Abstract
Flooding risk in cities has been recently exacerbated by increased urbanization and climate change, often with catastrophic consequences in terms of casualties and economic losses. Rainwater harvesting systems and green roofs are recognized as being among the most effective blue-green mitigation measures. However, performances of these systems have currently been investigated only at laboratory or very-small local scales. In this work, we assess the potential benefit of the extensive installation of these solutions on all the rooftops of 9 cities, with different climatological and geographical characteristics. Both surface discharge reduction and delay between rainfall and runoff peak generation have been investigated. Green roofs ensure a larger average lag time between rainfall and runoff peaks than rainwater harvesting systems, without significant differences between intensive and extensive structures. On the other hand, the cost-efficiency analysis, considering the entire urban area, shows a higher retention capacity with a lower financial investment for rainwater harvesting rather than for green roofs in most cases. For extreme rainfall events, large-scale installation of rainwater harvesting systems coupled with intensive green roofs over the entire city have shown to be the most efficient solution, with a total discharge reduction that can vary from 5% to 15%, depending on the city characteristics and local climate.
Highlights
There is a consensus among scientists that climate change is leading to an intensification of short-duration rainfall events, alternated with long dry periods [1,2,3]
The present study highlights and quantifies how the combination of rainwater harvesting (RWH) systems and green roofs (GRs) performs in terms of runoff reduction in urban areas
The study was conducted under some simplified hypotheses, such as neglecting possible additional costs for structural reinforcement in old buildings before the installation of GRs, it provides an overall comparison on performances of different blue-green solutions worldwide
Summary
There is a consensus among scientists that climate change is leading to an intensification of short-duration rainfall events, alternated with long dry periods [1,2,3]. Cities have become more and more urbanized, with a growth of urban density and impermeable surfaces [4,5,6]. The combination of these two phenomena makes our cities more prone to flood risk. E.g. green roofs, rainwater parks or permeable pavements, have been lately proposed and preferred to mitigate flood risk connected to extreme rainfall events at local
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