An Experimental Study to Evaluate the Effectiveness of Green, Purple (Detention) and Traditional Roofs for Urban Stormwater Management

This study aimed to evaluate the hydrological performance of a purple roof, which was conceptually and technically different compared to a green roof in managing stormwater in an urban environment. For this purpose, three types of roofs, namely (i) traditional metal roof (ii) green roof and (iii) purple roof were constructed at Western Sydney University’s Penrith campus with a sophisticated, precise data acquisition system. The hydrological performance of these roofs (in terms of runoff volume reduction, time to runoff commencement and time to peak runoff) was investigated. Data was collected during April 2021 to May 2022 (14 months). A total of 76 rainfall events were recorded during this period. The average rainfall for these events was 16.8 mm, with maximum and minimum values of 118.2 mm and 0.25 mm, respectively. In this study, rainfall events were categorised into four different groups based on the event rainfall depth: (a) small events (0 to <5 mm; number of events (N) = 27), (b) medium events (5 to <25 mm; N = 35), (c) large events (25 to <50 mm; N = 9) and (d) very large events (>50 mm, N = 5). The hydrological performance of the configured roofs was examined for various rainfall event categories. It was found that the purple roof performs better than other roof systems. For small events, the purple roof did not produce any runoff in many cases and delayed the start of runoff generation by 458 minutes and 50 minutes compared to traditional and green roofs, respectively, when events were consecutive. In the case of medium events, the purple roof delayed runoff by 324 and 54 minutes for traditional and green roofs, respectively, and for large events 316 and 78 minutes for traditional and green roofs, respectively. In terms of volume reduction, the purple roof worked as expected, which was a slow release of water over time, reducing the peak runoff. The time to reach peak runoff was slowest for the purple roof compared to the other roof types. This represents the proposed technology’s potential to reduce pressure on the urban stormwater system during rainfall. Purple roofs have the potential to overcome the need for traditional detention basins in new developments.