Analyzing Evidence of Sustainable Urban Water Management Systems: A Review through the Lenses of Sociotechnical Transitions

: Sustainability concerns and multiple socio ‐ environmental pressures have necessitated a shift towards Sustainable Urban Water Management (SUWM) systems. Viewing SUWM systems as sociotechnical, this paper departs from eight factors previously identified by transition research: Pressures , Context , Purposes , Actors , Instruments , Processes , Outputs , and Outcomes as a methodological framework for a structured review of 100 articles. The study seeks to analyze empirical cases of planning and implementing SUWM systems worldwide. A wide range of public actors—driven by social and environmental factors rather than by economic pressures—have initiated SUWM projects so as to locally fulfill defined social and environmental purposes. We provide evidence on the emergence of new actors, such as experts, users, and private developers, as well as on the diverse and innovative technical and societal instruments used to promote and implement SUWM systems. We also explore their contexts and institutional capacity to deal with pressures and to mobilize significant financial and human resources, which is in itself vital for the transition to SUWM. Planned or implemented SUWM outputs are divided into green (wet ponds, raingardens, and green roofs) and gray (rain barrels and porous pavements) measures. The outcomes of SUWM projects— in terms of societal and technical learning, and their institutional uptakes—are often implicit or lacking, which seemingly reduces the rate of desirable change.


Introduction
A wealth of socio-economic and environmental issues-in terms of resource scarcity and sinks, as well as climate change impacts-are placing a great amount of strain on conventional infrastructure systems, forcing them to overstep their sustainability limits [1][2][3][4].Urban water systems, and particularly underground-piped drainage systems, have for decades received significant criticism for not being able to adequately respond to the rising pressures, thereby acknowledging the necessity for a transformative change towards innovative solutions [1][2][3]5,6].Increasingly, open spatial systems mimicking natural principles are advocated as alternative system solutions that could restore natural hydrological flows.These systems could reverse the impacts of urbanization and alteration of engineering-based water flows, close the urban water cycle, enhance water security, and act as structural measures for urban climate change resilience [7][8][9].As an overarching concept, the envisioned solutions are commonly referred to as Sustainable Urban Water Management (SUWM) [3,6,10].These new systems encapsulate hydrological principles of detention and conveyance, retention, and infiltration of rainwater and runoffs, integrated into designed naturebased facilities, such as ponds, wetlands, porous pavements, swales, trenches, open canals, green roofs and walls, and bio-retention systems [7,[11][12][13].SUWM systems promise to deliver a wide range of socioeconomic, environmental, and ecological benefits and services, thereby significantly contributing to urban sustainability [7,10,14,15].Nevertheless, a transition towards SUWM systems has been relatively slow [3,16].Existing urban drainage and, generally, urban water systems are viewed as sociotechnical systems that imply seamless interconnectivity of physical artefacts, economics, politics, actors, and structures, and are therefore resistant to radical change.
Intensified water challenges, such as the increased pollution of receiving water bodies, diminishing water resources, and the increased occurrences of floods and droughts has ignited a process of regime change.Accordingly, cities are increasingly turning to initiatives that seek to implement and promote SUWM solutions [6,[17][18][19][20][21][22][23].A diverse array of terms has evolved to describe these new system solutions designed to fit for various purposes in dealing with urban water challenges in different urban contexts [11,24,25].Similarly, there has been an increase in the number of studies that focus on the transition towards SUWM and discuss a range of relevant technical and societal issues regarding the process of change (see [26] for a review).Factors that normatively and empirically hinder or facilitate desirable change include urban water pressures triggering change, contexts in which new solutions are sought, action processes for the transformation of urban water regimes, the actors involved, and the expected outputs and outcomes [26][27][28][29][30][31].
Despite the substantial body of academic work on the transition to sustainable policy approaches and the planning of urban drainage systems, the status of the knowledge available on the change required to cope with rising urban water issues and the impact of climate change is still in need of exploration.What lessons can be learned from worldwide empirical initiatives towards alternative solutions?Outlining the available knowledge in the relevant literature through a systematic review from the perspective of sociotechnical transition theory could serve to advance our understanding on the status and pace of change taking place in urban water systems and water drainage systems worldwide.Accordingly, this paper proposes a review of the literature on empirical evidence of the planning and governance of SUWM systems that have been advocated to complement and/or replace existing urban water systems.
Enhancing or bridging knowledge gaps through literature reviews so as to assess the process of urban water system regime change has gained the attention of many scholars.Some studies have focused on reviewing the evolving paradigms and terms used to describe the core principles of the newly sought solutions [11,32].The literature regarding the status of the implementation of sustainable initiatives, solutions, and challenges in particular geographic contexts [12], as well as recent trends on novel sustainable water systems [33], has also been reviewed.Other studies have centered around governance and management challenges to regime change [34], or the scope and type of barriers against the mainstreaming of alternative solutions [16].Moreover, other studies have specialized on specific issues, such as the spatial allocation of new systems for concluding on strategies and optimization tools [35], the societal benefits of the new solutions, and management criteria to control water pollution [36].In brief, departing from the observably slow pace of change, one can argue that these literature reviews sought to understand the factors facilitating or hindering the advocated change.However, these reviews tell us little on the status of available knowledge in relation to transition theory and prospects for change.
Through following a novel approach for our review, this paper contributes to the debate on the pace of transformation of existing systems in favor of sustainable approaches and practices regarding managing urban rainwater and runoffs.It does so by conducting a comprehensive systematic literature review of all of the factors considered significant for materializing regime change (Section 3).These factors-which form the methodological framework for the review process-are outlined from a previous study based on a literature review of relevant scholarly work on theoretical transition perspectives [26].These factors are described, defined, and translated into questions that were applied when reviewing the literature (Section 2).A key contribution of this paper is the analysis and discussion of the extent and ways in which the results (Section 3) are linked and relevant to the findings and conclusions of other studies in the field (Section 4).Finally, the paper draws conclusions (Section 5).

