A Review of Emerging Scientific Discussions on Green Infrastructure (GI)-Prospects towards Effective Use of Urban Flood Plains

Abstract

The goal of the present review is to collect data on trending scientific discussions on applying green infrastructure (GI) approaches to the effective use of urban floodplains and conceptualize potential future directions. A systematic literature review methodology was employed for this review. We reviewed 120 scholarly articles published between 2011 and 2022 under a predefined protocol. In this review, we discuss the trending dialogues on GI approaches and their applications. The research gap in applying GI approaches for macro-level urban-flood-plain management is addressed by (a) speculative arguments drawn from reviewed GI case studies, (b) an analysis of the trends’ strengths, weaknesses, opportunities, and threats (SWOT), and (c) presenting the concurrent ‘green–gray’ debate on neutral ground. Evidently, GI has its strengths and opportunities, as well as weaknesses and threats. The approaches to GI can be customized according to the application purpose, the regional or locational context, and the intended capacity. Following the analysis of emerging GI discussions, we position the current GI dialogues into four categories: (i) the green–gray continuum; (ii) GI for sustainable and resilient cities; (iii) GI as a resolution for urban issues; and (iv) the green–gray debate. In this classification, we strongly argue that placing GI in a more certain and instrumental position can be optimally achieved in the ‘green–gray continuum’ concept with a win–win scenario. Therefore, scientifically investigating the ‘green–gray continuum’ possibilities in a futuristic approach is strongly recommended.

1. Introduction

The urbanization of riparian zones occurs with great impact on the content, form, and function of flood plain landscapes [1,2]. The increase in the use of hard and impermeable surfaces has environmental impacts arising from alterations of the natural hydrological cycle [3,4]. Such an urbanization process increases the peaks of runoff and reduces the time to runoff concentration, challenging the capacity of existing drainage systems designed for lower runoff conditions [5,6]. With increasing severity of damage caused by urban storm water and floods, urban ecosystems are often misjudged concerning the degradation of ecosystem structures, functions, and services [7,8]. These problems often lead to the rapid conversion of natural green spaces into concrete-based storm-water- and flood-prevention infrastructure, which is also known as ‘hard’ or ‘gray’ infrastructure [9,10]. Conventionally, these gray-infrastructure approaches such as building dams, storm drains, levees, storm sewers, and culverts combined with sewer systems predominate storm-water- and flood-hazard-mitigation solutions [11]. The severity of damage caused by storm water and floods could be reduced by hard mitigation measures, but the problems are inevitably amplified repeatedly once human beings and nature adapt to the artificially generated stability and vulnerability levels, hence this vicious cycle continues [10,11].

The original purpose of combining urban impervious surfaces with conventional gray approaches was to divert storm-water runoff to reduce stagnation and flooding impacts [12,13]. However, in the current discussion of ‘urban sustainability’, there is much opposition to gray storm-water and flood-prevention-infrastructure approaches because of their mono-functional and ecologically inefficient nature [14]. These conventional approaches are further criticized for contributing to waterway erosion, ecosystem degradation in downstream areas, water pollution, limited carrying capacity in terms of storm-water mitigation, higher costs of implementation, and localized flooding [12,15]. In contrast, ‘green infrastructure (GI)’ emerged as a concept first towards enhancing urban livability and was later identified as a tool to enhance urban flood resilience [5,16,17,18]. Eco-sensitive approaches to storm-water management and flood mitigation are not novel worldwide [19]. The term ‘GI’ as a concept emerged in the United States of America (USA) in the mid-1990s and since then gained a wider popularity in recent years [7]. Many discussed features of GI approaches, such as rain gardens, bio-swales, constructed wetlands, urban tree canopies, and green roofs, are often widely acknowledged as effective storm-water-mitigation measures in urban landscapes [20,21]. These approaches have been praised as an all-inclusive package of desirable functions including, but not limited to, reduction in the storm-water-runoff flow rate, conservation of water quality, enhancement of air quality, carbon sequestration, and mitigation of the urban heat island effect, provision of ecosystem services, and provision of multiple socio-economic benefits [10,17,22,23,24]. In contrast to conventional urban-storm-water-management measures, GI approaches focus more on low-impact development (LID) of urban environments [8,16,25].

It is estimated that at least 233 cities worldwide are located within or close to riparian plains, with more than 663 million inhabitants vulnerable to high risks of flooding [26]. Enhancing urban flood plains’ connectivity with rivers and waterways is becoming crucial to prevent repeated flood damage in affected urban areas [11,27]. In doing so, densely populated urban areas critically require vital infrastructure that can withstand the ever-growing hazards posed by storm water and floods, especially those induced by climate change [28]. Managing urban flood plains therefore has drawn much attention in recent years with emphasis on the concern of ‘urban sustainability’ in terms of enhancing urban livability [10,14,29]. Even in this context, shifting towards GI has raised doubts among many stakeholders owing to the lack of post-evaluation of such measures, where their claimed benefits are not supported by scientific evidence, thereby casting doubt on their effectiveness [6,14]. Uncertainties concerning the hydrologic performance of GI and the lack of confidence in the public acceptance of GI approaches generate anxieties and challenges that limit the widespread adoption of GI [30]. Introducing GI approaches to restore urban spaces therefore has become strongly related to the economical valuation of nature rather than the claimed ‘multifunctionality’ of GI [22,31]. Therefore, wider adoption of GI approaches is impeded by uncertainties in a vast range of interdisciplinary barriers in decision-making [24].

Applications, criticism, and reviews of GI approaches vary in scope owing to the ambiguity of GI definitions, and therefore their potential utilization towards the sustainable management of urban flood plains has been widely debated [19,32]. There has been no systematic review of research studies to date, where the applicability of GI approaches for macro-level flood plain management is discussed in comparison to gray infrastructure on neutral ground. By conducting a systematic literature review on the trends in GI approaches over the past decade (2011–2022), we attempt to contribute to the above discussion. The goal of the present review is to collect data on trending scientific discussions on applying green infrastructure (GI) approaches for the effective use of urban flood plains and conceptualize potential future directions. The objectives are to identify (a) the significance of GI approaches with special emphasis on their strengths, limitations, opportunities, and threats and (b) potential and future directions of the application of GI approaches for the effective use of urban flood plains.

