What is GI: A new definition

Cities have always struggled with the need for green amenities. One can imagine Nebuchadnezzar realizing the need for some vegetation to keep Babylon cool, absorb air and water pollutants, and provide aesthetic services, and deciding to create the Hanging Gardens of Babylon that included exotic plants as well as constructed streams and topographic features (Polinger Foster Reference Polinger Foster1998). Cities worldwide have always integrated green and ecological elements into their core fabrics, like the wetland agricultural systems of chinampas in Mexico City (Merlín-Uribe et al. Reference Merlín-Uribe, González-Esquivel, Contreras-Hernández, Zambrano, Moreno-Casasola and Astier2013), and extensive systems of landscape alteration in Peru (Tomateo Reference Tomateo2021). The challenge of combining the benefits of dense urban form while maintaining pleasant and high-quality environments has long attracted creative and ambitious thinkers such as Nebuchadnezzar. A more modern example is the work of landscape designers and urban planners such as Frederick Law Olmstead and Ebenezer Howard working in the 19^th century to provide green amenities to rapidly developing cities in Europe and the USA (Eisenman Reference Eisenman2013). Struggles have emerged as these green visions intersected with hierarchical and unequal planning processes, like those that resulted in the displacement of a predominantly Black community during the realization of Olmstead and Calvert Vaux’s vision for New York’s Central Park. These processes have led to numerous intersecting concerns around the equity of GI strategies (Grabowski, McPhearson, and Pickett Reference Grabowski, McPhearson and Pickett2023). As urban development accelerated over the last century, and the challenges of maintaining green features in cities became more evident, conceptualizations of GI evolved to integrate human-engineered infrastructure systems with more “natural” ecosystems and processes.

A major change occurred in 2007 when the US Environmental Protection Agency (EPA) began encouraging the use of GI for compliance with the Clean Water Act’s National Pollutant Discharge Elimination System (NPDES) regulations. NPDES was designed in part to mitigate the damaging effects of stormwater runoff from impervious surfaces on the physical, chemical, and biological properties of receiving waters by regulating the outfalls of separated stormwater sewers and combined sewers (in which sanitary sewage and stormwater runoff are conveyed in the same pipes) as point sources of pollution. Of particular concern is the prevention of combined sewer overflows (CSOs) which occurs when a combined sewer systems capacity is exceeded during heavy rainfall events and raw sewage overflows directly into lakes and rivers. Because GI facilities can slow, store, and treat stormwater before it enters either sewer system type, it can help mitigate point source pollution discharge stemming from both combined and separated sewer systems. EPA’s acceptance of GI stimulated the development of numerous hybrid stormwater control measures in cities across the USA working to obtain NPDES permits (McPhillips and Matsler Reference McPhillips and Matsler2018).

The explosion of interest in GI features to slow, store, and treat stormwater in the USA underpinned the formalization of green stormwater infrastructure (GSI) approaches defined under Section 502 of the Clean Water Act as “…the range of measures that use plant or soil systems, permeable pavement or other permeable surfaces or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters.” These features reduce and treat stormwater where it is produced, while at the same time delivering ancillary environmental, social, and economic benefits. There is a clear contrast between these features and “gray” stormwater infrastructure including conventional piped drainage and water treatment systems designed to move stormwater away from urban features. GSI can save money. For example, the NYC Department of Environmental Protection estimated that by pursuing land conservation instead of treatment plant construction in the Catskill and Delaware watersheds they avoided over $8.5 billion dollars in costs (New York City Department of Environmental Protection 2010). Within the city itself, NYC DEP estimated that the use of GSI for controlling CSOs would save $1.5 billion compared to a straight gray approach (New York City Department of Environmental Protection 2010).