Selection of Articles: Keyword Search, Abstract Screening, and Text Analysis
The final sample of articles to be reviewed was identified through a four-stage selection process.In Stage 1, studies were selected through keyword searches (concluded on 3 May 2019) in the Web of Science v.5.32 (https://webofknowledge.com), while considering all databases from 1950 to 2019.Starting from the terminology reported in [11], the search included studies whose title, abstract, or keywords contained any of the terms used to indicate SUWM.The search keywords included terms such as Stormwater Control Measures (SCMs), Stormwater Quality Improvement Devices (SQIDs), Best Management Practices (BMPs), Sustainable Urban Drainage Systems (SUDS), Alternative techniques, Source control, Water-Sensitive Urban Drainage Design (WSUD), Low Impact Design, Integrated Urban Water Management (IUWM), Water-Sensitive Cities, and Green Infrastructures (GI) [11].Other relevant concepts were added to these, such as Sponge city, Nature-based Solutions (NBS), Blue Infrastructure (BI), Ecosystem Services (ES), Urban Rainwater Harvest (URH), Nature-close rainwater management, Open Storm/Rainwater Drainage or Management, and Blue-Green City (see Table A1 in Appendix A).These terms were used as search keywords to identify relevant peer-reviewed scientific articles.The search resulted in a total of 7002 'articles' and/or 'reviews' in the English language.
In Stage 2, in order to refine the search and identify those articles reporting on case studies of SUWM planning and implementation, we defined four types of inclusion criteria based on keywords.The inclusion criteria are meant to ensure that the study: (i) Addresses either planning, decisionmaking, or policy-making, (ii) includes a case study of SUWM planning and/or implementation, (iii) involves actors and/or stakeholders, and (iv) refers to some form of participation process.In fact, we were aware from the beginning of the study that hindering or facilitating factors towards sociotechnical transition are institutional rather than technical, and thus, we were interested in capturing the information on former factors.Appendix A.1 illustrates the additional keywords introduced to operationalize the four inclusion criteria.For example, we defined 23 keywords to represent the different types of actors, including inhabitants, activists, mayors, and investors.Similarly, we considered nine keywords as representing different levels of participation: From information to empowerment, or advocacy.Based on these four inclusion criteria, the sample of 7002 articles could be reduced to 4710, 5490, 2976, and 1094, respectively.Hence, in line with the research aim to be able to critically appraise and synthesize evidence on multitudinous factors that facilitate (or hinder) the transition towards SUWM, we identified a subset of articles as being particularly informative.We selected the articles that meet all four of the inclusion criteria, i.e., 471 articles out of 7002.
In Stage 3, after reading the title and abstract, the sample was further condensed to 138-the excluded articles not focusing on SUWM (e.g., concerning rural farming, strict biodiversity conservation, or forest management).Lastly, based on a content assessment conducted on all of the documents to check for relevance (i.e., to ascertain whether they actually report a case study of planning and/or implementing an SUWM system), the sample was further reduced to 100 articles (numbered in the Supplementary Materials) in Stage 4. This selection excluded 14 reviews whose main points were synthesized in the introduction, and 24 additional documents that were either irrelevant or unavailable.Figure 1 (and Table A1 in Appendix A) illustrates the selection process of the final sample of articles based on keyword searches, abstract screening, and full text analysis.While not exhaustive, we considered the sample of 100 articles to be sufficiently large as to gain useful insights into the empirical evidence on the planning and implementation of SUWM systems.

Review Framework
Despite the wealth of literature pertaining to transition processes, no broadly applicable analytical framework exists that captures the most significant factors for understanding transition trajectories-especially regarding socio-technical systems [37,38].Building on work conducted by [26], we here apply the framework presented in Table 1.By distilling the relevant literature, the framework identifies eight factors that are significant for sustainability transitions in general, and sociotechnical and SUWM transitions in particular [26].These factors are: Pressures, Context, Actors, Purposes, Instruments, Processes, Outputs, and Outcomes.In addition to these eight factors, Table 1 considers Location, Scale, and Terminology as complementary aspects that could enhance the proposed literature review.For each factor, Table 1 presents the guiding question(s) used to review the literature, explanatory notes on how to interpret said questions, and references.

Location
What is the geographical region of the case study and publication year?


Country and city in which the study area is located and year of publication. -

Scale
What is the extent of the analysis?
 Spatial extent of the study area in ha or km 2 and number of people. -

Terminology
What terms are used to refer to Sustainable Urban Water Management (SUWM)?


List the terms mentioned in the article to refer to SUWM.[3,11,13]

Pressures
What are the main pressures on urban water systems? List all environmental, social, climatic, and economic pressures mentioned in general.

Context
What has been the response to the pressures in the case study?How are the pressures in the case study articulated?
 Extract the information concerning the context described in the articles (often provided as a justification for the selection of the case study in the article).

Geography, Scale, Journals, and Authorship
On average, 14 articles per year have been published since 2015, six articles per year during 2012-2014, and one article per year during 2004-2011, making a total of 100 articles (Figure 2).The case studies were primarily located in North America (42 cases), Australia and New Zealand (23), Europe (28), Asia (10), Africa (5), and South America (1).The articles spanned 28 countries; the most represented countries were the USA (40 cases), Australia (21), Sweden (7), and the UK (5), followed by China, France, Germany, Indonesia, and the Netherlands (three cases each), as well as Denmark and New Zealand (two cases each).The remaining 17 countries were covered by one article (Figure 3).In terms of spatial scales, the case studies covered the following: Suburban/local (10), suburban/neighborhood (49), urban (41), and regional ( 14) scales (Figure 4).In the context of our study, local scale refers to a spatial extent of 3-10 ha, whereas the neighborhood scale could reach up to 3600 ha with a population of between 1300 and 100,000.The urban scale ranges between 50,000 and 5,000,000 people, with a spatial extent of between 5 and 5000 km 2 .The regional scale involves between 130,000 and 18,000,000 people, with a spatial extent of 10 to 10,000 km 2 .The 100 reviewed articles were published in 53 journals.Landscape and Urban Planning (eight articles) and Journal of Environmental Management (seven articles) were the two journals with the highest number of articles.Environmental Science and Policy and Water Science and Technology each contained six articles, followed by Sustainability, Water, and Water Resource Management with four articles each.Environmental Modelling and Software, Journal of Green Building, Journal of Hydrology, and Urban Water Journal contained three articles each.Regarding authorship, the majority of the articles (76) were co-authored by between two and five researchers, followed by articles with 6-7 co-authors (10), then articles with a sole author (6), and, finally, a cluster of articles had between 8 and 25 coauthors (8); see details in the Supplementary Materials.

