Implementing Nature-Based Solutions in the urban environment: benefits, limitations, and future challenges

According to the United Nations projections, the global population is expected to continuously grow in the next years, reaching 9.7 billion by 2050, and more than 70% of it will live in cities, with a consequent intensification of urbanization. In this context, the increase of short but intense rainfall events, as foreseen by the Intergovernmental Panel on Climate Change for many geographical regions, will lead to an increment of pluvial floods in urban areas, due to a higher and faster runoff generation. Besides a more traditional approach, where the sewer system is simply updated with larger pipes, multiple nature-based solutions, also called blue-green solutions, have been proposed and implemented to mitigate the runoff generation from impervious surfaces. These innovative solutions aim to reintroduce natural elements in the urbanized areas, and benefit from the soil retention capacity to store water during intense rainfall events and release it in the environment via evaporation, evapotranspiration, and infiltration processes. With the installation of these solutions, the volume of water collected by the drainage system is limited and, consequently, the pluvial flood risk is reduced. Among the most common nature-based solutions, it is worth recalling green roofs, green walls, rain gardens and retention and detention basins, which guarantee multiple benefits for the urban environment. Besides the runoff reduction, these structures can, in fact, help lowering the average temperatures, limiting the generation of the urban heat island, they improve the air quality, facilitating the CO2 sequestration, they increase the biodiversity, and they add aesthetic value to the cities, supporting the citizens physical and mental health. In this framework, this work presents a review of the most common and efficient nature-based solutions, installed in cities to mitigate pluvial floods and to ensure a sustainable development of the urban environment. Several nature-based solutions have proposed to mitigate pluvial floods and have shown high potential at local scale, while a large scale, involving the entire city, needs to be further investigated. Moreover, the integration of different blue-green solutions in the urban environment should receive further attentions to enhance the creation of smart and resilient cities.