The clear and distinct interests in GI for general urban greening and for stormwater control create confusion and a GI paradox (Grabowski et al. Reference Grabowski, McPhearson, Matsler, Groffman and Pickett2022). In an analysis of over 122 plans from 20 cities in the USA, Grabowski et al. (Reference Grabowski, McPhearson, Matsler, Groffman and Pickett2022) found over 140 unique definitions of GI in diverse types of city plans (comprehensive, watershed, sustainability, etc.). These definitions fit within three distinct conceptual orientations of GI: approaches focused on landscape conservation, stormwater management, or a broader integration of built systems with the natural environment. These concepts were somewhat aligned with specific plan types, as many cities use different definitions of GI in different types of plans. In some instances, a GI paradox emerged, whereby the adoption of a stormwater-focused concept prevented the implementation of broader landscape conservation approaches, exacerbating tensions between city agencies managing different aspects of a city’s GI. To facilitate a more robust interdisciplinary and inter-sectoral dialog, Grabowski et al. (Reference Grabowski, McPhearson, Matsler, Groffman and Pickett2022) proposed a new definition of GI as “…a system of interconnected ecosystems, ecological–technological hybrids, and built infrastructures providing contextual social, environmental, and technological functions and benefits. As a planning concept, green infrastructure brings attention to how diverse types of urban ecosystems and built infrastructures function in relation to one another to meet socially negotiated goals.” This definition motivates consideration of multiple types, functions, and benefits of GI and could provide an integrative point of departure for planning, design, and implementation of GI strategies in cities, encompassing built infrastructure systems providing stormwater, transportation, energy, and housing as well as broader urban greening objectives.

A key component of this definition involves viewing different features along a spectrum from almost entirely natural ecosystems such as relict forests and wetlands at one end to highly engineered features such as porous pavement at the other end, with features such as bioswales and green roofs in between (Figure 1). This “eco-techno spectrum” (Matsler, Miller, and Groffman Reference Matsler, Miller and Groffman2021) is useful for conceptualizing the range of GI features in a city. More fundamentally, it is useful as a basis for evaluation and analysis of the services provided by these features. There are multiple coherent patterns that fall out along this spectrum, including direct and indirect community interactions with different types of facilities, varied jurisdiction over the design, implementation, and maintenance of these features by different agencies with different missions, and differing sources of finance across the types of features. Information about this variation is critical if we are to evaluate the effectiveness of GI, as it will be evaluated on its effectiveness and efficiency at providing specific infrastructural services (Hoover et al. Reference Hoover, Meerow, Grabowski and McPhearson2021; Meerow and Newell Reference Meerow and Newell2017; Herreros-Cantis and McPhearson Reference Herreros-Cantis and McPhearson2021).

Figure 1. The eco-techno spectrum of green infrastructure organizes facilities by the proportion of the facility that consists of living, biological components vs human-made, technological components. From Matsler et al. (Reference Matsler, Grabowski and Elder2021).

Does GI work?

Evaluations of the effectiveness of GI features must be interdisciplinary and multiscalar. They need to consider the wide range of functions that are provided by these features at highly local (individual site) and larger (neighborhood, watershed, city) scales. Evaluations need to consider the wide range of stakeholders that are affected by the features, with a focus on the equitable distribution of amenities and disamenities.

Evaluation of GI has been a major focus of the Urban Resilience to Extremes Sustainability and Resilience Network (UREx-SRN) funded by the US National Science Foundation (McPhearson et al. Reference McPhearson, Cook, Berbés-Blázquez, Cheng, Grimm, Andersson, Barbosa, Chandler, Chang, Chester, Childers, Elser, Frantzeskaki, Grabowski, Groffman, Hale, Iwaniec, Kabisch, Kennedy, Markolf, Matsler, McPhillips, Miller, Muñoz-Erickson, Rosi and Troxler2022; McPhillips and Matsler Reference McPhillips and Matsler2018). This network views cities and their components such as GI as social-ecological-technological systems (SETS), which facilities multidisciplinary and multiscalar evaluations. UREx-SRN focused on 10 cities in the USA and Latin America, covering an extensive range of climate, governance, and socio-demographic characteristics.

