Jacopo Belli - September, 19, 2026

Green as Urban Infrastructure

1 Fletcher, T.D., Shuster, W., Sinclair, R., Vogel, J., Cloak, M., Breen, P., Deletic, A. e Burns, M.J., 2015. SUDS, LID, BMPs, WSUD and more – The evolution and application of terminology surrounding urban drainage. Urban Water Journal, 12(7), pp.525-542.

2 European Environment Agency, 2021. Nature-based solutions in Europe: Policy, knowledge and practice for climate change adaptation and disaster risk reduction. Copenhagen: EEA Report No 1/2021.

3 Giuffrida, A. (2026, January 27). ‘Situation is dire’ for Sicily town teetering on cliff edge after landslide. The Guardian. https://www.theguardian.com/world/2026/jan/27/niscemi-sicily-landslide-chasm-storm-cyclone-harry

4 Benedict, M.A. e McMahon, E.T., 2012. Green infrastructure: linking landscapes and communities. Washington: Island Press.)

5 Mell, I., 2016. Global green infrastructure: Lessons for successful policy-making, investment and management. London: Routledge.

6 Zevenbergen, C., Fu, D. e Pathirana, A., 2018. Transitioning to Sponge Cities: challenges and opportunities to address urban water problems in China. Water, 10(9), p.1230.

7 Chan, F.K.S., Griffiths, J.A., Higgitt, D., Xu, S., Zhu, F., Ng, C.N., Thorne, C.R. e Thorne, C.R., 2018. “Sponge City” in China. A choice of planning and constructing climate resilient frontier cities. International Journal of Disaster Risk Science, 9(4), pp.456-470.

8 Santamouris, M., 2014. Cooling the cities—a review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments. Solar Energy, 103, pp.682-703.

9 Gill, S.E., Handley, J.F., Ennos, A.R. e Pauleit, S., 2007. Adapting cities to climate change: the role of the green infrastructure. Built Environment, 33(1), pp.115-133.

10 Akbari, H., Pomerantz, M. e Taha, H., 2001. Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas. Solar Energy, 70(3), pp.295-310.

11 Santamouris, M., 2014. Cooling the cities—a review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments. Solar Energy, 103, pp.682-703.

12 Nowak, D.J., Crane, D.E. e Stevens, J.C., 2006. Air pollution removal by urban trees and shrubs in the United States. Urban Forestry & Urban Greening, 4(3-4), pp.115-123.

13 Tzoulas, K., Korpela, K., Venn, S., Yli-Pelkonen, V., Kaźmierczak, A., Niemela, J. e James, P., 2007. Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review. Landscape and Urban Planning, 81(3), pp.167-178.

14 Costanza, R., de Groot, R., Sutton, P., van der Ploeg, S., Anderson, S.J., Kubiszewski, I., Farber, S. e Turner, R.K., 2014. Changes in the global value of ecosystem services. Global Environmental Change, 26, pp.152-158.

15 Frantzeskaki, N., 2019. Seven lessons for governing nature-based solutions in cities for urban sustainability and resilience. Environmental Science & Policy, 93, pp.101-111.

16 Kabisch, N., Frantzeskaki, N., Pauleit, S., Naumann, S., Davis, M., Artmann, M., Haase, D., Knapp, S., Korn, H., Stadler, J. e Zaunberger, K., 2016. Nature-based solutions to climate change mitigation and adaptation in urban areas: Perspectives on indicators, knowledge gaps, barriers, and opportunities for action. Ecology and Society, 21(2), p.39.

Fig. 1: Integrated functioning of the Sponge City model

The worsening of extreme weather events and the constant increase in urban temperatures push contemporary urban planning towards a necessary evolution, which involves overcoming the traditional model of greywater management in favor of an approach based on "Green-Blue Infrastructure" (GBI). Through the lens of hydro-strategy, urban greenery is redefined not merely as a decorative element, but as a critical and multifunctional infrastructural asset. The paper aims to analyze how the adoption of the Sponge City model and Nature Based Solutions allows for achieving hydraulic and hydrologic invariance, mitigating the risk of flash floods and the tragic consequences of extreme weather events. The collateral ecosystem services are also quantified, from thermoregulation via evapotranspiration to atmospheric purification, highlighting the positive socio-economic return and the avoided costs for public administrations. Finally, an intersectoral governance model is proposed to overcome administrative silos, configuring the blue-green infrastructure as a water and thermal security shield for the cities of the future.

