Hydrological Behaviour of Extensive Green Roofs with Native Plants in the Humid Subtropical Climate Context

Lucia Bortolini,Giampaolo Zanin,Francesco Bettella

doi:10.3390/w13010044

Abstract

Different mitigation measures with vegetation have been proposed to sustainably manage rainwater, among which green roofs have demonstrated to be a valid solution in urbanized areas. Green roofs have gained interest also in Italy, but their spreading is generally based on application of ready-to-use packages, poorly tested in the specific climate conditions. A study was carried out to evaluate the green roof solution most suitable in the humid, subtropical climate context of Veneto Plain (north-eastern Italy) to reduce outflow volumes from building roofs into the urban drainage systems. Twelve different microcosm combinations of extensive green roof (three plant mixtures × two substrates × two storage/drainage layers) were tested and compared with gravel (considered as a conventional flat roof with gravel ballast). The tested drainage/storage layers were a preformed layer in recycled HDPE (PL) and an expanded perlite mineral layer (ML), and the growth medium layers were recycled brick substrate (RS) and volcanic substrate (VS). Three different mixtures of native plant species were transplanted: Sedum (SE), herbaceous perennial (HE), and suffruticose (SF). Results showed that all the green roof systems have a good ability to manage rainwater, with a retention ranging on average from 46.2% (SE-RS-PL microcosms) to 62.9% (SF-RS-ML microcosms) of the precipitation in the two-year period (September 2014–August 2016), against 15.4%, retained by gravel. Over the two-year period, the retained rainfall volumes were about 100% for all the light rainy events (<10 mm) and varied within a range of 48–95% for medium rainy events (≥10 and <25 mm) and 20–88% for heavy rainy events (≥25 mm), depending on rainfall depth and the antecedent weather period. The layer that gave the highest relative contribution to the stormwater retention was the vegetation layer, followed by the drainage/storage layers and then the substrate layer. In particular, SF plants decreased the outflows by 15.2% on average compared to SE, and ML layer retained more than 10% of precipitation compared to PL layer. At last, the analysis of variance showed that, within each layer, the more effective in water retention, able to generate less outflow volumes, was similarly suffruticose and herbaceous mixtures, the crushed bricks substrate, and the mineral drainage/storage layer.

Highlights

Green roofs are water retention measures to effectively counter the effects of the intense urbanization of the last decades that is causing many problems, such as the increase in air and surface temperatures, with the consequent urban heat island (UHI) effect, the intensification of the flooding phenomena, the accumulation of pollutants in the air and in the ground, and a reduction of the ornamental and landscape aspects inside and outside the urban areas [1,2].The modified urban microclimate creates problems for the health and well-being of the population, which is more severe in densely urbanized areas [3,4,5]
As for plant mix effect, the results show that the Sedum mixture was found to be significantly less effective in retaining rainwater, while the use of herbaceous perennial and suffruticose plants, instead, was an excellent solution to reduce outflows
The study and the performed tests clearly demonstrated the positive effect of extensive green roofs with native plants, compared with a traditional roof, to reduce the outflows originating from rainfall events in a humid, subtropical climate

Summary

Introduction

Green roofs are water retention measures to effectively counter the effects of the intense urbanization of the last decades that is causing many problems, such as the increase in air and surface temperatures, with the consequent urban heat island (UHI) effect, the intensification of the flooding phenomena, the accumulation of pollutants in the air and in the ground, and a reduction of the ornamental and landscape aspects inside and outside the urban areas [1,2].The modified urban microclimate creates problems for the health and well-being of the population, which is more severe in densely urbanized areas [3,4,5]. Green roofs are water retention measures to effectively counter the effects of the intense urbanization of the last decades that is causing many problems, such as the increase in air and surface temperatures, with the consequent urban heat island (UHI) effect, the intensification of the flooding phenomena, the accumulation of pollutants in the air and in the ground, and a reduction of the ornamental and landscape aspects inside and outside the urban areas [1,2]. Increasing the presence of plants through green roofs can mitigate the UHI effects thanks to evapotranspiration phenomena [6,7,8,9,10]. CO2 [20], and contribute to the increase in the ornamental and landscape value both inside and outside urban areas [21,22]. In Veneto region (north-eastern Italy), urbanisation and industrial expansion are common trends [25,26], and the increasing flood occurrence in this region is partly attributed to these land transformation processes [27], with a decrease in the drainage density of agricultural fields, water storage capacity, and peak flow buffering [28,29]

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Discussion

Conclusion