Terminology
Overall, the reviewed articles used 63 different terms to refer to SUWM.This variety of terms was grouped into 14 main clusters, including Blue/Green (Stormwater) Infrastructures and Low-Impact Development/Low-Impact Urban Design and Development.Figure 5 shows the frequency with which the terms in each cluster were used to refer to SUWM.In addition, Table A2 (in Appendix B) shows the frequency of mention of each of the 63 terms in the reviewed articles, as well as their grouping into 14 clusters.

Pressures
A considerable number of pressures were reported in the reviewed articles.These were described both in general terms (supported by references) and, more specifically, with respect to the case studies analyzed in the articles.Through an iterative process, we grouped the pressures into four categories: Environmental, social, climatic, and economic (adapted from [43]). Figure 6 presents the frequency of different types of pressures mentioned in the reviewed articles.The figure also specifies the different types of pressures by category.Indeed, the most frequently mentioned types of pressures were social and environmental, followed by climatic and economic.Table A3 (in Appendix B) illustrates the variety of pressures along with examples from the reviewed articles.

Contexts
We acknowledged a receptive and adaptive context as a key factor for transitions.Table 2 presents some selected extracts from the reviewed articles, which could indicate a receptive and adaptive context in the case study.The selected examples are meant to demonstrate the broad variety of adaptive contexts.However, it is worth bearing in mind that a more in-depth analysis of each case study would have been required so as to assess the actual receptivity of the contexts in question.

Short Description of Contexts
References Kiruna city has an ambitious planning strategy and a conducive political climate.
[67] Fort Collins is a progressive city with a Sustainability Services Department; it has adopted sustainability and climate action plans.The local community has a history of innovation and investment in its future focused on ecological and social values.[68] Since 2013, China has been promoting the planning and construction of the Sponge City, and has strengthened implementation of a CO2 emission reduction plan and a green economy strategy.
[69] Chesapeake Bay's natural resources add over 678 billion USD to the economy.Multiple legislative approaches through the 1972 Clean Water Act and later legislative amendments allowed effective monitoring, policy development, and [70] regulation of discharges.In 2009, President Obama enacted Executive Order 13508 to renew efforts to protect Chesapeake Bay.History of discrimination-Rochester Heights is the first African American neighborhood built partly within the Walnut Creek floodplain.In the mid-1990s, community leaders and advocates formed the Partners for Environmental Justice (PEJ), whose goal was to promote the growth and quality of life of Southeast Raleigh.[71] Portland is a leading city in its pursuit of Blue-Green Infrastructure (BGI).In 1999, the city adopted a Stormwater Management Manual (SWMM), and then initiated a Green Streets Policy in 2007.[72] Common resilience activities involving the built environment are often government-led, have limited public participation, and are dominated by interest groups/elite groups.Vulnerable communities in the USA have less voice with which to influence these types of actions, despite their facing the highest risk from environmental change and disasters.The disparities are deeply entrenched.[73] Chicago is working to comply with National Pollutant Discharge Elimination System Phase II requirements.It has been successful at implementing structural and non-structural Best Management Practices (BMPs) to treat stormwater runoff, and at utilizing Green Infrastructure's (GI's) broad appeal (the Stormwater Ordinance, the Green Roof Initiative, and the Green Alleys Program).[20] Australian Intergovernmental Agreement on a National Water Initiative incorporated the concepts of Water-Sensitive Urban Drainage Design (WSUD) into its urban water reform agenda, and defined WSUD as: 'The integration of urban planning with the management, protection, and conservation of the urban water cycle that ensures urban water management is sensitive to natural hydrological and ecological processes'.In South Australia, the former Department of Planning and Local Government developed the 'Water-Sensitive Urban Design Technical Manual'.[74,75] Melbourne is considered a world leader in Integrated Urban Water Management (IUWM) and, since 2009, has had a significant number of decentralized recycled sewage and stormwater harvesting reuse schemes planned.Five strategies were conducted over the 1997-2007 drought.[76] Daybreak, the largest built GI community in semi-arid Utah, USA.Daybreak is a master-planned community development in South Jordan, Utah, and one of the 'Top 500′ urban sites in the US.A demonstration community of comprehensive sustainable design, Daybreak's extensive parks and open space are integrated with stormwater management, and are enlivened by social and recreational programming.Daybreak is a project assessed in the 2011 Landscape Architecture Foundation Case Study Investigation program.[77] Water-sensitive urban design in Melbourne.In 2006, the state government of Victoria released a plan to improve the quality of the Yarra River.A total of 20 million dollars was allocated under the four-year plan to invest in stormwaterrelated WSUD and associated capacity building of 38 municipal councils across the region.
[78] Prince George's County has been a pioneer in promoting and implementing innovative and practical stormwater BMP and Low-Impact Development (LID) techniques, and has developed a number of tools to support analysis and decisionmaking.[79] The UK government strongly encourages local authorities to implement Sustainable Urban Drainage Systems (SUDS) for future development and regeneration sites.The 'Glasgow Surface Water Management Project' is a [80] Community Initiative established in 2000 by the Commission of European Communities.

Actors
Overall, a diverse range of actors were involved in the reviewed case studies.As shown in Figure 7, they include institutions (e.g., water utilities and municipal departments), different experts (e.g., water engineers, landscape planners, and social scientists), users (e.g., residents, marginalized groups, and non-governmental organizations (NGOs)), and businesses (e.g., developers).Interestingly, the majority of the studies involved or highlighted the key role of users (also referred to as consumers, landowners, and residents, among others), municipal decision-makers, as well as NGOs and other pressure groups.More specifically, Table A4 (Appendix B) provides examples of each category of actors from the reviewed articles.

Purposes
Different types of purposes were considered in the planning and implementation of SUWM systems.Following a classification proposed by [43], the purposes were here classified into environmental and ecological, social, and economic categories.As shown in Figure 8, the most frequently mentioned purposes were again social (e.g., flood mitigation, education and outreach, and recreation), followed by environmental (e.g., water quality of receiving bodies, biodiversity, and ecology), and economic (e.g., cost effectiveness and flexible implementation, green jobs, and investments).

Instruments
Figure 9 presents the range of instruments employed to support the planning and implementation of SUWM in the case studies.The different instruments are classified into five typologies.Decision support-the most frequently employed instrument type-includes hydrologichydraulic, water quality modeling, scenario development, and cost-benefit analysis.Planning instruments ranged from visions and strategies, through comprehensive plans, and up to the level of detailed designs and guidelines.Several studies also highlighted the application of instruments related to Policy, Governance, and Management, as well as Funding Schemes and Incentive Programs.Finally, one common instrument was Participatory Implementation, which took place through various forms of stakeholder engagement-often involving some piloting or experimentation schemes.Table A5 (Appendix B) shows a detailed example of the tools as mentioned in the reviewed articles.