2. Review Methodology

This review was carried out following a systematic literature review methodology to comprehend the intended objectives outlined in the introduction. This particular methodology was selected because it enables literature analysis in a systematic, transparent, and reproducible manner [33]. The methodology mainly consisted of the following five steps: (i) definition of the review objectives, (ii) selection of relevant research articles, (iii) evaluation of the selected articles, (iv) compilation of the collected data in a database, and (v) interpretation of the findings [33,34,35]. Accordingly, (i) the review objectives (as outlined in the introduction) were framed following a brainstorming session conducted by us. (ii) Springer Link, Taylor and Francis Online, Science Direct, Wiley Online Library, and Environmental Review bibliographic databases were utilized for article search. The search was limited to scholarly articles published in ISI indexed scholarly journals over the past decade (2011–2022). The Boolean operation was (“Green Infrastructure” AND “Urban Flood Plain Management”). The subject scope was mainly environmental sciences and related fields. The initially filtered scholarly articles were independently screened by us. Finally, only the articles (a) directly related to the review objectives and (b) case studies on the approaches of GI were selected for the review. (iii) selected scholarly articles (n = 120) were reviewed using a defined protocol to (iv) summarize the collected data in a Microsoft Excel database regarding (a) GI approaches in different regions worldwide, (b) diversity of GI typologies and, (c) analysis of strengths, weaknesses, opportunities, and threats (SWOT analysis) of GI applications. The (v) trends and possibilities for applying GI for effective use of urban flood plains were interpreted using the evidence gained by the analysis of the review findings.

3. An Overview of Temporal Trends of Selected Scholarly Articles and Geographic Distribution of Case Studies

The temporal trends of selected scholarly articles show an increasing attention to GI applications very recently (2020 onward). Only 4% of the scholarly articles found using the article search were published in the beginning of the decade (2011–2012). Interestingly however, at the time of data analysis in only the first six months of 2022, already 14 studies matching our search criteria were published, accounting for 12% of the total selected papers. This trend of increasing number of published research papers demonstrates a positive attention to GI applications worldwide (Figure 1).

The earliest articles in this period (2011–2022) included the USA (n = 4), Japan (n = 1), Canada (n = 1), Australia (n = 1), and Greece (n = 1) for case studies. The countries with the highest number of GI-related studies in the period of 2011–2022 (Figure 2 depicts only the number of articles with practical case study applications) are the USA (n = 30), the United Kingdom (UK), and the European Union (EU). There is a new temporal trend showing that GI research related to sub-Saharan African and Asian regions is emerging.

4. Trending Ideologies on GI Approaches for Broader Applications (Table 1)

While dealing with rapid and uncontrolled urbanization, most cities globally tend to have a negative relationship with nature, resulting in decreasing environmental quality with increasing urban density [36,37]. Diverse concepts have emerged worldwide over recent years on how to prevent the environmental quality degradation, such as green infrastructure (GI)/blue–green infrastructure (BGI), low-impact development (LID), nature-based solutions (NBS), sustainable urban drainage systems (SUDS), water-sensitive urban design (WSUD), and water-sensitive cities (WSC) [5,28,38,39,40,41]. When comparing different characteristics of the definitions of these concepts (Table 1), GI is widely considered a ‘core concept’ and is rapidly gaining ground against conventional storm-water- and flood-management approaches [14,42]. This GI concept and its approaches have highly attracted attention recently as ‘nature-inspired’ [3], focused on ‘living with and creating space for water and plants’, and are progressively adopted worldwide [6].

Table 1. Concepts on ‘greener’ urban development approaches—a comparison of trending terms in the literature.

Concepts	Characterization	Merits	Challenges	Sources
Green Infrastructure (GI) /Blue–Green Infrastructure (BGI)	Most definitions indicate GI includes all natural, semi-natural, and artificial networks of multifunctional ecological systems at all spatial scales; within, around, and between urban areas	Accepted as a ‘core-concept’; a multi-scaled approach; ‘connectivity’ is a key element; multifunctional role; nature Inspired	Context specific; financing and management related ambiguities; complexity in application due to broad definition	[43,44,45]
Low-Impact Development (LID)	Main focus is to keep land in an undisturbed state wherever possible and, if disturbance is necessary, to decrease impact on vegetation, soil, and aquatic systems	Control of haphazard development; low impact on ecosystems	Effectiveness; significantly subjective to climatic conditions, landscape characteristics, topography, soil type and especially management actions	[8,23]
Nature -Based Solutions (NBS)	Defined as ‘all actions which are inspired by, supported by, or copied from nature’	A holistic process; tackles multiple problems at the same time	A broad, rather an umbrella term; claims complex actions	[46,47]
Sustainable Urban Drainage Systems (SUDS)	Designed to let water either infiltrate or be retained in constructed structures to mimic natural disposal of surface water; based on natural hydrological processes	Restore natural features within the urban environment landscape	Functional complexity; implementation and monitoring concerns	[8,10]
Water -Sensitive Urban Design (WSUD)	Denotes the integration of ‘urban water systems’ with the ‘natural water systems’ as parts of the urban hydrological cycle	Holistic management of the urban water cycle; source control of storm water	Lack of continuous data to design the systems	[47]
Water-Sensitive City (WSC)	Falls under the umbrella notion of ‘sustainable and/or integrated urban water management’; focus on three pillars, namely (i) expansion of a multiplicity of water sources, (ii) provision of ecosystem services within the urban area, (iii) importance of the necessity for strong institutional capacities	Reduce dependence on a single water source; makes water provision resilient to future uncertainties	Depends on the institutional capacities specific to the context; political and management issues	[48]

Table 1. Concepts on ‘greener’ urban development approaches—a comparison of trending terms in the literature.

