Hydrologic Performance of an Extensive Green Roof under Intense Rain Events: Results from a Rain-Chamber Simulation

Elena Giacomello,Jacopo Gaspari

doi:10.3390/su13063078

Abstract

The water storage capacity of a green roof generates several benefits for the building conterminous environment. The hydrologic performance is conventionally expressed by the runoff coefficient, according to international standards and guidelines. The runoff coefficient is a dimensionless number and defines the water retention performance over a long period. At the scale of single rain events, characterized by varying intensity and duration, the reaction of the green roof is scarcely investigated. The purpose of this study is to highlight how an extensive green roof—having a supposed minimum water performance, compared to an intensive one—responds to real and repetitive rain events, simulated in a rain chamber with controlled rain and runoff data. The experiment provides, through cumulative curve graphs, the behavior of the green roof sample during four rainy days. The simulated rain events are based on a statistical study (summarized in the paper) of 25 years of rain data for a specific location in North Italy characterized by an average rain/year of 1100 mm. The results prove the active response of the substrate, although thin and mineral, and quick draining, in terms of water retention and detention during intense rain events. The study raises questions about how to better express the water performance of green roofs.

Highlights

The results prove the active response of the substrate, thin and mineral, and quick draining, in terms of water retention and detention during intense rain events
The performed 4 days of simulations with the use of rain chamber led to the following results describing the behavior of the green roof solution under investigation, demonstrating the consistency of the assumptions with real operational conditions which were carefully recreated for testing purposes
The estimated runoff coefficient was equal to 0.68; this value was lower than the runoff coefficient of extensive green roofs reported in green roof standards and guidelines, most likely due to the high water content in the substrate determined by previous days of rain simulations on the sample

Summary

Introduction

As evidenced by the Intergovernmental Panel on Climate Change (IPCC) [11], the correlation between the increase of average temperature and the recurrence of extreme weather events has been largely proven and many countries have already experienced problems of water scarcity or massive flooding that has become even more critical as a result of other very impacting phenomena, such as Urban Heat Islands (UHI) and heat waves [12,13,14,15] These events are often dependent on human activities: the way cities and buildings are shaped, the design choices, the use of materials and, above all, the very limited availability of green spaces within the built environment. This strongly reduces the chance of benefitting from the capacity of the natural environment to mitigate the intensity of weather events and climate conditions, and that is why the adoption of Nature-Based Solutions (NBS) [16] are largely encouraged within the EU action framework

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