Processes
In our analysis, the reviewed cases were tentatively associated with one or more stages of a generalized planning and implementation cycle.Figure 10 illustrates the number of articles that could hypothetically be linked to the different stages.Accordingly, a significant number of studies (over 35) could be linked to the stages of Analysis (B), Evaluation/Assessment (G), and Problem Analysis (A).We found Monitoring (F) to be the least frequent stage of the process.

Outputs and Outcomes
Numerous schemes exist for classifying outputs, i.e., the physical components of a SUWM.Table 3 presents seven different classification schemes identified within the sample, namely [43,[59][60][61][62][63][64].Particularly, the classification proposed by [59] assumes a gray-green stormwater infrastructure continuum, and considers both green and gray source control measures (SCMs), whereas the other classifications focus predominantly on green measures.Notably, the classification of [59] accounts for the type of construction material (concrete, earthen, plants), drainage area (city to parcel level), types of dominant hydrological processes involved (retention, detention, evapotranspiration, infiltration, and transpiration), dominant water quality processes (active treatment, settling, uptake, filtration, and straining), costs, and community benefits (flood protection, green space, parks, and aesthetic improvement).In Appendix B, Tables A6 and A7 present a detailed comparison of seven different classification schemes of gray and green measures identified in the reviewed articles.
[61] On-site detention tank; permeable pavements; ponds; rainwater harvesting; wetlands; green roofs; biofiltration systems, buffers; swales; bioretention devices.[62] Cisterns, sidewalk storage; porous pavements; filters; rain barrels; detention basins; rooftop gardens; buffers and strips; vegetated swale/swales; bioretention cells; tree preservation. [63] Porous paving; wet retention basin; constructed wetland; infiltration basin; grass filter; grass swale; extended detention basin.[64] Figure 11 shows the aggregated and detailed frequency of the different types of outputs considered in the literature.Generally speaking, green measures outweigh (by almost thrice the amount) gray measures.Specifically, wet ponds, grassed swales, and raingardens are the three most frequently mentioned green measures.Rain barrels, porous pavements, and detention tanks are the gray measures most often cited.In the reviewed case studies, the outputs were either planned, implemented, and/or evaluated.They consisted of both hypothetical (e.g., planned or modeled) and actual outputs (e.g., already implemented, installed, or existing).Notably, we identified 63 terms used to refer to different types of outputs-tentatively grouped here into green and gray measures.Figure 12 illustrates our results concerning outcomes within the sample.A key assumption here is that the outcomes of experimentation and transition are manifested in three primary aspects: Technical learning through developing and building a knowledge capacity, and the transition to a new 'technology'; social learning and change; and mainstreaming-replication, upscaling, and knowledge transferability to other contexts [16,28,45,49,51,52,56,65,66].Using our interpretation, the majority of the studies could be assumed to have achieved some level of technical (e.g., installing and running hydrological-hydrodynamic models) and social learning (e.g., action-research involving marginalized communities).Conversely, fewer articles could be associated with mainstreaming outcomes (Figure 12).

Geography, Scale, Journals, and Authorship
The years of publication and the geography of the articles reveal a rising academic interest for SUWM, particularly in the USA and Australia (where most of the empirical cases are concentrated).This is unsurprising considering these countries' sizes, the intense environmental and/or societal pressures in both contexts, e.g., [6,76,81], and the longstanding legislations and policy responses that have driven the planning and implementation of SUWM facilities at different scales [13,81].
In the European context, Sweden has placed the greatest focus on SUWM approaches.This could be explained by the emphasis that other studies place on the Swedish government's aspiration for the country to be a world leader for sustainable urban sociotechnical systems, such as environmentally adapted stormwater solutions, as a means of consolidating its position in the global market [82][83][84].Other European regions might have been under-represented in the results.Germany is a prime example of a country with a long and advanced experience of SUWM practices in [85,86].Over the past 40 years (with support from innovative policy approaches), SUWM-related technologies in Germany have evolved from experiments to common practices based on developed standards and norms that were also transferred to other contexts [85].
The results include a limited number of articles from Asia [21,[87][88][89][90], only one from Africa [91], and none from South America.This could be partially explained by the fact that studies on sociotechnical transition do not consider a shift from mainstreamed underground drainage pipe systems relevant for the Global South, where the 'modern infrastructure ideal' is largely absent [32,92].Others instead argue that the immediate challenges of the urban water sector in the Global South remain the lack of access to safe water, sanitation, and adequate drainage systems [3].
We also observed that most of the cases address the planning and implementation of sustainable urban water projects at either suburban/neighborhood (55) or urban (46) levels, but relatively less frequently at the regional, watershed, and river scales (12).One explanation for the lack of case studies addressing urban water challenges at the watershed/catchment scale may be due to our selection criteria, which limited our review to include case studies of urban water.Another explanation could be due to the lack of an integrated approach for urban water management because of organizational challenges regarding intra-municipalities and multi-scale collaborationschallenges often perceived as impediments to desirable transformation and outcomes [6,8,13,75,93,94].As explained by others [95,96], these challenges from the mismatch of political/judicial and hydrologic boundaries, if resolved, have the potential to significantly transform the policies and practices of governmental authorities towards sustainable outcomes, according to [8].

Terminology
Ref. [3], in line with others [11,13], describes SUWM as an overarching concept under which many terms are used to reflect a generalized goal to manage the urban water cycle so as to produce greater (and more plentiful) benefits than those delivered by traditional approaches.In the reviewed articles, we counted 63 different terms-although these articles do not necessarily use these terms consistently.Most of the articles would use one term, but mention 4-5 other terms with similar connotations to those used.The described terms are clustered into 14 groups.The most commonly used terms, as other studies have stressed [11], are 'Green Infrastructure' (GI)/42, 'Low-Impact Development' (LID)/35, 'Best Management Practice' (BMP)/23, 'Sustainable Urban Drainage System' (SUDS)/13, 'Water-Sensitive Urban Design' (WSUD)/18, and 'Integrated Urban Water Management' (IUWM)/13.
According to [11], the recent evolution of complex terminology has the potential to increase confusion and miscommunication.Nevertheless, it also promotes a shared local understanding and, as long as communication between disciplines and contexts remains uncompromised, can advance professionalism by being explicit and accurate in its application.Otherwise, according to [59], concepts under which operational measures and applications are not classified and accurately described pose analytical challenges in terms of the explicit and various inputs that decision-makers should consider for optimal water planning outcomes.Thus, for comparative analysis, the authors of [59] call for either a uniform and consistent classification scheme for SUWM technology or, rather than a specific term, suggest following the designs and practices of where an SUWM technology is implemented.
According to these premises and views, the 63 terms used within the sample could potentially add to the challenges faced by the facilitation of change.For instance, differing terms of communication and coordination between disciplines and global regions could complicate the development of appropriate knowledge and policy agendas.Perhaps it is precisely this confusion regarding the core aims and operational implications (in terms of the required type of water infrastructure and the management of urban water cycles) that has driven studies to seek a resolution to this issue [11,32].

