Quantifying the Quantity and Quality of Runoff from Urban Green Roofs
DESCRIPTION: Green roofs are increasingly seen as an established ‘green infrastructure’ technology that confers many environmental benefits. This is especially the case in urban areas where rooftops comprise a large fraction of the landscape, are typically low albedo and add to widespread impervious surfaces. The benefits of green roofs include urban heat island mitigation, reduced or eliminated roof façade heat transfer with associated building energy benefits, stormwater retention and detention, ecosystem service benefits and aesthetic amenity value, to name a few. Stormwater mitigation and subsequent receiving water quality improvement are increasingly perceived as an important function of this technology. 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 New York City, where average annual precipitation in New York’s Central Park is over 1200 mm for the 40-year historic period 1971-2010. 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. 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.
OUTCOMES: Our findings reveal the importance of green roof technical design, as well as substrate capacity, for stormwater retention at different storm sizes. 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+ (ammonium) 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 (phosphorus) 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 (calcium) and Na (sodium) 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.