Rooftop Rainwater Control - Combining Storage Tank, Vegetation and Rooftop Retention

Over the past few years, rapid urbanization, population growth and the projected climate change have contributed to global water scarcity and urban flooding. Thus, the rainwater harvesting system in buildings is getting popular for increasing water efficiency and reducing stormwater runoff. However, it is required to size the rainwater harvesting system adequately for improving its operation. An oversized system increases the capital cost, while an undersized system results in an unreliable water source. Therefore, the storage tank's sizing is the most critical objective for optimizing the overall system in a dense urban location. Rainwater harvesting system and the vegetated roof technology can be particularly promising to address the issue because the vegetated roof and blue roof perform very well as a stormwater management tool by providing reduced stormwater runoff generation. The research identified the factors involved in managing runoff from a complex set of rooftop arrangements, having partially vegetated and non-vegetated areas and developed a set of guidelines to optimize the rainwater harvesting system by utilizing vegetated roofs. The research analyzed four mid-rise buildings of the Ryerson University campus, and the results confirmed that an increase in the total percentage of vegetated area coverage reduces the rainwater storage tank size to a great extent. For a small institutional building, a 40% agricultural roof performs the best for meeting the outside non-potable water demand reducing the annual overflow from the tank. For 60% vegetated area coverage, the municipal service water must supplement the rainwater to meet the total water demand. In terms of substrate depth, 150mm yields the most reasonable benefit. Finally, a blue roof should be combined with a vegetated roof to maximize the stormwater retention and detention onsite.

Ryerson Department of  Architectural Science Toronto, CA.