The paradigm shift: green as a hydro-strategic infrastructure

For over a century, the development of Western cities has followed a purely "grey" planning logic. The traditional engineering approach has considered rainwater not as a resource to be managed, but as a nuisance to be removed in the shortest possible time (1). This philosophy has led to a hyper-impermeabilization of the soil, where concrete and asphalt have progressively replaced natural corridors, abruptly interrupting the local hydrological cycle and leading to increasingly arid soil. In contemporary cities, the water cycle is literally inverted compared to natural systems: while in a forest ecosystem about 50% of rainwater percolates into the ground and only 10% turns into surface runoff, in a densely urbanized environment, surface runoff often exceeds 55%, at the expense of deep infiltration, reduced to insignificant percentages (2). The consequences of this linear model manifest today through flash floods due to the collapse of undersized sewer networks, a chronic pollution of receiving water bodies due to the washing of road surfaces by first-flush waters, and a progressive depletion of underground aquifers that accelerates subsidence phenomena. The consequences of sub-optimal land management and hydrogeological risk can be particularly severe for local communities, as demonstrated by the case of the city of Niscemi in Sicily, where, in January 2026, 500 people were evacuated from their homes, with significant economic, human, and environmental repercussions (3).

Faced with the structural and economic limits of the grey model, a radical paradigm shift is required in which urban vegetation must be elevated to the rank of Blue-Green Infrastructure. This is a strategically planned network of natural and semi-natural elements designed to deliver a wide range of ecosystem services (4). The term "infrastructure" assumes here a purely functional value: exactly as a highway network manages traffic flows, the Blue-Green Infrastructure manages the flows of matter, energy and, above all, water (5). The real innovation lies in the inseparable fusion between the plant component (the green) and the water resource (the blue). The plant becomes the biological interface through which water is intercepted, retained, purified, and returned to the atmosphere or the aquifer. Therefore, effective urban water management cannot exist without the support of plant biomass, just as resilient urban greenery cannot exist without a dedicated water strategy that feeds and sustains it. In this way, urban infrastructures are more stable thanks to the absorption capacities of the soil, as well as the action of vegetation to retain and compact the soil to limit drainage risk.

To translate this infrastructural vision into concrete engineering and urban solutions, the global Sponge City model is an approach that aims to make urban areas capable of absorbing, storing, filtering, and releasing rainwater naturally, emulating the behavior of unanthropized soil (6). The core objective is the achievement of hydraulic and hydrological invariance, ensuring that the peak flow and volume of water runoff discharged from an area do not exceed those pre-existing urbanization. This goal is pursued through the integration of Nature-Based Solutions (NBS) into the urban fabric, acting as a widespread system of micro-reservoirs.   

Among these, rain gardens and bioswales collect street runoff by exploiting the microbiologically active soil and plant roots as a biological phytodepuration filter before percolation into the aquifer. In parallel, green roofs and living walls intercept up to 70-90% of annual precipitation falling on building roofs, reducing the overall volume and drastically delaying the time of concentration. 7 These elements are flanked by multifunctional public spaces, such as water squares and controlled flooding areas, which act as recreational parks in dry periods and as temporary lamination basins during flood peaks.

Fig. 2: Integrated functioning of the Residential Rain Garden model.

Among these, rain gardens and bioswales collect street runoff by exploiting the microbiologically active soil and plant roots as a biological phytodepuration filter before percolation into the aquifer. In parallel, green roofs and living walls intercept up to 70-90% of annual precipitation falling on building roofs, reducing the overall volume and drastically delaying the time of concentration. 7 These elements are flanked by multifunctional public spaces, such as water squares and controlled flooding areas, which act as recreational parks in dry periods and as temporary lamination basins during flood peaks. 