Pressures and Purposes, and Values Integrated in Designed SUWM Facilities
From the perspective of sociotechnical transition, socioeconomic and environmental pressuresas well as their intensity, scope, and frequency-cause disruptions and destabilize existing regimes, thereby creating opportunities for technological innovation and social structural change [39][40][41][42].On the role of pressures as a key factor in driving change, Furlong et al. (2016) presented clear evidence on how socio-environmental pressures have shaped the rise and fall of strategies supporting innovation and a transition towards SUWM within Melbourne's water sector.
The analyzed articles show that the types of pressures that have driven change in the urban water landscape correspond to the same types of purposes and values integrated in the designed SUWM systems.Social and environmental pressures play relatively equal and important roles in driving change-an issue that resonates with most of the literature addressing the sustainability limits of existing urban water systems from a predominantly environmental and societal perspective [1,3,13,95,97].Economic pressures play a less decisive role as a driving force for experimentation and innovation.Our results also lend weight to the notion that SUWM systems are primarily planned and designed for their social and environmental purposes/goals, rather than for economic values.Most of the mentioned economic values relate to energy savings, reduction of water users' bills, and increasing values of real estate property.It should be stated that these are context-dependent, and knowledge on whether alternative systems save energy and increase property values is still uncertain and subject to system performances and public acceptance and engagement [27].
Ref. [3,13], among others [6,59,96,98,99], addresses the issue of the lack of economic valuation input as a major constraining factor to technological innovation regarding sustainable urban water approaches.According to these views (and as opposed to gray infrastructure), SUWM solutions promise to deliver long-term intangible socio-environmental benefits and spatially distributed externalities (non-market valued) [100], although these can result in a lack of economic frameworks for the accounting and cost/benefit sharing, which are necessary inputs in decision-making processes.Ref. [59] uses the term 'co-benefits' to define them as ancillary positive ecological, environmental, and societal outcomes that coincide with the installation of structural SUWM facilities, grouped into hydrologic-, climate-, habitat-, and community-related benefits.Other scholars have referred to these co-benefits as 'added values' [14].Political ecologists [38,101] have advocated engagement with communities (considered knowledgeable subjects for valuing these co-benefits) as input into the decision-making processes for optimal sociotechnical infrastructure outcomes.However, the lack of a solid economic framework that incorporates costs (i.e., risks) and benefits has forced municipalities (in certain US regions, for example) to focus on compliance for management of water quality and quantity, and have thus prioritized gray water structural measures for minimizing project cost over green measures [59,101].More generally, based on evidence relating to Australia, Ref. [6] argued that there is an inherent conflict between neo-liberal and environmental policy agendas that support sustainable urban water policy.
Economic rationality for alternative solutions, as opposed to water pipe systems, plays a particular role.Decentralized systems become economically feasible where the risk of droughts and/or flooding is relatively high and imposes unforeseen socioeconomic consequences and costs.Replacing aging infrastructure is constrained by budget limitations and where developing markets for exporting innovative decentralized SUWM systems are sought [6,62].

Context and Instruments
In sociotechnical transition literature, a receptive context is one that is 'aware' of certain pressures and both 'acknowledges' and 'articulates' them [31,42].According to this general understating, the included case studies describe receptive contexts to different extents that cannot be quantified from the review.
An adaptive context and capacity is defined in terms of political, legislative, and financial support and the ability to mobilize resources (human and financial) for the networking and coordination of responses to articulate particular pressures [42,47].Other studies have stressed a particular human agency to materialize transition.Accordingly, pressures need to be taken up and translated by actor agency [29,[44][45][46].Considering these views, context adaptability varies both between countries and within them, e.g., in Onondaga County Metropolitan and the city of Philadelphia [102,103].Based on only the reviewed articles, all of the adaptability elements on context and capacity (including human agency) are found in Philadelphia, which, according to [103], is the first city in the US to attempt an entirely green approach to meeting federal regulations, and was subsequently recognized as a leader in transitioning to SUWM.Table 2 presents some examples of noteworthy findings related to the adaptability of context and transition perspectives.
Similarly, our results show the variety of instruments used to promote the shift towards a sustainable urban water approach.These include decision support tools for modelling and building scenarios; visioning and planning tools, policy designs, and legislations; and financial incentives and mechanisms.They also reveal the significance of a participatory approach involving relevant interest groups as a tool for allowing a space for experimentation and the development of technical niches and policy and societal innovations.The significance of these particular types of tools is stressed as a factor facilitating desirable sociotechnical changes [28,44,[52][53][54][55].
As shown in the reviewed literature-but also stressed in studies outside of the sample-the availability of technical knowledge and modeling tools is key [22,59,67]; however, these tools are neither fully utilized nor aid urban planning processes [35,67,93].Prior studies have also highlighted the critical need for socioeconomic and environmental assessment tools by incorporating socioeconomic factors alongside biophysical and planning factors [3,6,13,59,99].Contemporary efforts are being made to cover these research spots and construct decision support tools and methods for a holistic analysis approach by incorporating urban planning polices and socio-economic factors alongside biophysical models for optimal and long-term outcomes [22,67,94].

