Fig1: Green Roof Technology (http://dcgreenworks.org/programs/rainwater-conservation-and-reuse/green-roofs-2-0/)
Global warming, a major treat to the modern civilization, is speculated to increase the rate and magnitude of natural disasters, especially storms, in recent years. Thus, elevated level of storm water is one of the prominent concerns in the world, particularly in urban areas. The burning question is: How can we control this situation?
On Thursday, 3 Nov, Dr. Elizabeth Fassman-Beck, Associate Professor at Stevens Institute of Technology, New Jersey, presented her research on integration of green roof technology in the development of storm water management to the Department of Plant Science and Landscape Architecture. Her research focuses on improving the effectiveness and impact of green roofs/living roofs (Fig 1; vegetation and engineered media that sits atop a roof’s waterproof membrane) in urban areas by measuring water retention (rainfall capture) and detention (peak flow mitigation).
Urban storm water runoff is the result of impermeable roads, buildings, sidewalks, and less infiltration and evapotranspiration due to infrastructure, which negatively affects our environment and water by polluting with particulates, sewage, heavy metals, nutrients, bacteria, etc. Dr. Fassman-Beck showed that green infrastructure can be an effective solution to overcome the negative impact of storm-water runoff. She reported that retention, using a green roof can reduce runoff up to 67% of rainfall over a period of 28 months. Day-to-day retention can vary significantly depending on evapotranspiration of that area. Another aspect of controlling storm water is detention to slow the rate of flow. Studies of green roofs show more than 80% peak flow mitigation was achieved in Portland, 87% in North Carolina, 93% in New Zealand, 50% in Germany and 60% in UK. The question is, how can engineers calculate reduction rate? There are several hydrologic models are available. Each of them has different level of accuracy and necessitates different levels of effort and produces different types of information. In spite of its inaccuracy, regulatory agencies are still using “Curve Number” method. Conversely, USEPA SWMM 5.1 offer substantial improvements in accuracy and a greater range of useful information that can allow us to predict runoff from a rooftop, or from an entire watershed, under a wide range of climate conditions.
Considering the long term economic and environmental impacts, green roof or vegetated roof cover can be a valid management tool for addressing the current problems of elevated storm-water.