The hydro-strategy applied to urban greenery thus transforms the city into a porous ecosystem, where water ceases to be a liquid waste to be expelled and returns to being the lifeblood of the city's infrastructure. This approach embodies the principles of the circular economy applied to water resources, overcoming the traditional linear economic model based on the "take-make-dispose" logic, in favor of a closed cycle of reuse and valorization that will be resumed in the conclusions.

The multifunctional ecosystem: climatic, environmental, and socio-economic benefits

The effectiveness of a Blue-Green Urban Infrastructure is not limited solely to the mitigation of the hydraulic risk described previously, but extends to the generation of a wide range of interconnected ecosystem benefits, capable of radically transforming the livability and economy of urban centers. The real engine of this transformation is the water resource retained in the city fabric, which stops being a danger and becomes the biological fuel necessary to activate environmental regulation services. Many contemporary cities are afflicted by the Urban Heat Island (UHI) phenomenon, in which summer temperatures in densely built centers can exceed those of surrounding rural areas by 5-10°C (8). In this context, urban greenery acts as a true natural air conditioner through the combined process of shading and evapotranspiration. The latter, in particular, represents an exquisitely hydro-strategic dynamic: plants absorb water from the soil through their roots and release it into the atmosphere as water vapor through their leaf stomata. This change of state from liquid to gas requires a considerable amount of thermal energy, which is extracted from the surrounding environment, resulting in an effective lowering of the air temperature. It has been shown that a 10% increase in urban tree cover can reduce the surface temperature by more than 1°C, mitigating the lethal impacts of heat waves on the population (9). Unlike artificial air conditioning systems, which extract heat from interiors to pour it outside, thereby fueling the torrid microclimate of the streets, the plant infrastructure dissipates thermal energy sustainably, provided that the urban water system is planned to ensure its constant supply.

Internationally, the empirical application of these methodologies offers evident feedback and opens a crucial debate on urban planning choices. The case of Paris is emblematic: through the Oasis project, the French capital has initiated the transformation of school courtyards from expanses of impermeable asphalt to veritable microclimatic hubs. This strategy is based on de-paving (débitumisation) and the introduction of natural and permeable materials capable of retaining rainwater in the soil, nourishing the new vegetation and drastically reducing the local temperature during heat waves. A radically different approach, based on the engineering modification of urban albedo, can be found in the experience of Los Angeles, the metropolis that has chosen to mitigate extreme temperatures by coating miles of roads and building roofs in white through the application of special reflective resins (cool pavements and cool roofs), capable of repelling solar radiation and lowering the surface temperatures of the asphalt by up to 10°C (10). Although these techniques improve the lives of citizens in the city, when analyzed through the lens of urban hydro-strategic planning, these solutions show some structural limits due to their mono-functionality. Although the increase in albedo effectively mitigates surface thermal accumulation, it leaves the problem of managing water flows, sewer overload, and soil impermeabilization entirely unresolved. An integrated approach does not limit itself to repelling solar radiation but transforms the water resource into an active agent of latent cooling through evapotranspiration, offering a simultaneous response to the city's hydraulic and thermal crises.

Parallel to thermal regulation, the biological architecture of the GBI plays a fundamental role in the purification of the atmospheric matrix and in combating global climate change (11). The widespread presence of arboreal and shrubby biomass within cities constitutes a mechanical and chemical filter for the main atmospheric pollutants deriving from vehicular traffic and domestic heating. Leaves, thanks to their roughness and thepresence of waxes and trichomes, intercept and capture fine particulate matter (PM10 and PM2.5), permanently removing them from the airborne cycle until precipitation washes them away, directing them towards soil filtration systems (12). At the same time, through the process of photosynthesis, urban vegetation sequesters atmospheric carbon dioxide (CO2), storing carbon in woody tissues over the long term and directly contributing to local climate neutrality goals. This purification service is not disconnected from the water component: an adequate state of hydration of the plants is the necessary condition for the stomata to remain open to absorb polluting gases, such as nitrogen oxides and tropospheric ozone, without the plant entering water stress, confirming how the environmental health of the city depends on hydro-strategic efficiency.