Actors
Our results emphasize the diversity of actors involved in new urban water projects.Experts and users are emerging actors and have (or are thought to have) important roles in the promotion and support of new water projects.However, it is unclear from the review whether experts are consultants or scientists.The analyzed articles do not focus on actors' values, vested interests, human agency, influence, or roles, despite these being deemed important factors in enabling/stabilizing or constraining the shift towards new sociotechnical systems and defining outcomes [3,15,28,29,51,52].Still, our review highlights the extended range of actors involved, including relevant experts, as well as the communities and users required by new water infrastructure solutions-a finding that resonates with other studies that stress the variety of actors involved and the vital role of close interdisciplinary collaboration at various levels to achieve maximal benefits [1,15,[48][49][50]98].
Findings concerning actors also illustrate that SUWM niches and projects are mainly developed by public organizational actors at different levels (national, municipal, and water agencies).Business actors have minimal and restricted roles in urban development projects, a finding supported by the fact that most of the socio-environmental co-benefits that SUWM solutions promise to deliver are non-market valued externalities.
The business sector's marginal role is also explained in the literature pertaining to sustainable entrepreneurship, which stresses the importance of public support and funding and cautions against the potential for substantial socio-technical changes towards sustainability initiated by industries [104].According to this view, industries are not motivated to change business models and bear high financial risks for developing innovative infrastructure services in the absence of economic and supportive legal frameworks that accurately define costs, risks, and benefits.The leadership role of public authorities and landscape and urban planning agencies in the transformation process towards SUWM has been emphasized by other studies [26,85,86].
Three aspects are worth remarking upon from these results.First, the mapping exercise of involved actors shows the key roles of four types of actors: Water engineers, urban planners, landscape architects, and environmental (and, to a lesser extent, social) scientists.This resonates with the ideals of SUWM that place water as visible features of a city, and has thus required the early integration of water infrastructure, as well as urban and land use planning, for the design and implementation of SUWM projects [1,5,67,94].This shift in the roles of traditional actors is supported by other studies.Indeed, a shift from the current water infrastructure planning approach that often occurs in isolation from the broader planning of our urban environments and other social services has become a prerequisite for the optimal and effective system design, location, and management of space [49,55,94].
The second aspect relates to the visibility of academic experts in the landscape of developing niches and projects for sustainable urban water-a non-traditional role in planning conventional urban water systems.This role is likely significant for circumscribing the uncertainty and risks present in new water projects, and has been often discussed in the relevant literature [3,13].
Third, the results show a more significant role played by water engineers compared to urban and landscape planners.These roles and their significance are likely contextual.Other studies have shown planners to take a more leading role than water engineers [26,67], which is perhaps an issue related to the types of drivers and defined goals of urban water policy and projects.

Processes
Most of the reviewed articles focus on either the first or second phase of planning cycles; i.e., identifying and recognizing a problem, defining goals and opportunities, and/or problem analyses at different levels.Less emphasis was placed on processes of planning and system design or the implementation of SUWM projects.Most noticeably, analyses of monitoring processes and system performance were less explored by the literature.This is perhaps due to the literature review's limitation of focusing only on planning, and not necessarily reflecting on the type and frequency of processes that occur in real-life contexts.A few articles have implied the existence of monitoring programs, although these have not been described or focused upon [8,22,74,105].
Generally, many studies have highlighted the lack, or sub-optimality, of monitoring processes of constructed water systems.One explanation is given by [59,103] in their report that maintaining and monitoring system performance is labor intensive and thus entails high operation costs.Others highlight different factors by addressing governance challenges to the maintenance and monitoring of new urban water projects [13,34,75,96,97,99,103].These include fragmentation and shifted roles of responsibility, as well as the lack of an institutional capacity to accommodate new water projects in terms of organizational relations, appropriate legislations and mandates, adequate funding and financial mechanisms, operational guidance and standards, and the workforce skills necessary for the maintenance, operation, and long-term commitment to a multi-decadal planning cycle.According to sociotechnical transition premises, the lack (or limited number) of these types of processes could likely compromise learning outcomes, cause the up-take of experimentation results, and defer the urban sustainability agenda [16,38,52,57,58,106].
From our review, we can claim that programs for evaluating systems exist, from which SUWM assets databases, technical manuals, design practices, and tools to support decision-makers have been developed [74,76,79,93,103,107].
Additionally, over 40 articles in our sample address the processes of evaluation and assessment of, for example, public acceptance, economic values, environmental impacts, design and implementation of water projects with respect to policy designs, and stormwater mitigation, etc.However, excluding a handful, e.g., [67], articles that address and analyze the evaluation processes of system performance by actors and planned and designed the systems in an integrated approach are largely missing from our literature review.
The sample does not cover in-depth processes of urban planning and water system design, implementation, operation and maintenance, governance processes for evaluating system performance for social and technical learning, and institutionalization stressed in sustainability transition.Ref. [99] put forward similar remarks and explained that, since SUWM approaches are relatively new and involve an increased level of complexity, there are wide knowledge gaps in their planning, design, implementation, operation, and management.However, we do not claim that our literature review is comprehensive or inclusively captures the developed knowledge of SUWM due to the aforementioned limitation.As such, there exists a need for examining empirical cases that focus on these processes so as to inform policy and practice.

Outputs and Outcomes
According to [59]'s scheme that we followed in order to classify SUWM technologies in the literature, the green measures that are either planned, modeled, implemented, or existing are double those of gray measures.The implication here is that there is a shift towards experimenting, complementing, or replacing gray for green measures regarding SUWM.This corresponds to the proposed claims that the urban water sectors in Australian and US cities (where most of the empirical cases are concentrated) are in stages of transition [6,15,16,75,76,92,99,102,103].
In the United States, however, scholars do not refer to these shifts as transition, but instead use such terms as 'successful cities' [20], 'first city' [103], 'national leader city' [102], or 'leading city' [72,95] in transitioning to SUWM.Concerning technical functions, most gray and green structural measures are primarily designed for detention and retention or, to a lesser extent, as a secondary function.
Regarding the outcomes, the reviewed articles do not generally discuss/analyze or expressly cover processes of social and technical learning or the institutionalization of learning outcomes as core activities in long-term planning processes assumed by transition theorists.Nevertheless, we have assumed, as argued or implied by certain articles [74,79,93,94,103], that technical and organizational learning for developing knowledge capacity is manifested in the experimentation of using (aforementioned) diverse instruments and tools, and is gained through action processes in different phases of the planning cycle.According to this interpretation, we could associate the examined case studies with significant technical learning and knowledge capacity building (approximately 60 articles), social learning in terms of actors and organizational design (55 articles), social learning that has had the capacity to induce institutional re-adjustments (50), and alterations of governance structures and decision-making processes (30).However, fewer case studies discuss mainstreaming as outcomes (6).Despite the significant technical and organizational learning, such edification is not sufficiently adequate to enable a stabilized breakthrough in the urban water sector due to the few articles that, explicitly or implicitly, discuss or describe mainstreaming, i.e., replication, upscaling, and knowledge transferability to other contexts [20,102,103,105].
Only a handful of articles have shown that social and technical learning outcomes are taken up through the provision of strategies, policies, long-term plans, funding schemes, practices, and technical standards, tools, and guidelines for the implementation of urban development projects that have led to societal and organizational changes [85,95,107].This is in line with [94], who argued that, despite more than 25 years of Australian development in this area, SUWM systems have not yet reached maturity due to sub-optimal outcomes of planning practices and disappointing system performances.According to them (page 154), the experiences indicated that 'reactive and incremental approaches to planning are ill-suited to guide a transition towards widespread adoption of Water Sensitive Urban Design (WSUD) approaches'.
Supported by the types of described outcomes, as well as the few cases of mainstreaming, learning outcomes could be said to be unsystematic-except for a few case studies [93,107].To facilitate such change, there is a need for decadal experimentation [49], collective action and reflection among actors and organizations involved in experimentation, and the institutionalization of continuous learning and changes to its operations in order to achieve long-term desirable sustainability outcomes [103,108,109].

