The analysis of green roof's runoff volumes and its water quality in an experimental study in Porto Alegre, Southern Brazil

The green roofs are structures characterized by the application of vegetation cover in the buildings, using adequate waterproofing and drainage systems. It allows the reduction of surface runoff and delay in peak flow, contributing to the mitigation of flood events in urban areas. Therefore, this study aimed to evaluate the effect of the use of vegetal coverings on the surface runoff, taking into account quantitative and qualitative aspects, using an experimental module installed in the city of Porto Alegre, Brazil. The experimental station consisted of four modules: two horizontal modules with and without vegetation cover and two modules with slopes of 15 degrees with and without vegetation cover. It was evaluated 19 precipitation events, and it was verified the volumes drained in each module after 3, 6 and 12 h from the beginning of precipitation. The water samples were collected in order to analyse the quality of the runoff from the experimental modules. The results have shown that the use of vegetal coverings can provide better distribution of the surface runoff, as well as a decrease of the speed of excess water release with no surface runoff in the first 3 h after the onset of rainfall in the horizontal module. Additionally, it was proved the reduction in drained volumes, with the flat module with vegetation cover being capable of retaining completely precipitations with volumes of approximately 22 mm. The vegetation cover module in roofs was the one that has presented better results regarding the reduction of the flow, presenting an average retention percentage of 91.7% for the first 3 h, indicating that the slope is an important factor. The physical-chemical analysis of the water shows that for all analysed modules, it is possible to use water for non-potable uses, although the water quality of the modules with vegetation cover is lower when compared to the water coming from the module without vegetation cover.