Greenroofs for stormwater runoff control : experiences from two sites

As cities expand and encroach onto rural lands, large impervious surfaces in the form of buildings, parking lots and roads cover the landscape. These impervious surfaces generate increased quantities of stormwater runoff which flow rapidly into municipal conveyance systems bypassing the natural infiltration processes resulting in polluted water and altered natural hydrologic systems. By replacing the footprint area of a building with a rooftop garden, greenroofs may offer an opportunity to control runoff and remove pollutants from rainwater. In this report we examined two greenroofs, one in Petaluma, California and the other in Portland, Oregon to determine their hydrological characteristics and how they perform as stormwater management techniques. We found that both roofs reduce the volume of stormwater runoff by an average greater than 60% while also reducing peak flows and increasing runoff time. Our results show that greenroofs are viable on-site stormwater control devices.