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.

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Globally, many cities have an overreliance on gray infrastructure, which does little to prevent flooding, and further contaminates the runoff, exacerbating its environmental effects. Green infrastructure (GI) has emerged as an alternative, which works by utilizing natural hydrology through the incorporation of permeable surfaces, retention, and water filtration. GI practices can include structures such as green roofs, permeable pavements, and rain gardens. Rain gardens are among the highest-priority GI practices in urban areas, due to their soil suitability, space requirements, and high nitrogen removal from stormwater. They are built with layers of gravel, geotextile fabric, engineered soil, and a hardy plant species. Stormwater is captured and retained, before eventually exiting via outlets, sewer piping, or evapotranspiration. This analysis of rain gardens includes an examination of stormwater infiltration and storage of a hypothetical design at a NYC location. Furthermore, "greened acres" (equivalent impervious area for stormwater) are presented for the project, to highlight the effectiveness of stormwater quantity management. Additionally, runoff quality was experimentally measured using a physical model of a rain garden, where nitrogen concentration is measured before and after to determine change in runoff quality. This concept is based on a previous green roof study, where nitrogen removal in soil was determined after wood mulch (which is known to use up excess nitrogen) was added. This previous experiment showed a 22% reduction in nitrates after a week of soil mixed with wood mulch. The modelled rain garden was able to capture 80% of the nitrogen in the inflow for a single storm event, but only 60% for a second storm event. Additionally, previously present nitrogen leached from the soil into the runoff, significantly reducing its effectiveness. Further research of the nitrogen removal by rain gardens is recommended.