Wet season hydrological performance of green roofs using native species under Mediterranean climate

Abstract

Urban areas generate considerable amounts of stormwater runoff due to a high percentage of impervious surfaces. In Mediterranean climates, during winter, large volumes of rainfall over short periods of time can cause flooding. Green roofs are emerging as a tool for stormwater management under the Sustainable Urban Drainage Systems philosophy. This paper investigates the influence of rainfall patterns and types of native vegetation cover upon the rainfall-runoff relations of a green roof under a Mediterranean climate during the autumn/winter period. Test beds were installed on a rooftop in Lisbon, Portugal, incorporating a substrate layer 150mm deep, three native vegetation covers and one non-vegetated control. Rainfall and runoff data were monitored over a 6-month autumn/winter period. Results show that the vegetated systems reduced the stormwater runoff, attenuated peak flow and delayed runoff. Overall, 69 of 184 events did not produce runoff, median retention ranged from 55 to 100% and median peak attenuation from 97 to 100%. The combination of shrubs, grasses and mosses proved to be the most effective vegetation cover. Antecedent substrate moisture and plant rainfall interception significantly influenced green roof stormwater performance for all the vegetation covers. Estimations based on the experimental green roof data, an empirical model and a detailed spatial analysis, revealed that, by greening 75% of the flat roof area of the municipality of Lisbon, approximately 166 500–224 000m3 of water could be retained, relieving the drainage systems and preventing floods.