An Investigation into the Hydrologic and Geochemical Processes Contributing to Green Roof Performance Public
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Low Impact Development (LID) techniques for site design are increasingly being utilized to mitigate the negative impacts associated with stormwater runoff, and green roofs are one such application. The ability of green roofs to reduce the total and peak volumes of stormwater runoff has been fairly well documented, but performance varies in different climate zones, and there is limited information available regarding green roof effectiveness in New England, a region whose weather patterns are notoriously variable from season to season and often even day-to-day. Additionally, there are questions regarding the impact that green roofs have on water quality, especially regarding phosphorus. While many green roofs have been found to leach phosphorus into stormwater runoff within the first few years after installation, it is assumed that this phenomenon will not continue after the green roof vegetation has been established. However, it is still unclear whether or not this assumption is valid, and very few research projects have focused on providing the necessary insight into the hydrologic and geochemical processes that are contributing to this observed problem. The Nitsch/Magliozzi Green Roof, located atop WPIâ€™s newest residence hall, was donated to enhance the sustainability of the building and to foster continued research and education. This roof provided an opportunity to better characterize the relationship between rainfall and runoff volumes, phosphorus sorption/desorption in the growing medium, and plant uptake processes. Comparisons of grab samples of stormwater from both the green and non-green portions of the roof within the first few seasons following installation confirmed that phosphorus was leaching into the runoff, and some seasonal trends were observed. For example, the highest concentrations (3-13 mg/l P-PO4-3) were observed during an especially rainy summer. In order to gain a better understanding into the nature of this occurrence, laboratory experiments on sections of this same green roof were designed and set up in WPIâ€™s greenhouse. A series of simulated rainfall events were conducted, a mass balance approach was used to analyze flow, and the phosphorus content of the water, plants, and soil were assessed. For flow attenuation, the green roof panels performed as expected under different rainfall and antecedent moisture conditions. Additionally, the greenhouse experiments provided improved insight into the nature of the relationship of phosphorus between the flow conditions, plant uptake, and soil processes, as well as its distribution throughout a storm. The laboratory data further provides a basis for estimating performance of a green roof and its long-term impact on stormwater quality. In a broader context, the findings also serve to inform future extensive green roof designs and subsequent research efforts.
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