Concepts	Characterization	Merits	Challenges	Sources
Green Infrastructure (GI) /Blue–Green Infrastructure (BGI)	Most definitions indicate GI includes all natural, semi-natural, and artificial networks of multifunctional ecological systems at all spatial scales; within, around, and between urban areas	Accepted as a ‘core-concept’; a multi-scaled approach; ‘connectivity’ is a key element; multifunctional role; nature Inspired	Context specific; financing and management related ambiguities; complexity in application due to broad definition	[43,44,45]
Low-Impact Development (LID)	Main focus is to keep land in an undisturbed state wherever possible and, if disturbance is necessary, to decrease impact on vegetation, soil, and aquatic systems	Control of haphazard development; low impact on ecosystems	Effectiveness; significantly subjective to climatic conditions, landscape characteristics, topography, soil type and especially management actions	[8,23]
Nature -Based Solutions (NBS)	Defined as ‘all actions which are inspired by, supported by, or copied from nature’	A holistic process; tackles multiple problems at the same time	A broad, rather an umbrella term; claims complex actions	[46,47]
Sustainable Urban Drainage Systems (SUDS)	Designed to let water either infiltrate or be retained in constructed structures to mimic natural disposal of surface water; based on natural hydrological processes	Restore natural features within the urban environment landscape	Functional complexity; implementation and monitoring concerns	[8,10]
Water -Sensitive Urban Design (WSUD)	Denotes the integration of ‘urban water systems’ with the ‘natural water systems’ as parts of the urban hydrological cycle	Holistic management of the urban water cycle; source control of storm water	Lack of continuous data to design the systems	[47]
Water-Sensitive City (WSC)	Falls under the umbrella notion of ‘sustainable and/or integrated urban water management’; focus on three pillars, namely (i) expansion of a multiplicity of water sources, (ii) provision of ecosystem services within the urban area, (iii) importance of the necessity for strong institutional capacities	Reduce dependence on a single water source; makes water provision resilient to future uncertainties	Depends on the institutional capacities specific to the context; political and management issues	[48]

4.1. Nature-Inspired GI

The requirements raised for higher urban livability led to considerations on a climate-adaptive, ‘polycentric’ approach to urban planning, which may facilitate ‘diverse actions’ through ‘multiple scales’ [49,50]. This argument has led to the lengthy discussions on GI as a sustainable and multifunctional approach to storm water and flood management [51]. According to the European Commission, ‘GI including parks, green roofs, rain gardens, constructed wetlands, and detention basins is a strategically planned network of natural and semi-natural areas co-operating in combination with other urban environmental features designed and managed to deliver an extensive range of ecosystem services’[44].

The term GI is also defined as a concept involving approaches to provide as many green spaces as possible in planning urban areas while aiming to maximize the benefits from those green spaces, and the widespread implementation of GI will lead to more dispersed and source-management applications of future storm-water-management systems [52]. There are terminological variations of GI, whereby the most frequent is blue–green infrastructure (BGI) [6,38,53]. BGI potentially provides both hydrological and ecological protection systems for the urban landscape while generating resilient and adaptive measures towards storm water and flood mitigation [6,24]. Furthermore, the goal of planning BGI is to recreate a nature-oriented urban water cycle while supporting the enhancement of city amenities, by providing green spaces with water management systems [6]. With the focus on addressing ‘interconnected green and blue space networks’ within the urban domain, ‘GI’ is often taken as the common terminology representing GI and/or BGI. The term GI has been extensively promoted as a ‘smart concept’ on preserving and enhancing the remaining natural spaces in today’s cities [10].

Attempts to equate GI to LID make GI an umbrella term, which accommodates all urban storm-water-management concepts that claim to be ‘greener approaches’ [5,54]. Recent GI approaches highlight the significance of designing storm-water-mitigation measures that have a wider role in providing ecosystem services than LID [52], which results in GI having wider and advanced approaches. The concept of NBS is also explained to be similar to that of GI in some instances, where both NBS and GI are described as a ‘creative combination of natural and artificial structures in order to achieve specific goals especially urban-flood-impact mitigation’ and theorized as ‘hybrid sociotechnically engineered systems’ [28,55,56]. However, for a ‘big picture’ of a neighborhood or a watershed, GI is a comparatively broader concept that focuses more on a synchronized effort to utilize both natural elements (e.g., trees and water) and processes for creating healthier urban environments than LID [23]. GI is often explained as a multiscale concept grounded on the landscape and on urban ecological ideas on urban landscape structures, functions, and transformations where ‘connectivity’ is a key element [2,18,29,32]. Therefore, in restoring and/or maintaining urban landscapes, GI aims towards explicit goals [57] through ‘multifunctional approaches’ that can augment the resilience of urban regions facing emerging environmental risks [58].

4.2. Typologies of GI Approaches

As GI is rapidly attracting much attention because it encompasses multiscale and multifunctional approaches, GI has generated different typologies of its approaches [59]. The commonly adopted classifications of GI approaches in applied research can be categorized into three main groups: (i) functional (services), (ii) structural (form/morphology), and (iii) configuration-based (spatial inter-relationship) attributes [60]. Broad-scale classifications based on the functional attributes of GI approaches can be summarized as (i) extensive, (ii) semi-intensive, and (iii) intensive; these classifications distinguish GI approaches by the implementation material (e.g., soil layers), related vegetative capacity, and requirements for maintenance [61]. On the basis of structural attributes, GI approaches are classified into four wider groups largely adopted by many researchers: (i) tree canopy, (ii) green open spaces, (iii) green rooftops, and (iv) vertical greenery systems [62]. Configuration- or spatial inter-relationship-based classifications of GI approaches are strongly associated with discovering how diverse patterns and physical interactions form a multifunctional GI network [60].