Conclusions
In this section, we review the worldwide empirical initiatives covered in this study, return to our research objective regarding the status of available knowledge on the transition towards alternative SUWM solutions to cope with rising urban water stresses and the impacts of climate change, and present our key findings.
The paper is distinctive compared to other literature reviews.To fulfill the objective of the study, we followed an innovative methodological framework that analyzed the main factors viewed as significant for materializing sociotechnical regime change, which are both multiple and interdependent.Thus, this approach has both strengths and weaknesses.Regarding its strengths, the paper outlines available knowledge and advances our understanding of the status and the pace of change in SUWM systems worldwide.It does so through an in-depth analysis of each of the factors outlined in the methodological framework.Nevertheless, analyzing specific causal relations between these factors, such as the relation between purposes and outputs/outcomes or the relationship between context, actors, and outcomes, is lacking.Such an analysis would have produced interesting and valuable results and conclusions; however, this was not possible because the reviewed articles rarely, if at all, thoroughly addressed these factors together.
One of the current study's key findings is the wide diversity in (a) the terminology used to describe the key principles of SUWM; (b) the typology of actors involved in planning processes; and (c) the technical and societal instruments used to mobilize human and financial resources, as well as their coordination so as to plan, spatially locate, and design new systems.
Compared to the substantially important roles played by social and environmental factors, another clear finding is that economic pressures are less significant in driving the change towards SUWM facilities, and economic values are often lacking as input to decision-making processes and in defining the purposes and goals for planning these systems.This also serves to explain the dominate role played by public actors at different levels in promoting new SUWM systems, and the decreased involvement of business actors as initiators, niche developers, and experimenters.When there is a lack of supportive legal and economic frameworks that accurately define costs, risks, and benefits, business actors tend to feel less motivation to bear high financial risks for developing innovative infrastructure services.
The difficulty in analyzing the capacity and adaptability of the contexts in which empirical cases are examined as being subjected to the authors' interpretation, rather than a straightforward criteria, should be noted.Despite this, we can still provide examples of cities where there have been (a) longterm political, legislative, and financial support and commitment; (b) a clear mandate and defined responsibilities; (c) a coordination between human resources and effort; and (d) the inclusion and engagement of interest groups, the mobilization and sustaining of financial resources, and the adoption of human agency as leaders for the transitioning to SUWM infrastructure.The study shows a few cities in the USA and Australia (and, to a lesser extent, certain European countries) in which their context is adaptable, and their leadership of the process of change has been successful in achieving sustainable approaches and SUWM facilities.However, as our literature review is dominated by scholarly work on Australia and the USA, we are well aware that our analysis may be biased in favor of these countries and cities.
The review has also shown the wide range of available societal and technical instruments to promote SUWM, as well as the importance of local context in terms of the institutional and knowledge capacity to design instruments for the development of technical policy and societal niches.
Two main conclusions follow from the study regarding the actors involved.First, a shift towards sustainable urban water projects is mainly driven by politicians and public bodies in response to a multitude of local environmental and social concerns, which, in turn, define their purposes and are manifested in the design of water projects.Second, the shift towards urban water sustainability necessitates a multi-disciplinary approach so as to integrate a diversity of existing and emergent actors in the landscape of urban water management, namely experts and users.
What emerges from our study is the lack of focus or in-depth analysis of processes regarding the monitoring and evaluation of SUWM systems performance, social and technical learning, and the institutionalization of outcomes.This could reflect real-life contexts due to the highlighted profound challenges of the analyzed articles, as well as being due to the scope of our study focusing mainly on planning.However, the study shows that a few cities have managed to shift to sustainable urban water systems, thereby implying that processes coming after the first and second planning phases are likely to be locked in by institutional hurdles.This could lead to a compromising of the desirable outcomes and a deferral of the urban sustainability agenda.
Despite the decadal experimentation in SUWM systems, we can generally conclude the existence of well-recognized challenges, such as governance issues, bridging technical and societal knowledge gaps for optimal planning outcomes and design, and a lack of economic frameworks for accounting and sharing costs, risks, and benefits among societal actors.The study also highlights two uncovered areas in empirical research: The lack of in-depth empirical research regarding both governance structures, and the processes beyond the experimentation and implementation phases of SUWM infrastructure facilities.
Refined by: DOCUMENT TYPES: (

Figure 1 .
Figure 1.Selection of the final sample based on keyword searches, abstract screening, and full text analysis (see also Appendix A).

Figure 2 .
Figure 2. Number of articles per year published during 2004-2019.

Figure 3 .
Figure 3. Case studies of SUWM planning and/or implementation described in the reviewed articles grouped by country (dots placed on the capital cities).

Figure 4 .
Figure 4. Spatial scale of the case studies reported in the reviewed articles.

Figure 5 .
Figure 5. Frequency of the terms used in the reviewed articles to refer to SUWM.

Figure 6 .
Figure 6.Type and frequency of pressures mentioned in the reviewed articles.Lighter colors represent pressures mentioned in general, while darker colors refer to pressures in specific case studies.

Figure 7 .
Figure 7. Type and frequency of different types of actors mentioned or involved in the reviewed articles.

