Stormwater Detention and Retention Abilities of Green Roofs

With growing urbanization comes increased impervious areas and decreased open land for stormwater BMPs. Urbanization also increases stormwater runoff volumes and peak flow rates while decreasing infiltration and evapotranspiration. Green roofs consist of layers of specially designed and selected materials combined with shallow-rooted living plants to form a biological system that mitigates stormwater and decreases the heating and cooling load within structures. The departments of Agricultural and Biological Engineering and Horticulture at the Pennsylvania State University have combined efforts to quantify the stormwater retention capabilities of a specific green-roof system. This green-roof system consists of a conventional flat roof covering, a 12-mm thick Enka-drainage layer, 89 mm of porous medium, 25 mm of Porous Expanded Polypropylene (PEPP), and Sedum sperium planted at a spacing of 75 mm on center. The combined layers of this green roof had a maximum retention of 31 mm (25%), a maximum detention of 30 mm (24%), and a saturated hydraulic conductivity of 11 mm/s. Rainfall and runoff data were collected at the Russell E. Larson Agricultural Research Center at Rock Springs, Pennsylvania. Three replications each of a control conventional roof and the experimental green roof system, both at 1:12 slope were monitored between July and October 2002. When compared to the conventional roof, the green roof detained runoff for up to five hours. The green roofs retained between 20% and 25% of two 25-mm October rainfall events.