This complex of ecological functions translates directly into a tangible impact on the socio-economic dimension and the quality of life of citizens, offering a Return on Investment (ROI) that far exceeds the construction and maintenance costs of biological works. From a public health perspective, access to structured green spaces and the presence of natural elements in the urban landscape are associated with a drastic reduction in cardiovascular and respiratory diseases, as well as an improvement in psychological well-being, with a clear decrease in stress, anxiety, and depression levels in the population (13). Blue-green infrastructures redefine public space, transforming marginal or degraded areas into places of social aggregation, also countering the phenomenon of commodification of public space, stimulating active mobility, and mending the community fabric. This social regeneration in turn generates a strong economic value of a direct and indirect nature. From a real estate perspective, buildings adjacent to parks, tree-lined avenues, or green roofs register a significant increase in their market value, changing the economic attractiveness of entire neighborhoods.

However, the most relevant economic aspect for public administration budgets lies in the so-called "avoided costs". A correct evaluation of ecosystem services shows how investment in GBI linearly reduces expenses for health treatments of the population, energy costs for summer cooling of buildings, and, above all, the multimillion-dollar compensations linked to flood damages caused by catastrophic rains (14). Managing water through greenery is not an aesthetic luxury, but a strategy of urban macroeconomic stability that transforms emergency management costs into preventive structural investments.

From theory to practice: hydro-urban governance and future perspectives

The transition from a linear and rigid urban model to a porous and resilient system is not solely an engineering or botanical challenge, but primarily represents a complex challenge of governance and political planning. The main obstacle to the large-scale implementation of Blue-Green Infrastructures lies in the historical separation of competencies within public administrations, where stormwater management (entrusted to hydraulic engineers), the design of public spaces (led by architects and urban planners), and greenery maintenance (managed by agronomists and park services) traditionally operate in isolated and non-communicating "silos". 15 To overcome this institutional fragmentation, it is urgent to define an integrated policy framework in which water plans and general urban planning tools are no longer conceived as independent layers but merge into a single hydro-urban vision.

From a regulatory point of view, this requires the introduction of stringent and binding criteria within municipal building regulations, making the adoption of Nature-Based Solutions (NBS) mandatory both in new construction projects and in urban regeneration interventions.16 In this perspective, planning must embrace the principles of the circular economy of water resources at a neighborhood scale: green infrastructure must not be seen as a factor of further consumption of drinking waterm, an unsustainable dynamic in scenarios of increasing scarcity, but must be fed through intelligent systems for the collection, phytodepuration, and reuse of local grey and meteoric waters, ensuring the survival of vegetation even during periods of prolonged severe water stress.

This bureaucratic and technical evolution lays the foundations for the formulation of a hydro-strategic vision for urban centers, elevating the GBI to a fundamental "Water Security Shield" for the resilience of territories. In an era characterized by the polarization of meteorological events, the blue-green infrastructure can operate as a dynamic and bidirectional regulator.

On the one hand, it acts as a hydraulic shock absorber during extreme rainfall; on the other, it configures itself as a "Water Thermostat" useful for the thermal resilience of cities. The capacity of greenery to cool the environment through evapotranspiration, in fact, depends in a linear and direct way on the amount of water stored in the soil: a hydro-strategically planned city accumulates the water resource during rainy months within its spongy fabric to then use it as a natural refreshment during summer heat waves, stabilizing the local microclimate. Allowing water to infiltrate vertically through rain gardens and draining areas means transforming the city's subsoil into an immense decentralized strategic storage basin, which constitutes the only true insurance policy against periods of drought.

The ecological transition of cities, ultimately, is measured both by the square footage of green spaces and planted trees, and by the ability to govern water flows through biological processes, overturning the twentieth-century assumption of water removal to coordinate the philosophy of its retention with landscape enhancement. Designing urban greenery means, today more than ever, doing hydro-strategy: an essential political and technical act to secure the territory, guarantee supply, and protect future generations from the extremes of climate change.

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