Aerial view of a green roof

A new study published in the Journal of Environmental Management and co-authored by School of Environmental and Forest Sciences Professor L. Monika Moskal found that the use of green roofs in the Pacific Northwest effectively manages and reduces stormwater runoff.

A green roof is when the roof of a building is partially or completely covered in plants, vegetation and a growing foundation. It is among the most popular type of green infrastructure used in cities because of the low cost to create and maintain it, and they’re built on space that wouldn’t otherwise be used.

The study looked at the effectiveness of green roofs in the Pacific Northwest, and the results showed that the modeling approach the authors used is effective in assessing the watershed-scale hydraulic impacts of the green roofs in large cities.

The abstract of the study reads, “Green roofs are among the most popular type of green infrastructure implemented in highly urbanized watersheds due to their low cost and efficient utilization of unused or under-used space. In this study, we examined the effectiveness of green roofs to attenuate stormwater runoff across a large metropolitan area in the Pacific Northwest, United States. We utilized a spatially explicit ecohydrological watershed model called Visualizing Ecosystem Land Management Assessments (VELMA) to simulate the resulting stormwater hydrology of implementing green roofs over 25%, 50%, 75%, and 100% of existing buildings within four urban watersheds in Seattle, Washington, United States. We simulated the effects of two types of green roofs: extensive green roofs, which are characterized by shallow soil profiles and short vegetative cover, and intensive green roofs, which are characterized by deeper soil profiles and can support larger vegetation. While buildings only comprise approximately 10% of the total area within each of the four watersheds, our simulations showed that 100% implementation of green roofs on these buildings can achieve approximately 10–15% and 20–25% mean annual runoff reductions for extensive and intensive green roofs, respectively, over a 28-year simulation. These results provide an upper limit for volume reductions achievable by green roofs in these urban watersheds. We also showed that stormwater runoff reductions are proportionately smaller during higher flow regimes caused by increased precipitation, likely due to the limited storage capacity of saturated green roofs. In general, green roofs can be effective at reducing stormwater runoff, and their effectiveness is limited by both their areal extent and storage capacity. Our results showed that green roof implementation can be an effective stormwater management tool in highly urban areas, and we demonstrated that our modeling approach can be used to assess the watershed-scale hydrologic impacts of the widespread adoption of green roofs across large metropolitan areas.”

The full study can be viewed here.