Abstract
Drainage from the 27,316-m2 Jacob K. Javits Convention Center (JJCC) green roof was investigated in the field to quantify the system’s long-term rainfall-runoff response. The JJCC hosts one of the largest extensive green roofs in the United States. Utilizing four years of rooftop monitoring data collected using a weather station, custom designed and built drainage systems, three Parshall flumes equipped with pressure transducers, and weighing lysimeters, this study quantified the 25.4-mm-deep green roof’s ability to decrease the volume and peak rate of runoff. With parameters derived from the site, the Environmental Protection Agency Stormwater Management Model (EPA-SWMM) predicted event total runoff volume and event peak runoff rates to within +10% to −20% and +25% to −15% of the observations, respectively. The analysis further indicated that approximately 55% of the cumulative precipitation that fell on the JJCC extensive green roof during the monitoring period (warm weather months, June 2014–November 2017) was captured and retained. The average percent retained on an event-basis was 77%, and average event runoff coefficient was 0.7, implying a substantial reduction in the volume and rate of runoff generated from the roof compared to the pre-green roof condition, when most, if not all, of the precipitated water would have immediately resulted in runoff. Our research suggests that, on average, 96% of rainfall events 6.35 mm or less were retained within the green roof, whereas 27% of the total event volume was retained for events greater than 12.7 mm in depth. A sensitivity analysis suggests if the substrate depth were increased, better stormwater capture performance would be achieved, but only up 127 mm, whereas increased precipitation coupled with warmer temperatures as a result of climate change could decrease the performance by up to 5%, regardless of substrate depth. An equivalency analysis suggested that even shallow green roofs can significantly reduce the required stormwater detention volume that New York City requires on new development. This particular green roof appears to be more than 18 times as cost-effective as a subsurface cistern would be for managing an equivalent volume of stormwater in Midtown Manhattan.
Highlights
Built even in ancient times [1,2], green roofs have emerged as one of the most effective methods of distributed storm water control in contemporary cities, where storm water runoffWater 2018, 10, 1494; doi:10.3390/w10111494 www.mdpi.com/journal/waterWater 2018, 10, 1494 can cause flooding, sewer surcharges and overflows, and pollutant loading to receiving waters [3,4,5,6].In dense cities with limited undeveloped space, one key advantage of green roofs over other types of green infrastructure (GI) is that they can be retrofitted onto existing buildings [7]
As the pilot phase of GI implementation in many US cities comes to an end, street-level runoff capture sites are increasingly scarce, making rooftops, which typically account for 20–25% of impervious cover in American cities [8,9], one of the big opportunities for decentralized urban storm water management
Intensive green roofs with thicker soil layers can accommodate larger vegetation resulting in higher retention rates, but often require building reinforcements or retrofits to support the additional weight, which increases the initial costs of installation
Summary
Built even in ancient times [1,2], green roofs have emerged as one of the most effective methods of distributed storm water control in contemporary cities, where storm water runoffWater 2018, 10, 1494; doi:10.3390/w10111494 www.mdpi.com/journal/waterWater 2018, 10, 1494 can cause flooding, sewer surcharges and overflows, and pollutant loading to receiving waters [3,4,5,6].In dense cities with limited undeveloped space, one key advantage of green roofs over other types of green infrastructure (GI) is that they can be retrofitted onto existing buildings [7]. Built even in ancient times [1,2], green roofs have emerged as one of the most effective methods of distributed storm water control in contemporary cities, where storm water runoff. Green roofs can be classified based on the thickness of the substrate layer as either intensive (substrate thickness > 150 mm) or extensive (substrate thickness < 150 mm) [10,14,15]. Intensive green roofs with thicker soil layers can accommodate larger vegetation resulting in higher retention rates, but often require building reinforcements or retrofits to support the additional weight, which increases the initial costs of installation. Extensive green roofs have much thinner substrate layers and can be installed at a lower cost, often without the need for reinforcements [7,16]
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