# Nature-Based Solutions in Urban Areas: A European Analysis
## 5 Discussion
Although it is not the focus of this research, it is worth noting that a major advantage of implementing NbS is their high cost-benefit ratio over traditional solutions [25, 28, 29], as they are solutions that represent a flexible approach to sustainable inclusive growth at an affordable cost [8]. Thus, NbS represent an important tool for the development of a regenerative, shared, and circular economy in cities. A study of the identification of NbS shares in the city of Amsterdam addressed NbS classification according to the level of applied technology (this reinforces the existence of other NbS classification methods). An example of high-tech NbS classification is the intervention of a tree-lined central square (IJburg Island), with great efforts due to the enormous amount of vegetation and the creation of a water retention system. On the other hand, an urban park (Roofpark Orly Square) classified as low-tech NbS transformed a grey space into a green space, with the intention of retaining rainwater and capillary irrigation [55]. Another example of low-tech NbS is urban gardens characterized by low-impact solutions, particularly in Lisbon, where the cost of gardening is lower than in other European regions [70]. However, with respect to differences in classification, many articles approached the initiatives in an urban environment with the same logic adopted in the present work. Research shows positive results of microscale interventions, for example, green roofs generate benefits in food production, energy savings (10-30%), and rainwater storage for cultivation and local cooling [56]. Among NbS solutions, facade vegetation and green roofs on buildings play an important role in temperature regulation, with improvements in thermal load, thermal comfort, and thermal storage of the building [67], as well as in reducing the harmful effects of heat waves on human health [85]. Therefore, buildings play an important part in these processes, not just in terms of carbon reduction but also in terms of adaptability and resilience in urban areas [103]. Furthermore, when deployed on a large scale, green roofs show great potential to develop a robust flood control network [60; 103] and reduce runoff [67], in addition to potential to increase biodiversity [100]. Tree planting along streets or in urban parks has the greatest impact on heat mitigation and \"greenness\" (benefits in terms of restoration and mental health related to the amount of natural and seminatural areas that people experience in their surroundings, either by seeing them or directly accessing them) [100]. Small gardens also demonstrate cooling potential, as evidenced in the heavily urbanized region of Lisbon [69]. With great potential for the urban environment, as mentioned above, the actions of \"green buildings\" and \"urban gardens and green spaces between buildings\", characterized by the same scale of implementation, represent actions with less frequency in the sample of the analysed case studies. This may mean that large European investments in NbS initiatives in the urban environment occur in projects with larger scales of implementation. On a mesoscale, studies reported projected in the city of Amsterdam, with an alternative of ditches and filter strips (water decelerating green strip), which allows for local cooling and reduced runoff and storage capacity [56]. In Ljubljana, public orchards and nectar gardens with the planting of fruit trees (mesoscale) have increased livelihoods in the city by providing additional green areas, creating recreational areas, and generating a local cooling effect during hot summer periods [56]. This scale of implementation corresponds to most actions in an urban environment, representing approximately 45% of the total cases analysed. The actions of \"parks and urban forests\" and \"community gardens and allotments\" are highly correlated with the mesoscale. With the adoption of macroscale solutions, for example the Gorla Maggiore Water Park, there was the possibility of peak flow reduction of 86% (downstream flooding) [56].