That the urban fabric is highly complex makes it difficult to draw a precise boundary between the two realms of natural and artificial elements [57,63]. Consequently, it is almost impossible to decide what GI is and what it is not, when following the broader classifications [60,61,62]. Therefore, it is unrealistic to suggest a ‘universal’ classification that covers all research purposes and expectations worldwide [60]. Therefore, it is evident that GI approaches are multifunctional and can be flexibly customized according to the application purpose, regional or locational context, and the intended function.

5. Highlights of Research on GI Approaches for Storm Water and Flood Management

As a vital component of urban runoff control, urban flood mitigation, and the control of urbanization impacts on hydrological cycles, the importance of GI development is increasingly getting recognized worldwide [3,5,30,52]. The implementation and management of GI involves multiple obstacles, owing to the complexity of urban storm-water management, for which the transition from ‘gray to green’ is intensively debated [51]. However, despite these challenges, many exemplary GI-implementation projects have revealed potential benefits of unit-based, small-neighborhood, and city-level GI applications [64]. As a result, many cities around the world have commenced utilizing GI to achieve the goals of storm-water management and urban revitalization [22,24]. A hypothesis shared among GI advocates today is that GI equals the sustainable development of urban areas [14]. There are many GI typologies adopted worldwide with the broader perspectives of ‘green solutions’ provided by the concept of GI [59,65].

5.1. Adopting GI–Strong Potential and Success Stories

The USA claiming to be the origin of the term ‘GI’, showcases interesting examples of GI best practices [24,66]. Portland, a city in Oregon, claims to be the first city to have applied GI in the USA [24,30]. The city leads in GI implementation in conjunction with gray infrastructure and proves that GI can be implemented at a considerably lower cost than gray infrastructure [11,24,30,49]. An example of GI in New York is the Central Park, whose ‘New York Green Infrastructure Plan’ aims to promote the city itself as an environmentally resilient city [67]. Chicago claims to be one of the first communities to implement a green alley program on a larger scale whose quantifiable success has been achieved since 2006 by installing more than 100 green alleys in diverse types of neighborhoods [68]. Bicester in England is the United Kingdom (UK)’s first ‘eco-town’ where 6000 new zero-carbon houses will be implemented under a government policy that allocates 40% of the area for green space with GI for a ‘linked and wider’ countryside [28]. London, UK is considered one of the world’s greenest cities with approximately 49% of the city comprising blue and green spaces, and GI is a core component in delivering social, cultural, ecological, and economic values [59]. The national ‘Sponge City Program’ in China relies on GI adoption and demonstrates the effective mitigation of urban floods [55,69,70]. This concept of a ‘Sponge City’ is explained as a comprehensive terminology comprising LID, SUDS, WSUD, and GI approaches, and especially the ancient Chinese concepts of nature [28,69]. A holistic perspective of adopting the ‘green–gray continuum’ is showcased by Singapore through its ‘Active, Beautiful and Clean (ABC)’-movement, with an extensive utilization of the city’s ‘greening’ plans to exemplify the links between economic progress and ecologically profound urban design [59]. Japan as a country that traditionally utilized nature in proactively dealing with disasters officially adopted the concept of GI in 2015 in the government’s ‘National Spatial Strategy’, recognizing the potential of GI as a method to restore local habitats and utilize ecosystem services, thereby regulating rising temperatures; and as a means to generate sustainable and smart landscapes [71].

5.2. Concluding Arguments from Applied GI Case Studies (Figure 3)

The success stories of GI applications collectively show that GI is directly connected to the sustainable development goals (SDGs) that can be achieved via ‘a mix of natural approaches’ [52]. Although GI applications strive to reach four main sustainability goals namely, (i) hydrological, (ii) ecological, (iii) social, and (iv) economic, the priority order of these goals varies in terms of specific context and time [72] Moreover, it is evident that actual GI in practical settings is mostly applied in the ‘parcel’ or ‘unit’ scale [21,73,74]. Even though there is a lack of applied GI research on macro level watersheds or flood plains, these GI applications in the parcel or unit level provide evidence that such measures can sensibly be implemented in available urban public spaces without compromising the primary functions of those public spaces [39,54,68]. The disputed complex interconnections between diverse environmental matrices within contemporary urban riparian ecosystems [46], can potentially be restored by utilizing the networking attributes of GI [75]. The case studies conducted for on-site storm-water management using GI approaches predominantly show that storm-water harvesting and reuse must be pursued more earnestly in urban fabrics, since they are highly productive and efficient [76,77,78]. Significantly, interactions between GI implementation and ecosystem services based on co-benefits enhance the communication of the overall GI implantation values [20,79]. It has been empirically and descriptively proven that when the co-benefits of GI are included, the feasibility of implementing GI for flood mitigation can be substantially improved [67,80]. There are many tools and models that were developed for testing the applicability of GI, but they are yet to be calibrated and/or applied in a practical setting [81,82]. Moreover, owing to the lack of research there is still ambiguity regarding the relative efficacy of positioning a few large structures (e.g., embankments and dams) versus many small GI features (e.g., rain gardens and bioswales) dispersed in a watershed in managing urban flood issues [83]. In applying larger scale GI, the support from science and politics towards a holistic development is extensively required [84]. The engagement of stakeholders, shared learning, and two-way communication between the public and authorities are remarkably emphasized towards successful GI implementation, monitoring and maintenance [45,85]. Figure 3 shows an intertwined conceptualization of highlights from concluding arguments of the reviewed GI case studies.

Figure 3. An intertwined conceptualization of highlights from the reviewed GI case studies’ concluding arguments.

Figure 3. An intertwined conceptualization of highlights from the reviewed GI case studies’ concluding arguments.

6. SWOT Analysis of GI towards Effective Use of Urban Flood Plains

The introduction of GI to an urban environment largely depends on human choices (in terms of the complex involvement of stakeholders), yet the current GI trends represent the human desire for environmentally responsible changes [3]. Although combining green and gray infrastructure approaches has been the focus of dialogues on GI, the potential of such an approach in terms of macro-level adoption remain mostly inconclusive [7,51,59,86]. The integration of such potential based on a SWOT analysis is formulated in present research considering the applicability of SWOT analysis in (i) synthesizing and classifying opposing ideas [87,88], (ii) identifying internal (strengths and weaknesses) and external (threats and opportunities) aspects relevant to GI implementation [89,90,91] towards the evaluation of potential GI applications.