Figure 8 .
Figure 8. Type and frequency of different purposes associated with SUWM in the reviewed articles.

Figure 9 .
Figure 9. Type and frequency of instruments for implementing SUWM mentioned in the reviewed articles.

Figure 10 .
Figure 10.Different stages of generalized planning and implementation cycles addressed in the articles.

Figure 11 .
Figure 11.Type and frequency of outputs mentioned in the sample.In the aggregated graph (left), the darker color represents hypothetical output (planned or modeled), while the lighter color refers to actual outputs (already implemented, installed, or existing).

Figure 12 .
Figure 12.Number of articles that could be associated with outcomes in terms of technical and social learning and mainstreaming.

Table 2 .
Examples of receptive and adaptive contexts in the reviewed articles.

Table 3 .
Types of green and gray outputs according to seven classification schemes reported in the reviewed literature.For a detailed comparison, see TableA6in Appendix B.

Table A2 .
Frequency of the terms used to refer to SUWM systems and their grouping into 14 clusters.

Low-Impact Development (LID) (systems) Low-Impact Urban Design and Development (LIUDD) 38
Altered hydrological response; pressures to the hydrological cycle; disruption of hydrology; hydrological disruption and devegetation; disturbs the natural water cycle; disturbance of natural hydrological regimes; dramatic changes in the hydrologic regimes of whole watersheds; transforms local hydrologic behavior; urban water socio-ecological systems in a degraded and unsustainable regime.Changes in runoff behavior; increased frequency and magnitude of storm-flows and reduced infiltration to feed baseflows; greater discharges and runoff volumes, high runoff volumes and flow rates, higher peak flows, larger volumes, shorter times to peak and accelerated transport of pollutants; increased runoff rate and volume, increased surface runoff volumes and high flows, increases in volume and peak flow. Urbanization,

urban development and sprawl and its impacts
Financing aging infrastructureFinancing the replacement of aging water infrastructure, budget shortfalls; High physical capital investments in centralized systems, demanding massive public investment.Financial and institutional constraints (e.g., in developing countries), municipal infrastructure lags urban growth; Reduced infrastructure financing to compliance with environmental regulations.Resource and capacity constraints (coarse environmental datasets, data scarcity).

Long-term economic crisis/decline 
Post-industrial cities struggling to meet the diverse needs of vulnerable populations, handicapped by eroded tax and infrastructure user bases.Economic and political crises (e.g., in Indonesia, 1997-1998);

Table A5 .
Examples of five types of instruments: Decision Support, Planning Instruments, Policy and Governance, Funding and Incentives, and Participatory Implementation.Scenario; Scenario-based approaches; Scenarios of LID implementation; Criteria weighting scenario; Scenario interpreted in terms of flood assessment, and potential for enhanced recharge in an urbanized catchment; SCMs  A catchment-based structure plan incorporated into the District Plan; Architectural and site program for the project, restoration measures, and its physical expression (2003-2007); State Environmental Quality Review Act (SEQR), Environmental Impact Statement (EIS), and approved mitigation plan;  Detailed site development plans for implementation by subcontractors (standards and natural infrastructure to be provided);  Example of design for the Pollok area.SUDS Option Decision Support Tools = suitability of SUDS for implementation in study areas;  IUWM project design;  Assessing impediments and constraints in the stage of design and development;  State Environmental Quality Review Act (SEQR) process, Environmental Impact Statement (EIS), and approved mitigation plan  Design Manual developed based on experience of 250 Green Streets projects;  Compulsory rainwater management and ecological construction guidelines for all public construction projects;  GI good practice catalogue;  Operational guidelines and a plan of action to help adopting effective strategies to improve the hydrological budget, water availability, and water quality;  Manuals and guidelines, e.g., the "Urban Stormwater Management Manual for Malaysia"; "Guidelines for installing a rainwater collection and utilization system";  Formal or informal rules to regulate gardening practice;  Special planning provision to ensure new developments meet the LSC project's stormwater runoff objectives (collaboration with MW and council);  Stricter regulations and data-driven approaches (Chicago);  The new Swedish design standard SWWA (new stormwater systems to resist a 10 year design storm event without surface ponding);  The Manatee County Fertilizer Ordinance.Management system for the urban greenways; Setting-up a municipal committee to manage the greenways; A municipal charter of collaboration and permanent dialogue for climate resilience;  A two-tiered hybrid (hierarchical, market, and network governance approaches) model, consisting of a local hydrological district and a city-level agency;  "Waterscape" management plan for the Pirai River watershed to strengthen the provision of watershed ES to the city of Santa Cruz; Annual budget of the city council allocated to WSUD in capital works;  About 10 million USD/year earmarked for GI from stormwater fees;  Estimated cost 19.96 million AUD;  Assembling the different ownership parcels of the Long Dock site (1997);  Collaborative land purchase by the City and NGOs;  Land provided by the municipality, funding for gardening tools and materials;  EPA grant funding to assist in stormwater management study; EPA grant (Christina Basin Clean Water Partnership);  Funding for works from Melbourne Water, the Victorian Water Trust, Australian federal and Victorian state government;  Funding from Central Government;  Housing and Urban Development (HUD) funding: 60 million USD in Community Development Block Grant;  Retrofitting existing public buildings with GI by municipalities (e.g., Chicago and Minneapolis);  Financial incentives (an economic-incentive program); Government subsidies to decrease impervious cover in dense city-center;  Water Stormwater Offset Scheme to provide a market for Total Nitrogen (TN) removal;  Carrots: Direct Provision; Rebates and Cost Shares; stormwater credits;  Community engagement through market-based instrument to install SCMs; Consultation with water professionals, utilities, and other stakeholders. National online survey with participants from private and public organizations and professional associations  National survey to collected experimental feedback on BMPs from users/professional  On-line survey to local professionals  Stakeholders' interactive discussions in strategic management;  Stakeholders' consultation to build a (shared) vision of sustainability in the urban water sector  Widespread support from citizens, including a vote of confidence to secure Council funding for the next ten years. Awareness building to engender change in the management of stormwater runoff from properties; Awarenessraising through media and social communication.Water resource conservation in school curriculum;  Citizen-municipality interactions; Large parks, Green spaces "Greened acres"; Forest; Green open spaces; Green spaces; Large parks; Large, permanent forest preserved; Open space; Park; Peri-urban forest; Parkland (landscaped), Pockets of nature; Urban greenspace (to alleviate flooding). Scenarios