1. Urban challenges: why we need nature-based solutions

In the present day and age, we tend to think about cities and nature as two incompatible opposites: on one side, the grey, sterile, polluted city, overcrowded with people and cars; on the other side, the green, fertile nature, filled with life. Recently, however, more and more researchers, urban planners and citizens have started talking about living infrastructure, biophilic urbanism and rewilding: all these terms refer to the use of natural elements in the cities to deliver ecosystem services benefitting both man and biodiversity. In fact, over the years it has become clear that the current urban planning models and the sole use of architectural and engineering solutions are not enough to tackle the challenges of living in cities. Air and noise pollution, floods, the heat-island effect, a general lack of beauty and green spaces, respiratory diseases, obesity, depression, are all phenomena we have come to accept as a natural consequence of living in cities. Shouldn’t things be different?

As the IUCN defines them, nature-based solutions are “actions to protect, sustainably manage and restore natural and modified ecosystems in ways that address societal challenges effectively and adaptively, to provide both human well-being and biodiversity benefits.” I would like to name a few of these solutions and show how their implementation would benefit urban areas.

2. Green walls and green roofs

Both green walls and green roofs are green infrastructures combining architectural elements and vegetation.

Green walls, or vertical gardens, are structures installed on an indoor or outdoor wall consisting of a growth medium such as soil or hydroculture felt, a hydration and fertigation delivery system and a layer of selected vegetation, chosen primarily based on the local conditions of humidity and sun exposure. Green walls deliver several benefits:

  • Aesthetic improvement: it is the first reason for their installation. Indoor vertical gardens could be seen in several luxury buildings and shops across all the major cities of the world and have become a trend in interior design. They are also installed outdoors, but, given the not so cheap costs of realisation, they are less common. A wonderful example of an outdoor green wall is the Musée du Quai Branly, in Paris. If the technology were cheaper, it could be installed on more buildings to guarantee a better equilibrium between concrete and vegetation.
  • Pollutant reduction: it is well known that the presence of vegetation helps to reduce the concentration of particulate matter in the atmosphere by dispersing the particles that collide with the leaves and by trapping them in the waxy and hairy leaf surface (when it rains, the particles are then washed away into drains and consequently into water-treatment plants). Several studies are now concentrating on the quantification of pollutant reduction and on the selection of the most suitable plant species. Green walls could therefore help to reduce particulate concentration in those densely populated areas the space for public green is limited, thus providing a sanitary benefit to the citizens.
  • Noise reduction: vegetation acts as a noise insulator and for this reason improves the inhabitants’ life quality.
  • Thermal insulation: vegetation would provide better energy efficiency for the buildings.
  • Heat island effect reduction: evapotranspiration reduces heat emanating from asphalt and concrete, improving the citizens’ life quality in summer, and reducing the need for air conditioning.
  • Ecological benefits: green walls could potentially have positive ecological impacts, such as the attraction of birds and insects, but further research is needed.

Green roofs are roofs covered with vegetation, a growing medium and a waterproofing membrane. Two typologies exist intensive roofs, which are thicker and can support heavier plants and trees, but require more maintenance; extensive roofs, meaning they are shallower and require less maintenance, and are usually planted with vegetation resistant both to humidity and to a lack of water, such as Sedum. Some of the benefits they provide are like those of green walls, while others are exclusive to green roofs:

  • Noise reduction, heat-island effect reduction, thermal insulation.
  • Use of unused surfaces: in the cities, where the room for private gardens is limited, the installation of green roofs comes with the possibility to use that new surface in the free time. In addition, they contribute to reducing the consumption of soil, a non-renewable resource.
  • Water retention: the vegetation and the absorbing layers retain the water and return it back to the drain later after the rain event. This helps the water-treatment plants to face high-intensity rain events without going over-capacity and to reduce the pressure on sewage. In addition, it could be possible to use the collected water for sanitary ware, given that nowadays in Italy we still use perfectly drinkable water for that purpose.
  • Ecological benefits: evidence shows that green roofs host a greater species diversity than conventional roofs, but it is still unclear whether they could be ecologically relevant in the context of the urban landscape, thus more studies are necessary.

3. Rain gardens

Rain gardens are a typology of sustainable urban drainage systems, which combine both engineering and natural elements to manage water in urban and peri-urban areas. The base concept underpinning SUDs is the sponge city: born in China, this model aims at the absorption of water during storm events and the later restitution in drains. This may seem obvious, but the more frequent flooding events have made clear that the stress put on drains, sewages and water plants exceeds their management capacity. With the expansion of cities and the increasing soil impermeabilization, this problem will only get worse. SUDs involve permeable surfaces in parking lots, green roofs, bioswales, wetlands, retention basins, rain gardens. However, despite their potential, they still are not commonly employed, also due to the lack of long-term studies on their effectiveness.

Rain gardens are conceptually very simple: they are gardens planted in a slight depression, often with a drainage pipe connected to the sewers. Thanks to the lower infiltration rate of the soil and to the plants, they retain the water for 24-48 hours thus reducing the stress on the water treatment facility. They also provide many more benefits:

  • Low costs of installation and management.
  • Water depuration from metals and organic pollutants thanks to the bioremediation mediated by bacteria in the soil or in the zeolite or gravel beds.
  • Heat-island effect reduction.
  • Aesthetic improvement of the city: in fact, they are virtually indistinguishable from common gardens.
  • Creation of habitats for pollinators.

4. Final considerations

Nature-based solutions are a very fascinating concept, and their strength lies in their low costs of maintenance and in the provision of several benefits: considering the growing pressure that cities will bear soon, there is a lot of room for their use. Urban planners and governors should implement and incentivise these solutions, while researchers should focus on the quantification of the benefits they provide and on the improvement of their efficacy.

Several cities in Italy are densely populated and suffer from a lot of the abovementioned problems, some of which are getting worse because of climate change. Unless we want to be left behind by Europe, China, and the rest of the world, now is the time to invest and experiment with nature-based solutions and green infrastructures.

 

Source: Written by Laura Besana, MSc student in Environmental Sciences and Technologies as published on Linkedin