6.1. Strengths and Weaknesses of GI (Table 2)

Table 2 summarizes the strengths and weaknesses of GI applications. The major benefits of GI are its multifunctional nature, connectivity, and applicability in multiscale levels with a manifold of flexible typologies [2,18,32,62] which are considered basic GI attributes. The recognition of GI as technique for management, reduction, and utilization of storm-water runoff [17] which delivers a variety of environmental, social, and economic co-benefits, especially ecosystem services, is vastly discussed in terms of GI ‘multifunctionality’ [92]. The provision of benefits for (i) restoring natural hydrology, (ii) ground water recharge, (iii) decreasing the storm-water runoff flow rate, and (iv) water utilization efficiency via rain water harvesting is the key functional benefit of GI [8,23,93]. In urban GI, (i) removing atmospheric pollutants, (ii) increasing air humidity, (iii) sequestering carbon, and (iv) controlling noise and odor are frequently discussed [22,23,42]. Utilizing GI approaches in revitalizing dilapidated urban areas evidently (i) increases adjacent property values, (ii) restores ecosystems and habitats, (iii) generates spatial cohesion, (iv) supports climate change adaptation, and (v) protects fragile nature spots in urban areas and fights against further loss of urban green spaces [10,14,16,22,71]. Visiting places with GI would provide psychological, health, and social benefits, including (i) the sense of community, (ii) recreational and educational values with new cultural opportunities, and (iii) personal well-being improvement, which are recognized by the public in community-based evaluation research [94,95]. Simple GI interventions in an urban fabric can provide greener and smarter networks with (i) functioning open spaces, and (ii) safer links to public destinations (e.g., alleyways to railway stations) making the neighborhoods (i) more livable, (ii) walkable, and (iii) vibrant [68]. Other benefits include (i) calmer traffic, (ii) promoting the use of public transport, and (iii) energy efficiency in buildings [28]. Emerging case studies show that GI alongside gray infrastructure is significantly cost-effective in terms of financial and economic benefits [7,30,42]. Hence, urban GI is explained as a system that addresses (i) geological, (ii) hydrological, (iii) biotic, (iv) circulatory, and (iv) metabolic systems in an urban fabric sustaining natural landscape processes while stimulating socio-economic development [29,45].

The application of GI is always greatly affected by context-specific factors including but not limited to (i) landscape characteristics, (ii) topography, (iii) climatic circumstances, and (iv) soil type, which are considered a weaknesses of GI [16,23,96]. If the context is not understood before planning GI, unexpected engineering and design failures will affect the anticipated co-benefits [59]. For example, although a GI facility meets the required aesthetic level, it may fail to deliver the regulating ecosystem services [14]. Another example is that a GI facility allows free human movement, but it may lead to soil compaction, decrease soil porosity and infiltration capacity, and increase the amount of runoff [8]. Engineering and design failures of GI may lead to serious consequences through (i) numerous small-scale terrain changes caused by massive but faulty systems of GI; (ii) errors in the design or during the construction of a GI facility that may lead to excessive maintenance requirements; and (iii) ending up as a stand-alone facility that is not connected to the overall hydrological system of the site [16]. GI may also become a water pollution source with climatic conditions always uncertain: if prolonged dry weather is followed by a heavy storm, the tendency of polluted storm water to accumulate in the ‘first flush’ is inevitable [10,68]. GI may not be able to address the filtering of certain pollutants (e.g., pathogens, nitrate, and salts), which may lead to ground-water contamination [42]. Ecosystems are not always beneficial and may also be hostile or unsafe at times [97]. GI has the following main weaknesses: (i) tree roots may damage for example sewer pipes and buildings; (ii) if not maintained properly, GI will become a vector of poisonous or toxic animals, invasive species and diseases; (iii) very tall trees may block commuters’ view; (iv) during strong winds there is risk of branches falling; (v) larger GI facilities with vegetation may require higher energy consumption for pumping water; (vi) pollen may become an allergy issue; (vii) dark places at night may be conductive to crime; (viii) if not properly managed, GI may lead to unclean and unpleasant environs with increased vegetative debris; (ix) there may be an increase water-related risks in communities; and (x) it requires high maintenance and cleaning costs [7,10,97]. The values of a certain GI facility may strongly depend on the technology it uses; however, once particular technological conditions change, GI will lose its value [7]. Poorly planned GI will also lead to greater social inequality especially for marginalized populations in urban areas who might be (i) forced to relocate or (ii) prohibited from enjoying the GI co-benefits, thereby resulting in socially unbalanced outcomes [28,98,99]. In the context of rapid urbanization, land is a scarce resource in an urban fabric, but GI may require large spaces and legal barriers will become a hindrance [42,100]. The multiscale and multifunctional nature of GI sometimes results in conflicting functions [2].

Table 2. SWOT analysis of trending GI applications related to effective use of urban flood plains—internal factors (strengths and weaknesses).