Interdisciplinary analyses of GI in these 10 cities found five challenges to the realization of comprehensive GI benefits (McPhearson et al. Reference McPhearson, Cook, Berbés-Blázquez, Cheng, Grimm, Andersson, Barbosa, Chandler, Chang, Chester, Childers, Elser, Frantzeskaki, Grabowski, Groffman, Hale, Iwaniec, Kabisch, Kennedy, Markolf, Matsler, McPhillips, Miller, Muñoz-Erickson, Rosi and Troxler2022). The first challenge was a lack of cohesive GI design and implementation guidelines and standards across different “silos” within cities. The use of GI has stimulated a great flowering of creative design and strategies, but a lack of guidelines and standards creates variation in implementation and challenges for evaluation. This variation underlies the second challenge, that is, the need for regionally specific codes and standards (Matsler, Grabowski, and Elder Reference Matsler, Grabowski and Elder2021). Guidelines and standards for dry cities in Latin America (e.g., Mexico City) need to be different than those that apply in wet temperate cities (e.g., Baltimore). Development of these guidelines and standards should be adaptive and flexible however, as new forms and approaches develop. They must all be comprehensive and encompass the wide range of social and ecological issues affected.

A third challenge is a lack of attention to environmental justice and equity (Cousins Reference Cousins2021). There are frequent disconnects between the populations and communities that benefit from and appreciate the amenities of GI and the communities that suffer disservices and burdens associated with GI (Hoover et al. Reference Hoover, Meerow, Grabowski and McPhearson2021; Heck Reference Heck2021; Walker Reference Walker2021). Environmental justice is a great challenge across cities, and there is hope that a SETS-based approach to evaluate GI can provide an approach for addressing inequity in other areas (McPhearson et al. Reference McPhearson, Cook, Berbés-Blázquez, Cheng, Grimm, Andersson, Barbosa, Chandler, Chang, Chester, Childers, Elser, Frantzeskaki, Grabowski, Groffman, Hale, Iwaniec, Kabisch, Kennedy, Markolf, Matsler, McPhillips, Miller, Muñoz-Erickson, Rosi and Troxler2022).

A more disciplinary challenge is the fact that design metrics for GI do not consider extreme events and implications for sustained performance and support of human well-being. This omission is especially important for GSI features that are designed to process runoff from small- or medium-size storms. Extreme events are often responsible for the majority of environmental damages and negative social outcomes and are increasing with climate change (Rosenzweig et al. Reference Rosenzweig, Ruddell, McPhillips, Hobbins, McPhearson, Cheng, Chang and Kim2019).

A final cross-cutting challenge identified by the UREx-SRN analysis was a lack of streamlined financing mechanisms. Many groups (municipalities, real estate developers, nonprofits) are interested in GI, but a source of funds for design, implementation, and interdisciplinary and multi-scale evaluation is often lacking. And while many municipalities have extensive experience with estimating costs and potential bids for other infrastructure projects, the cost uncertainty of GI strategies hinders the creation of dedicated budgets (Dickson et al. Reference Dickson, Arnold, Dietz, Lefevre, Kinnear and Boyer2018). There is a clear need for research about marketing methods to stimulate provision of the missing funding through private action or government taxation powers.

While the UREx-SRN analysis identified general challenges facing the function of GI, there has also been extensive analysis of the performance of individual sites in specific neighborhoods. For example, work in NYC addressed concerns about accumulation of petroleum hydrocarbons and heavy metals in GI features connected to streets (Figure 2). This work showed that features more directly connected to streets, for example, with curb cuts, have higher levels of contamination. However, these features still maintained high levels of microbial function relevant to water quality (Figure 3). There is a need to consider how long high levels of microbial function will be sustained if contaminants continue to accumulate. These results highlight the clear need for ongoing, detailed evaluations that consider how ecological function affects or serves human well-being.

