Factors Controlling the Hydraulic Efficiency of Green Roofs in the Metropolitan Area of Milan (Italy)

Franco Salerno,Benedetta Barozzi,Riccardo Minoia,Diego Copetti,Nicolas Guyennon,Laura Marziali,Niccolò Pirola,Alice Bellazzi,Gianni Tartari,Nicola Colombo,Lucia Valsecchi

doi:10.3390/su132413638

Abstract

Green roofs (GRs) are considered sustainable solutions for the adaptation of urban water management to climate change. The use of GRs is particularly promising in urban environments like the Metropolitan Area of Milan, the most urbanized area in Italy. In this work, we evaluated the subsurface runoff coefficient at the event-time scale, for more than one year of observations, of 68 small-scale test beds comprising different configurations of green roofs (e.g., different vegetations, types and depths of growing media, and different slopes) installed in the Metropolitan Area of Milan. The objectives of this study are three-fold. Firstly, the controlling factors of the hydraulic have been assessed for efficiency. We calculated a mean drainage flow rate of 51%, finding that growing media play a significant role in determining the drainage flow during the spring, at the beginning of the vegetative period. During this season, water retention in fertilized beds increases significantly. At the beginning of the summer, the vegetation cover is able to significantly reduce the drainage flow, playing an even more crucial role with respect to the growing medium material. However, we found that the vegetation type (grass field and Sedum) does not play a significant role in the retention processes. Secondly, the delay of the peak flow rate was determined. We found a precipitation peak delay from 1 to 2 h, which would be sufficient to guarantee environmental benefits for urban drainage. Finally, the factors controlling the hydraulic efficiency of GRs for individual precipitation events were assessed. We found that soil moisture and cumulated precipitation are both significant factors determining the drainage flow rate. In conclusion, we point out that soil moisture is one of the main parameters characterizing GR drainage and should be further considered in future research efforts devoted to the analysis of GR performance.

Highlights

Assessing the effects of climate change and developing mitigation/adaptation measures are of primary importance in urban environments, as cities will comprise two-thirds of the global population by 2050 [1]
The mean drainage flow of the 48 test beds (TBs) located at the IRSA-CNR following 24 precipitation events during the 2018–2019 period was calculated to be equal to 51%, i.e., on average, half of the water is used for evapotranspiration processes and/or retained in the growing medium layer, while the other half is released into the urban drainage system
Green roofs may present a solution for minimizing the impact of urbanization on the hydrologic cycle and for sustainably managing water resources in urban environments

Summary

Introduction

Assessing the effects of climate change and developing mitigation/adaptation measures are of primary importance in urban environments, as cities will comprise two-thirds of the global population by 2050 [1]. The use of sustainable solutions as an alternative to conventional techniques has become a general goal of urban water management. To move in this sustainable direction, nature-based solutions (NBS) have been suggested as a potential eco-solution [6,7,8,9]. The critical point of this system involves the hydraulic capacity of the wastewater treatment plant (WWTP), which may be exceeded during rain events In such cases, a portion of untreated wastewater is directly discharged into receiving surface waters, impairing the ecological quality of aquatic ecosystems (e.g., [12,13,14,15])

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