Internal Factors
SWOT Component		Details	Sources
Supportive	S–Strengths	Green infrastructures (GI) incur multi-functionality and co-benefits ◦ Storm-water management and preserve runoff benefits ◦ Provide ecosystem services ◦ Water and air purification ◦ Visual quality and urban livability enhancement ◦ Provide health benefits ◦ Protect natural and fragile spaces in urban fabrics ◦ Provide urban public spaces with cultural opportunities	[2,7,8,10,14,16,17,18,22,23,28,29,30,32,42,45,62,68,71,92,93,94,95]
		Enhance connectivity and spatial cohesion of urban elements
		Applicability in multiple scales (site/unit scale, street scale, neighborhood scale, city scale, watershed scale)
		Comprise flexible classifications and typologies
		Promote sustainability by incorporating multiple disciplines
Obstructing	W–Weaknesses	▪ Context specific factors influence efficiency of GI implementation ◦ Landscape characteristics ◦ Topography and soil conditions ◦ Location-specific climatic conditions	[2,7,8,10,14,16,23,28,42,59,69,97,98,99,100]
		▪ Planning, designing, and engineering errors occur with a high tendency due to a low testing rate of mechanisms of GI
		▪ May become potential catalysts for urban issues due to design and maintenance failures ◦ Vegetative debris; home for pests, poisonous animals, and pathogens ◦ Unintended pollution source (water, air, visual) ◦ Crime spot generation ◦ Damage to urban built elements (e.g., damage by roots) ◦ Risks (e.g., branches falling and traffic accidents) ◦ Pollen allergies
		▪ Conflicting functions due to multifunctioning nature

Table 2. SWOT analysis of trending GI applications related to effective use of urban flood plains—internal factors (strengths and weaknesses).

Internal Factors
SWOT Component		Details	Sources
Supportive	S–Strengths	Green infrastructures (GI) incur multi-functionality and co-benefits ◦ Storm-water management and preserve runoff benefits ◦ Provide ecosystem services ◦ Water and air purification ◦ Visual quality and urban livability enhancement ◦ Provide health benefits ◦ Protect natural and fragile spaces in urban fabrics ◦ Provide urban public spaces with cultural opportunities	[2,7,8,10,14,16,17,18,22,23,28,29,30,32,42,45,62,68,71,92,93,94,95]
		Enhance connectivity and spatial cohesion of urban elements
		Applicability in multiple scales (site/unit scale, street scale, neighborhood scale, city scale, watershed scale)
		Comprise flexible classifications and typologies
		Promote sustainability by incorporating multiple disciplines
Obstructing	W–Weaknesses	▪ Context specific factors influence efficiency of GI implementation ◦ Landscape characteristics ◦ Topography and soil conditions ◦ Location-specific climatic conditions	[2,7,8,10,14,16,23,28,42,59,69,97,98,99,100]
		▪ Planning, designing, and engineering errors occur with a high tendency due to a low testing rate of mechanisms of GI
		▪ May become potential catalysts for urban issues due to design and maintenance failures ◦ Vegetative debris; home for pests, poisonous animals, and pathogens ◦ Unintended pollution source (water, air, visual) ◦ Crime spot generation ◦ Damage to urban built elements (e.g., damage by roots) ◦ Risks (e.g., branches falling and traffic accidents) ◦ Pollen allergies
		▪ Conflicting functions due to multifunctioning nature

6.2. Opportunities of and Threats against GI

Table 3. SWOT analysis of trending GI applications related to effective use of urban flood plains—external factors (opportunities and threats).

External Factors
SWOT Component		Details	Sources
Supportive	O–Opportunities	▪ Increasing number of scientific research on GI	[1,14,16,18,22,23,24,30,42,59,64,71,82,103,104]
		▪ Trending attention from governments, institutions, and policy planners
		▪ Innovations, novel models, instruments, and tools for simulation
		▪ Increasing awareness of the stakeholders
		▪ Attitude changes towards GI, as urban attraction catalysts, co-benefit generators, and economic revitalization instruments
		Valuation of GI co-benefits is trending in business, commerce and real-estate sectors
		▪ Climate-change-related concerns and theories tend to embrace GI concepts
		▪ Perceiving GI as the connector of multiple disciplines in spatial planning is trending
		▪ Government- and political-attention-based promotions, subsidies, and efforts on GI implementation
		▪ Emerging discussion on green and gray infrastructure-coupling for resilience enhancement
Obstructing	T–Threats	▪ Complexity of government and organizational procedure	[1,3,6,14,16,22,28,30,42,71]
		▪ Conflicting interests of institutions and stakeholders
		▪ Political agendas and poor decision making
		▪ Lack of knowledge on GI of decision makers
		▪ Bias towards large-capital-based short-term-impact-generating projects
		▪ Fear of adapting novel ideas and innovations
		▪ Land-acquisition-based issues due to higher urban land values
		▪ Uncertainties concerning design climate conditions with climate change related future scenarios

summarizes the opportunities of and threats against GI applications. The implementation of GI is largely based on (i) biophysical, (ii) social, (iii) economic, (iv) planning, and (v) governance contexts [82]. The attraction towards GI in governmental and policy planning has become considerable in recent years [30]. Adopting GI with its multifunctionality in the areas of (i) spatial planning, (ii) development policies, (iii) environmental policies, (iv) disaster risk reduction policies, (v) health policies, (vi) consumer management, (vii) urban agriculture management, and other multiple policy interventions is important worldwide [71,101,102]. It is argued that cities with less dense populations (due to certain issues such as aging) tend to adopt GI to transform them into potential ‘attraction catalysts’ of urbanization [64]. With multiple stakeholder requirements for implementing GI, active and passive agents are increasingly more informed about the co-benefits of GI, altering the negative perceptions [102]. Economic valuation of GI co-benefits is becoming increasingly instrumental for business (e.g., value-added real-estate marketing) and research (e.g., social research on GI as a relaxation opportunity), revealing many innovative techniques and tools [24,71]. Evidence and best practices for GI from science are possible with the increase in GI research activity towards sustainable GI implementation [14,42]. Concurrent concerns on climate change and resilient cities may encourage GI applications [16]. Opportunities for integrating storm water and flood management into urban revitalization, and economic recovery via GI may lead to an extremely strong fiscal capacity [22]. Accumulating scientific evidence on the benefits of GI is increasingly becoming a core rationale for finance-source diversification [59]. The employment of subsidies, refunds, and legislations significantly boosts GI adoption in many countries worldwide [23,68]. Dialogues are ongoing regarding viable means of (i) media interventions; (ii) public discussions; (iii) public review phases for policy, regulation, and legislation mandates; and (iv) innovative instruments (e.g., tools, models, and technologies) and training supporting GI [1,18,103,104].