Figure 2. Boxplots showing the effect of green infrastructure designs: enhanced tree pits (ETP), street-side infiltration swales (SSIS), and vegetation swale (VS), on total petroleum hydrocarbon (TPH), lead (Pb), and zinc (Zn), respectively. The middle bar is the median, the box extends from the 25% to the 75% quartile, and horizontal bars show minimum and maximum values. Significant differences (P < 0.05) are indicated by different letters. Dashed line is the contamination threshold (n = 12, 15, 33 for ETP, SSIS, and VS, respectively). The level of “connectivity” to the street is ETP > SSIS > VS. From Deeb et al. (Reference Deeb, Groffman, Joyner, Lozefski, Paltseva, Lin, Mania, Cao, McLaughlin, Muth, Prithiviraj, Kerwin and Cheng2018).

Figure 3. Boxplots showing the effect of green infrastructure designs: enhanced tree pits (ETP), street-side infiltration swales (SSIS), and vegetation swale (VS), on soil microbial biomass carbon and nitrogen (N) content, microbial respiration, potential net N mineralization and nitrification, and denitrification potential (DEA), respectively. The middle bar is the median, the box extends from the 25% to the 75% quartile, and horizontal bars show minimum and maximum values. Significant differences (P < 0.05) are indicated by different letters (n = 12, 15, 33 for ETP, SSIS, and VS, respectively). The level of “connectivity” to the street is ETP > SSIS > VS. From Deeb et al. (Reference Deeb, Groffman, Joyner, Lozefski, Paltseva, Lin, Mania, Cao, McLaughlin, Muth, Prithiviraj, Kerwin and Cheng2018).

A major site-scale concern is maintenance. Informal surveys of GI features in many cities reveal accumulation of trash, clogging of flow paths, and degradation of biological features, for example, plants (Figure 4). Maintenance issues can often be seen in underserved neighborhoods, which combined with expectations that volunteers maintain facilities and can create further environmental justice issues (Hager et al. Reference Hager, Belt, Stack, Burgess, Grove, Caplan, Hardcastle, Shelley, Pickett and Groffman2013; Hoover et al. Reference Hoover, Meerow, Grabowski and McPhearson2021; Riedman Reference Riedman2021).

Figure 4. A green infrastructure feature in an underserved neighborhood in Baltimore, MD, USA. This feature is designed to reduce runoff to receiving waters well outside this neighborhood, which loses parking spaces. The feature has accumulated trash, despite a “please don’t litter” sign and plant material has degraded. Photo by Neil Bettez.

How can we tell if GI is working?

The above discussion about just how well GI functions highlights the need for interdisciplinary and multiscalar evaluation systems. These systems must account for the wide range of biophysical and social components of GI and must be valid for the wide range of features and locations where these features occur. Further, to meet the needs and demands of multiple stakeholders, they must be technologically and scientifically valid, efficient to carry out, and understandable to a wide range of stakeholder audiences.

For example, an evaluation system to monitor natural and nature-based shorelines was produced recently in New York State in the USA (Wijsman et al. Reference Wijsman, Novem Auyeung, Brashear, Branco, Graziano, Groffman, Cheng and Corbett2021). The state was making large investments in natural and nature-based shorelines to increase resilience to large storms such as Hurricane Sandy. These investments created a strong need for a framework capable of evaluating the full range of these features, which is similar to the range of features along the eco-techno spectrum (Figure 1). These features are deployed across the highly diverse shorelines of New York State, that is, Atlantic Ocean, Hudson River, and Great Lakes. The framework needed to have a strong scientific and technical foundation and be relevant to and understandable by agencies, practitioners, and civic groups. Most importantly, the framework had to encompass the multiple functions and effects of these features; hazard mitigation, structural integrity, ecological processes, and socio-economic and community resilience. Wijsman et al. (Reference Wijsman, Novem Auyeung, Brashear, Branco, Graziano, Groffman, Cheng and Corbett2021) describe the development of this framework with scientific working groups, interactions with stakeholders across the state, and field testing. The framework is currently being applied and further tested with the hope that it will serve as an example that will be useful over the long term, in many locations.

Sandy is coming back: What are we going to do about it?

