual and functional aspects but worked well for addressed urban challenges and delivered ecosystem services.Source: §4.3 · p. 29
• The proposed tool quantitatively scores NBS for urban challenges and ecosystem services, enabling users to explore co-benefits or identify the NBS best addressing a specific issue.It provides scores for each NBS and visualizes scores for individual challenges or services.Source: §4.3 · pp. 29–30

Mainstreaming NBS

• A shared “Nature-Based” language across research, public and private sectors could facilitate knowledge transfer, replication, civil-society engagement, and more resilient cities.The article identifies the gap between technological development, practical applications, and civil-society values as difficult to address without common NBS conceptualization.Source: §5 · pp. 30–31

Contributions

• The methodology evaluates 32 NBS across urban challenges and ecosystem services and provides a robust, expandable classification scheme for systematic knowledge representation.The scheme supports integration of databases beyond the four analysed H2020 projects.Source: §5 · p. 30
• The classification reconceptualizes NBS as ensembles forming complex living systems with other NBS, grey infrastructure, or conventional technologies.A holistic perspective may diversify ecosystem-service provision, close resource loops, and compensate possible disservices.Source: §5 · p. 30

Terminology and definitions

• The green factor and non-intensive resource use are key aspects practitioners associate with NBS, informing clearer and more comprehensive definitions.Further mixed quantitative-qualitative research could explain terminology preferences and examine ‘nature’ as an ‘empty signifier’ in NBS debates.Source: §5 · pp. 30–31

Limitations and future research

• The proposed common list, terminology, and classification are provisional and require refinement to distinguish NBS interventions from other green planning and management approaches.Excluded items, especially those lacking urban-challenge or ecosystem-service assessments or appearing in only one project, warrant review.Source: §5 · p. 31

Assessment priorities

• The integrative assessment indicates a need for stronger scientific evaluation of NBS performance for socio-economic challenges and supporting and provisioning services, especially technological and interventional NBS.These assessment gaps are particularly relevant to technological and interventional solutions.Source: §5 · p. 31

MOTIVE

• NBS terminology is complicated by confusion between NBS, plant-dependent benefits or functions, non-natural nature-inspired interventions, overly intensive practices, and planning tools that support implementation.The passages explicitly note that benefits or functions are not NBS, some interventions are inspired by nature without employing nature, and planning tools are not NBS.Source: MOTIVE · pp. 41–42
• The first web-based survey selected names receiving more than 50% of valid votes and carried the two most-voted names without consensus into a second survey.Table A.3 states the vote-selection rule and identifies the first survey as the basis for the second web-based survey.Source: Table A.3 · p. 44, MOTIVE · p. 45, NBS 10 · p. 45

NBS 10

• The classification includes NBS categories spanning habitats, soil conservation, rivers and streams, erosion control, green parking pavements, and other urban green infrastructure.Examples include habitat creation, soil improvement, river re-meandering, erosion-control systems, green parking pavements, and green roofs.Source: NBS 10 · pp. 45, 46, 50
• NBS31 recorded 36.8 votes with p=0.03 and favored diverting and deflecting elements at 63.2%, while NBS32 recorded 38.5 votes with p=0.04 and favored vegetated grid paving at 61.5%.These results are reported alongside the one-sample chi-square testing framework using a significance level of 0.05.Source: Table A.4 · p. 49, NBS 10 · p. 50
• Rain gardens and swales manage stormwater through infiltration, storage, filtration, treatment, and conveyance, with swales emphasizing runoff treatment during conveyance and rain gardens during infiltration.Rain gardens use layered soils, mulch, and climate-suitable native plants; swales are gently sloped vegetated channels directing runoff from roads, paths, or car parks.Source: NBS 10 · pp. 52–53
• Constructed wetlands replicate wetland processes with vegetation, soils, and microbes to treat wastewater and stormwater for aquifer recharge or non-potable reuse.They include free-water-surface systems and subsurface horizontal- or vertical-flow wetlands.Source: NBS 10 · p. 53
• Vertical and rooftop greening solutions range from climbing-plant facades and panel-based living walls to mobile gardens, planter walls, pergolas, and extensive, intensive, or semi-intensive roofs.Intensive roofs support greater vegetation diversity, deeper substrates, public access, and higher maintenance than extensive systems, while semi-intensive roofs combine both approaches.Source: NBS 10 · pp. 54–55
• Biodiversity-oriented solutions create or preserve habitats, shelters, street trees, green corridors, urban forests, parks, gardens, and other green spaces across urban scales.Examples include pollinator areas, habitat modules, tree-lined streets, linear parks, large parks exceeding 0.5 ha, and pocket parks smaller than 0.5 ha.Source: NBS 10 · pp. 56–57
• Community gardens, urban orchards, pre-existing vegetation, composting, and soil improvement support food production, ecological and social qualities, habitat protection, waste processing, and reduced implementation or maintenance costs.Community gardens cultivate vegetables, fruits, and flowers, while pre-existing vegetation is preserved to protect habitats and reduce green-area costs.Source: NBS 10 · p. 58
• Soil and slope bioengineering uses deep-rooted vegetation, mats, wooden structures, and surface roughening to stabilize steep slopes and prevent erosion, landslides, and sedimentation.These techniques generate natural protection against wind- or water-driven soil loss.Source: NBS 10 · p. 59
• River restoration techniques widen, deepen, or reopen channels to increase storage capacity, optimize flow dynamics, and reduce flood risk and erosion.Covered watercourses can be reopened by removing concrete layers.Source: NBS 10 · p. 60
• Floodplain reprofiling or extension provides additional flooding space, reducing flood risk while enabling recreational or agricultural use during low water levels.Techniques include excavating the lateral riverbed and dividing discharge into branches.Source: NBS 10 · p. 60
• Fluvial engineering with rocks, trunks, and willow branches redirects and deflects flow, alters sediment dynamics, and protects riversides against erosion.Placed near riverbanks or within channels, these elements affect channel length and depth.Source: NBS 10 · p. 60
• Vegetated permeable pavements allow runoff to soak through planted modular structures for storage or infiltration across parking areas, roads, paths, sidewalks, and street-furniture zones.They are typically lower-cost and lower-maintenance than traditional pavements.Source: NBS 10 · p. 61