In one of the most compelling arguments, all challenges to GI are categorized into five distinct categories including (i) standards, (ii) regulations, (iii) socio-economic factors, (iv) financing, and (v) innovation [28]. Another argument is that the challenges to GI are (i) general issues of project management that affect all local governance and infrastructure management aspects, or (ii) issues specific to GI [30]. The complexity of government project-implementation mechanisms poses managerial challenges to adopting GI measures [3]. Even though greater attention is given to GI, it is still not a priority agenda in terms of politics and decision-making because of (i) lack of knowledge on GI among decision makers, (ii) the fear of embracing new ideas, (iii) the tendency towards adopting short-term projects, and (iv) lack of legislation to support GI [6,30]. Getting numerous stakeholders involved across the spectrum of community, administration, and politics is an exhausting and costly challenge for GI [22]. It becomes an even more complex challenge since these stakeholders are always entwined with countless hydrological, social, economic, environmental, and aesthetical (i) considerations, (ii) constraints, and (iii) drivers [3]. The availability of multiple institutions with unclear legal obligations and overlapping mandates is another challenge in this context, as well as coordinating issues in both horizontal and vertical hierarchies [22]. Land availability in the context of rapid urbanization is discussed as one of the most pressing challenges to implementing GI considering (i) the land acquisition difficulties, and (ii) long-term financing issues [14,71]. Hidden and unknown stakeholders who benefit from chaotic situations in cities in the developing world can become a threat to implementing GI [14]. Climate-change-related uncertainties are another prime challenge to utilizing GI, where inadequate experience with the construction and maintenance of GI is a greater hindrance to dealing with such uncertainties [16,42]. There are arguments, that an environment that is restored to mimic a previously available optimal environment using GI is most likely to be unsustainable in futuristic challenges [1].

7. Green and Gray Debate and Emerging Dialogues on GI for Effective Use of Flood Plains

Generally, gray infrastructure is employed with the primary purpose of capturing and treating storm water [105]. It conventionally includes hard engineering approaches (e.g., embankments, tunnels, storage reservoirs, large-scale pumping systems, and channelized and concrete-lined waterways) to convey storm water towards treatment units or to discharge it [39]. Gray infrastructure is often criticized as monofunctional, providing only particular intended benefits [32]. With limited budget available, urban administrators have to make decisions on gray infrastructure implementation against natural elements in urban regions [22,32]. Gray infrastructure rarely provides co-benefits (e.g., ecosystem services) as often pointed out, while emphasizing the unintended negative impacts arising with its implementation [106], such as reducing ecosystem services offered by storm water [21], loss of biodiversity [107], and many others. However, it is true that gray infrastructure measures have been thoroughly tested and evidently found to be reliable in coping with moderate rainfall events; moreover, prospects in implementing them are high given the availability of abundant design methods and public acceptance [58,80].

On the other hand, GI uses vegetation, water, soil, and landscape design to infiltrate runoff, and to entrain pollutants towards restoring the natural hydrologic function and improving the water quality in the urban fabric [20]. In planning and implementing GI, ‘network connectivity’ is considered a core concept where multidisciplinary inputs are taken into account [108]. Providing co-benefits [109] is one of the main features that attract attention towards GI, where its multifunctionality (e.g., social, economic, and environmental benefits) is essentially recognized [110]. It is argued that GI can become a powerful instrument against the rapid loss of nature against haphazard urbanization by protecting fragile green spaces, alleviating further nature loss, and creating innovative urban green spaces [71,86]. Furthermore, GI can be positioned flexibly into urban, natural, and engineering requirements while facilitating the collaboration of stakeholders beyond intended practices [95]. Hence, making the public aware of the prospective social and economic returns from investing in urban GI is currently a trending research issue [59,111]. Yet, alongside these promising advantages, GI is subject to opposition and criticism [42,53,59].

7.1. Dilemma of GI for Macro-Scale Flood Plain Management

Uncertainties of GI approaches, which hinder the wide-spread adoption can be classified into (i) societal barriers, (ii) institutional issues, and (ii) procedural obstacles [51]. Results of multiple studies pinpoint negative attitudes towards GI implementation, and resistance to moving away from conventional methods as the key barriers to GI implementation [6,22,28,51,59]. Tangible or measurable hydrologic performance of GI is constantly debated and government authorities are less confident about the public acceptance of GI [30]. The large-scale adoption and implementation of GI are considerably limited particularly owing to the economic uncertainties related to the costs and benefits of GI approaches versus conventional gray infrastructure [23,112]. The vagueness surrounding financing (e.g., initial capital, maintenance, and perceived costs), public expectations and behavioral variations, and environmental phenomena (e.g., hazards and climate change) significantly and adversely affect the implementation of GI [30]. The under-investment in GI is often rationalized as due to the long-term direct returns of GI investment [113]. It is difficult to convince stakeholders regarding the co-benefits of GI given the long-standing ideologies of conventional methods [59,71]. Dislocation of marginalized communities and underserved settlements in cities under the ‘green-washing paradigms’, are misinterpreted as GI, which hinders effective GI implementation [28]. Opposition to utilizing GI alone for storm water and flood alleviation argue that GI is only appropriate for small-scale, frequent rainfall events but unrealistic for large-scale flood control [16,42]. Hence, even with its benefits, GI is considered impractical to entirely replace gray infrastructures owing to its limited capacity [56].

7.2. Emerging Discussion on a Mutually Complementing ‘Green–Gray’ Approach

Regarding larger-scale GI adoption, there is ongoing discussion on integrating the positive aspects of gray infrastructure and GI to complement each other [14,17]. In designing the guidelines for the ‘green–gray’ applications, a ‘six-word’-principle is gaining attention: (i) infiltrate, (ii) detain, (iii) store, (iv) cleanse, (v) use, and (vi) drain [103]. This approach would couple the long-term tested reliability of gray infrastructure with the sustainability, multifunctionality, and flexibility of GI [22,39,114]. Since ‘resilience’ is the most important factor for confronting pressing urban challenges [115], if the constructed and natural elements mutually reinforce each other, urban ecosystems may regain their capacity to be self-sustaining [7,116].