In October 2012, Hurricane Sandy flooded streets, inundated tunnels, and caused power outages across NYC. There were 44 fatalities in the city and an estimated $19 billion in damage and lost economic activity (https://www1.nyc.gov/site/cdbgdr/about/About%20Hurricane%20Sandy). Due to oceanographic and geological factors, NYC occupies a sea level rise hotspot (Sallenger, Doran, and Howd Reference Sallenger, Doran and Howd2012) and will be subject to increasing intensities of localized extreme precipitation events (New York City Department of Environmental Protection 2017). Although there is considerable uncertainty about increases in the intensity of coastal hurricanes and extratropical cyclones (Garner et al. Reference Garner, Mann, Emanuel, Kopp, Lin, Alley, Horton, DeConto, Donnelly and Pollard2017), there is a strong sense that more Sandy-like superstorms are on the way. How cities like New York will prepare for these storms, and other intersecting climate-related hazards is perhaps the most important question facing coastal cities over the next 100 years (Orton et al. Reference Orton, Lin, Gornitz, Colle, Booth, Feng, Buchanan, Oppenheimer and Patrick2019).

A crucial decision point in preparing for major storms is the use of gray versus GI. This decision can be framed as a contrast between “fail-safe” and “safe-to-fail” approaches to hazard management (Kim et al. Reference Kim, Eisenberg, Bondank, Chester, Mascaro and Shane Underwood2017). In the traditional fail-safe approach, large walls, concrete channels, and other engineered structures are used to protect human settlements. If these structures fail, for example as they did during Hurricane Katrina in New Orleans in 2005, the damage can be catastrophic. Concern about these failures is increasing as both the climate and the structure and function of cities becomes harder to predict. This concern has led to the consideration of safe-to-fail approaches that prioritize protection of key services, minimizing the consequences of extreme events and facilitating rapid recovery and resumption of services. Safe-to-fail approaches are often less centralized than fail-safe approaches and require interdisciplinary approaches to develop. Of particular interest is the need to “design for exceedance” or undergo a process of examining the consequences of systems’ experiencing conditions exceeding their design thresholds, necessitating advancements in systems modeling approaches that incorporate gray and green elements. These systems models could be combined with economic analyses of the costs, benefits, synergies, and trade-offs to provide a more balanced estimate of the net benefits and costs associated with different GI strategies.

To ecologists, safe-to-fail approaches are extremely appealing as they are deeply rooted in ideas about resilience and diversity that have emerged from the study of natural ecosystems. But how will this “play out in the real world”? Will decentralized GI features work as well as large flood walls during a large storm? Perhaps more importantly, will people have confidence in these features as they make decisions about where to live and invest? Will the features be distributed equitably, for example, which neighborhoods will get esthetically pleasing GI versus unsightly earthen berms? Can significant greening investments provide local resilience benefits but displace current residents to more risky areas (Gould and Lewis Reference Gould and Lewis2016)? What role should planned retreat play in making space for extreme events? For example, one approach to increasing flood resilience may be “buyouts” of land in floodplains which will require difficult decisions requiring significant attention to the social dynamics of planning such interventions. As a new Federal Technical Working Group on the costs and benefits of nature-based solutions takes shape in the USA (White House 2022), we must remain attendant to the social preferences that influence the valuation of different types of infrastructure systems supporting distinct patterns of development and economic activity.

In NYC, the decision point between fail-safe and safe-to-fail approaches involves trade-offs between a series of enormous flood barriers and construction of a large number of natural and nature-based features across the City (Aerts et al. Reference Aerts, Wouter Botzen, de Moel and Bowman2013). Making these decisions will require extensive scientific and technical expertise and interaction with a large and diverse group of stakeholders (McPhearson, Hamstead, and Kremer Reference McPhearson, Hamstead and Kremer2014). A heated debate continues over the desirability and feasibility of planned retreat as part of making space for extreme weather and GI in NYC and other coastal cities worldwide, which largely appears to only be feasible after disasters are experienced (Braamskamp and Penning-Rowsell Reference Braamskamp and Penning-Rowsell2018). Hopefully, the advances in design, planning, and evaluation that we discuss here will be useful for a more anticipatory process in New York and other coastal cities.