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

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.

Green Roofs as an Urban NbS Strategy for Rainwater Retention: Influencing Factors—A Review

“…Along the same line, Rosatto et al 2015 [45] described a decrease in storage capacity of the studied extensive GRs (15 cm media depth) (from 68% to 16%) with increasing precipitation intensity (ranging from 21 mm to over 90 mm precipitation events). Results presented by Bortolini et al 2021 [46] once again corroborated that GR systems present a high capacity to retain precipitation, with retained rainfall volumes varying depending on the intensity of the events: ≈100% for light precipitation (<10 mm); 48-95% for medium precipitation (≥10 and <25 mm) and 20-88% for heavy precipitation (≥25 mm). This differences in retention capacity not only depends on the amount of precipitation (rainfall depth) but also on the weather conditions before the event occurs.…”

“…Baryla et al 2018 [55] concluded that GR substrates with a mixture of mineral and organic materials in their composition showed higher retention abilities compared to growing substrates composed of only mineral materials. On the other hand, Bortolini et al 2021 [46] described a GR holding capacity between 46.2% and 62.9% against 15.4% retention by gravel. Another study reported that a substrate layer composed of fine tile duplicated the capacity to retain rainwater compared to the same substrate layer composed of coarse tile [38].…”

Simulating the impact of nature-based solutions on runoff control by using i-Tree Hydro: a case study in Padua (Italy)

“…A multi-year study conducted in Veneto (Bortolini and Zanin, 2018) has shown that a bioretention area with ornamental herbaceous plants (rain garden) with a surface equal to 10% of a drainage area (a building roof) can manage more than 95% of the total runoff, throughout infiltration and evapotranspiration, effectively avoiding also irrigation request during the summer periods. In another study conducted in the same area, Bortolini et al (2021) highlighted the positive effect on runoff reduction of various extensive green roof solutions. Depending on the stratigraphy and the type of vegetation, the reduction in runoff can exceed on average over 60% in the year, even if the effectiveness is less during the most intense rainy events, especially in periods characterized by lower temperatures in which the evapotranspiration phenomena are reduced (from November to February).…”

A Critical Review of Existing Methods to Evaluate the Performance of Nature-Based Solutions (NBS) on Commercial Roofs (CR) to Mitigate Urban Flooding

“…In a subtropical climate in northeastern Italy, ref. [50] constructed 36 platforms raised 1000 mm (3.3 ft) above ground for a full surface area of 0.44 m 2 for two years (2014-2016). The objective was to investigate the effect of different vegetation types (herbaceous perennial (HE), Sedum (SE), and suffruticose (SF)), substrate types (120 mm in depth volcanic substrate (VS), 120 mm in depth recycled substrate (RS)), and drainage (PL)/storage layers (ML) on the hydrological performance of GR.…”