There are claims that GI is complementary to ‘gray infrastructure’ in terms of protection against flood, temperature regulation, water and air cleansing, wastewater treatment, and sustainable energy availability, providing locally sourced food, and nature-based health benefits [32]. Attempts to balance the stresses between flood disaster mitigation and the protection of ecosystem services through GI and gray infrastructure coupling are emerging in the USA, the European Union (e.g., UK), and China [11,108,117]. From a long-term perspective, the ‘green–gray’ combination would be a systematic win–win solution in a continuum [10,55]. Additionally, incorporating the socio-demographic aspect of environmental justice into the ‘green–gray’ discussion is also emerging [36,118]. Another argument is to move away from the conventional ‘bounce-back’ theory of resilience via the ‘green–gray continuum’, and to move instead towards a ‘bounce-forward’ approach through collective urban management efforts to achieve ecological and social resilience [88]. A ‘resilient urban fabric’ results from the interconnected trio of the built form (urban and landscape design), GI (ecosystems), and knowledge-to-action (social inclusion); therefore, the strategic placement of ‘green and gray’ elements in the urban fabric is gaining interest in discussions, towards optimal results [88,119]. In the context of emerging discussions on a ‘green–gray continuum’, a focus is on ‘integrated-design-matrix’ approaches for flood resilience [17]. The potential improvement of the stressful lives of urban inhabitants by the positive aspects of ‘nature-inspired GI’ is becoming a major selling point for GI [94]. Whatever the major implementation purpose is, GI proposals are therefore deliberated as key modules of a hierarchy of urban spaces (site, street, neighborhood, district, and city scale) for the betterment of urban environments [59].

7.3. GI Dialogues in a Futuristic Approach (Figure 4)

The design and planning, collaboration, co-creation, and coordination approaches towards minimizing GI implementation uncertainties with science-based evidence are increasingly becoming popular [28,120]. Driven by the challenges to climate-change adaptation, urban authorities are focusing on GI-based storm-water management in spatial and policy planning [16,78,121] which is a positive and futuristic trend [18]. In this present review, we position the current GI dialogues on two axes, into four categories: (i) green–gray continuum, (ii) GI for sustainable and resilient cities, (iii) GI as a resolution for urban issues, and (iv) green–gray debate as illustrated in Figure 4. The vertical axis represents the spectrum of urbanization where the effective use of urban flood plains and haphazard development are considered the boundaries. The horizontal axis represents the hierarchy of external challenges, which we conceptualize as emerging with the imbalance between the levels of knowledge and acceptance by stakeholders towards financing and implementing measures against challenges. In this classification, we strongly argue that placing GI in a more certain and instrumental position can be optimally achieved in the ‘green–gray continuum’ concept with a win–win scenario as shown by the analysis results obtained in the present review. Therefore, we propose that the effective use of urban flood plains with GI is achievable through scientifically investigating the ‘green–gray continuum’ possibilities in a futuristic approach.

Figure 4. Positioning the current scientific GI discussions in a conceptual framework.

Figure 4. Positioning the current scientific GI discussions in a conceptual framework.

8. Research Gaps and Opportunities for Future Advancements

In order to progress the use of GI in the future, the rising concept of a ‘green–gray’ approach to GI demands bridging the current research gaps. The key points discussed by many of the researchers point out the need for GI-sustainability studies that take into account the various social, economic, and ecological dimensions and the importance of evaluating the overall sustainability of GI initiatives [11]. Regulation, financial support, and negotiation with stakeholders are necessary for the realization of the other components, which depend on the mentioned three components when considering GI sustainability [3,30]. The process of local citizen participation would benefit rather practical solutions [2]. In doing so, it may be important to examine the concept of social responsibility when assessing the findings of prior and future studies on community-related GI implementations [32]. Important next steps include longitudinal research to see if people’s use and views of urban green spaces (for their physical and mental health) have changed, as well as research to find out what role GI plays in planning and designing urban landscapes [16]. On the other hand, case studies have demonstrated that the tested theoretical assumptions and principles of GI implementation are significant, but that more specification and optimization are required. For practical implementation, the multi-functionality of GI must be discovered, validated by scientific data, and advocated by stakeholders who will benefit. Typically, the literature on flood-risk management does not address how GI could be branded outside of flood-risk-management schemes [6]. In order to develop infrastructure that is scientifically sound and supported by urban stakeholders, a coordinated effort should be made to remove some bio-physical uncertainties and barriers through better data and enhanced scientific knowledge [6,16,22]. More deliberately addressing ecological functions as well as specific functional-accessibility indicators, and measures on the quality of GI would contribute to a better understanding of GI planning and may motivate practitioners to develop examples of best practices on applying the ‘green–gray’ approach [62,80,107]. In addition, future research needs to collect more data and look into the benefits of GI’s multi-functionality in more depth. It also needs to translate ecological, environmental, and social impacts into monetary terms to make the public more aware of sustainable GI implementations using the ‘green–gray’ approach [23].

9. Conclusions

The concept of a ‘green–gray continuum’ makes it possible and helpful to use GI in order to make efficient use of urban floodplains. The primary conclusions drawn from the GI case studies that were examined for this review all point to the fact that GI may be adapted to fit the requirements of a given application, regardless of the place or region in question or the function that is meant to be performed. In addition to its functional capabilities of storm-water management and water balance in urban fabrics, GI’s primary benefits include multi-functionality, connectivity, and adaptability. These benefits are in addition to its functional capabilities. The term ‘GI’ is related to a number of underlying uncertainties, some of which could be addressed by the utilization of developing prospects. The significance of GI applications has been the subject of a significant amount of research recently, particularly in nations that are transitioning toward climate-resilient cities. In addition, it is strongly recommended that scientific research into the potential of the ‘green–gray continuum’ should be conducted using a futuristic approach.