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Evaluation of Green Roof Water Quantity and Quality Performance in an Urban Climate
OConnor, T., P. Culligan, T. Carson, S. Gaffin, R. Gibson, R. Hakimdavar, D. Hsueh, N. Hunter, D. Marasco, AND W. McGillis. Evaluation of Green Roof Water Quantity and Quality Performance in an Urban Climate. U.S. EPA Office of Research and Development, Washington, DC, EPA/600/R-14/180, 2014.
This effort, which supports SSWR 4.2, supports EPA's continued effort to show the utility of green infrastructure in the urban environment.
In this report we present an analysis of water benefits from an array of observed green roof and control (non-vegetated) roof project sites throughout NYC. The projects are located on a variety of building sites and represent a diverse set of available extensive green roof installation types, including vegetated mat, built up, and modular tray systems, as well as plant types. Moreover the projects have been monitored for a few years and are being observed in an urban climate. For water retention performance, we monitored runoff from four full-scale green roofs, including one built up system, one modular tray system and two vegetated mat systems. We gathered roof runoff data for over 100 storm events for each green roof over a period of 23-months. Our main findings for water quantity performance include: (i) runoff from green roofs has a quadratic relationship to precipitation depth, where the percent retention decreases as storm size increases; (ii) the relationship between precipitation depth and green roof runoff depth (runoff volume divided by rooftop drainage area) can be described by a Characteristic Runoff Equation (CRE) for each roof; (iii) the CRE can be used with historic rainfall data to reduce bias in reported green roof retention performance, which might arise due to a bias in the distribution of storm events during a monitoring period; (iv) the modular tray system captured the lowest percentage of precipitation among all green roof systems for storms 0-20 mm in depth, and the highest for storms above 30 mm; (v) multi-year predictions show that on an annual basis, the built up system will retain more rainfall than the modular tray system, which will retain more rainfall than the vegetated mat systems. Our findings reveal the importance of green roof technical design, as well as substrate capacity, for stormwater retention at different storm sizes. For water quality performance with respect to stormwater runoff, we undertook a 16-month survey of stormwater runoff quality from five full-scale green roofs, including two built up systems, one modular tray system and two vegetated mat systems. For comparison, we also surveyed the chemical composition of runoff from five non-vegetated (control) roofs as well as local precipitation. In total we collected and analyzed over 100 water samples. Our results show that the pH of runoff from green roofs was consistently higher than that from the control roofs and precipitation with observed average pH’s equal to 7.28, 6.27 and 4.82 for the green roofs, control roofs and precipitation, respectively. Thus, the green roofs neutralized the acid rain. In general, we observed lower NO3- (nitrate) and NH4+ concentrations in green runoff than control roof runoff, with the exception of runoff from the built up system, which had higher NO3- concentrations than the control roof runoff. Overall, total P concentrations were higher in green roof runoff than control roof runoff. Finally, with respect to micronutrients and heavy metals: we either detected these constituents at very low concentrations or not at all (concentrations were below the detection limit), with a few exceptions. One exception related to the detection of boron in runoff from one of the vegetated mat systems, and another related to the detection of Ca and Na in runoff from all five green roofs. Based on our results, we estimated that annual mass loading per unit rooftop area of NO3-, NH4+ and total P discharging from all five green roofs was considerably less than that from their respective control roofs, due to the ability of green roofs to retain precipitation. Thus, green roof implementation could improve urban stormwater and subsequently urban receiving water quality if achieved at large areal scales.